Biological Evolution and Genetic Variation Updated April 09, 2016. What Is Evolution? Biological evolution is defined as any genetic change in a population that is inherited over several generations. These changes may be small or large, noticeable or not so noticeable. In order for an event to be considered an instance of evolution, changes have to occur on the genetic level of a population and be passed on from one generation to the next. This means that the genes , or more specifically, the alleles in the population change and are passed on. These changes are noticed in the phenotypes (expressed physical traits that can be seen) of the population. A change on the genetic level of a population is defined as a small-scale change and is called microevolution. Biological evolution also includes the idea that all of life is connected and can be traced back to one common ancestor. This is called macroevolution. What Evolution Is Not Biological evolution is not defined as simply change over time. Many organisms experience changes over time, such as weight loss or gain. continue reading below our video 10 Best Universities in the United States These changes are not considered instances of evolution because they are not genetic changes that can be passed on to the next generation.

Is Evolution a Theory? Evolution is a scientific theory that was proposed by Charles Darwin . A scientific theory gives explanations and predictions for naturally occurring phenomena based on observations and experimentations. This type of theory attempts to explain how events seen in the natural world work. The definition of a scientific theory differs from the common meaning of theory, which is defined as a guess or a supposition about a particular process. In contrast, a good scientific theory must be testable, falsifiable, and substantiated by factual evidence. When it comes to a scientific theory, there is no absolute proof. It's more a case of confirming the reasonability of accepting a theory as a viable explanation for a particular event. What Is Natural Selection? Natural selection is the process by which biological evolutionary changes take place. Natural selection acts on populations and not individuals. It is based on the following concepts: Individuals in a population have different traits which can be inherited. These individuals produce more young than the environment can support. The individuals in a population that are best suited to their environment will leave more offspring, resulting in a change in the genetic makeup of a population. The genetic variations that arise in a population happen by chance, but the process of natural selection does not.

Natural selection is the result of the interactions between genetic variations in a population and the environment. The environment determines which variations are more favorable. Individuals that possess traits that are better suited to their environment will survive to produce more offspring than other individuals. More favorable traits are thereby passed on to the population as a whole. Examples of genetic variation in a population include the modified leaves of carnivorous plants , cheetahs with stripes , snakes that fly , animals that play dead , and animals that resemble leaves . How Does Genetic Variation Occur in a Population? Genetic variation occurs mainly through DNA mutation , gene flow (movement of genes from one population to another) and sexual reproduction . Due to the fact that environments are unstable, populations that are genetically variable will be able to adapt to changing situations better than those that do not contain genetic variations. Sexual reproduction allows for genetic variations to occur through genetic recombination . Recombination occurs during meiosis and provides a way for producing new combinations of alleles on a single chromosome . Independent assortment during meiosis allows for an indefinite number of combinations of genes. Sexual reproduction makes it possible to assemble favorable gene combinations in a population or to remove unfavorable gene combinations from a population.

Populations with more favorable genetic combinations will survive in their environment and reproduce more offspring than those with less favorable genetic combinations. Biological Evolution Versus Creation The theory of evolution has caused controversy from the time of its introduction until today. The controversy stems from the perception that biological evolution is at odds with religion concerning the need for a divine creator. Evolutionists contend that evolution does not address the issue of whether or not God exists, but attempts to explain how natural processes work. In doing so however, there is no escaping the fact that evolution contradicts certain aspects of some religious beliefs. For example, the evolutionary account for the existence of life and the biblical account of creation are quite different. Evolution suggests that all life is connected and can be traced back to one common ancestor. A literal interpretation of biblical creation suggests that life was created by an all powerful, supernatural being (God). Still others have tried to merge these two concepts by contending that evolution does not exclude the possibility of the existence of God, but merely explains the process by which God created life. This view however, still contradicts a literal interpretation of creation as presented in the bible. In paring down the issue, a major bone of contention between the two views is the concept of macroevolution .

For the most part, evolutionists and creationists agree that microevolution does occur and is visible in nature. Macroevolution however, refers to the process of evolution that takes place on the level of species, in which one species evolves from another species. This is in stark contrast to the biblical view that God was personally involved in the formation and creation of living organisms. For now, the evolution/creation debate continues on and it appears that the differences between these two views are not likely to be settled any time soon. The Theory of Evolution in 2 Minutes - YouTube The Theory of Evolution in 2 Minutes Want to watch this again later? Sign in to add this video to a playlist. Need to report the video? Sign in to report inappropriate content. The interactive transcript could not be loaded. Loading... Rating is available when the video has been rented. This feature is not available right now. Please try again later. Uploaded on Aug 9, 2009 ... "The Theory of Evolution in 2 Minutes: What it is, what it says, how it works" by --- • --- In biology, evolution is change in the genetic material of a population of organisms from one generation to the next.

Though changes produced in any one generation are small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a process that can culminate in the emergence of new species. Indeed, the similarities amongst species suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence. The basis of evolution is the genes that are passed on from generation to generation; these produce an organism's inherited traits. These traits vary within populations, with organisms showing heritable differences (variation) in their traits. Evolution itself is the product of two opposing forces: processes that constantly introduce variation, and processes that make variants either become more common or rare. New variation arises in two main ways: either from mutations in genes, or from the transfer of genes between populations and between species. In species that reproduce sexually, new combinations of genes are also produced by genetic recombination, which can increase variation between organisms. Two major mechanisms determine which variants will become more common or rare in a population. One is natural selection, a process that causes helpful traits (those that increase the chance of survival and reproduction) to become more common in a population and causes harmful traits to become more rare.

This occurs because individuals with advantageous traits are more likely to reproduce, meaning that more individuals in the next generation will inherit these traits. Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of the variants best-suited for their environment. The other major mechanism driving evolution is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role that chance plays in whether a given trait will be passed on as individuals survive and reproduce. Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when studies of the fossil record and the diversity of living organisms convinced most scientists that species changed over time. However, the mechanism driving these changes remained unclear until the theories of natural selection were independently discovered by Charles Darwin and Alfred Wallace. Darwin's landmark work On the Origin of Species of 1859 brought the new theories of evolution by natural selection to a wide audience. Darwin's work soon led to overwhelming acceptance of evolution among scientists.

In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, which connected the units of evolution (genes) and the mechanism of evolution (natural selection). This powerful explanatory and predictive theory directs research by constantly raising new questions, and it has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth. • --- All clips/images used in this video are either copyright-free or covered under "fair use" for nonprofit educational purposes (Title 17 § 107 of the USC). Music: "Operation Evolution" by Dimrain47 . Phys.org - evolution evolution 28 minutes ago in Plants & Animals 3 Scientists at the University of Chicago have created the first genetically modified animals containing reconstructed ancient genes, which they used to test the evolutionary effects of genetic changes that happened in the ... Jan 13, 2017 in Evolution 800 It has long been suspected that humans and the urban areas we create are having an important—and surprisingly current and ongoing—effect on evolution, which may have significant implications for the sustainability of ...

Jan 03, 2017 in Environment 235 A new study from the George Washington University's Center for the Advanced Study of Human Paleobiology (CASHP) found that whereas brain size evolved at different rates for different species, especially during the evolution ... Jan 02, 2017 in Evolution 1071 You've likely heard about being in the right place at the wrong time, but what about having the right genes in the wrong environment? In other words, could a genetic mutation (or allele) that puts populations at risk for ... Dec 30, 2016 in Evolution 1323 Dec 20, 2016 in Plants & Animals 4 Dec 20, 2016 in Social Sciences report 503 Dec 13, 2016 in Social Sciences 14 Dec 08, 2016 in Archaeology & Fossils 419 Dec 06, 2016 in Archaeology & Fossils 37 Next » Evolution In biology, evolution is change in the genetic material of a population of organisms from one generation to the next.

Though changes produced in any one generation are small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a process that can culminate in the emergence of new species. Indeed, the similarities amongst species suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence . The basis of evolution is the genes that are passed on from generation to generation; these produce an organism's inherited traits. These traits vary within populations, with organisms showing heritable differences (variation) in their traits. Evolution itself is the product of two opposing forces: processes that constantly introduce variation, and processes that make variants either become more common or rare. New variation arises in two main ways: either from mutations in genes, or from the transfer of genes between populations and between species. In species that reproduce sexually, new combinations of genes are also produced by genetic recombination, which can increase variation between organisms. Two major mechanisms determine which variants will become more common or rare in a population. One is natural selection, a process that causes helpful traits (those that increase the chance of survival and reproduction) to become more common in a population and causes harmful traits to become more rare.

This occurs because individuals with advantageous traits are more likely to reproduce, meaning that more individuals in the next generation will inherit these traits. Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of the variants best-suited for their environment. The other major mechanism driving evolution is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role that chance plays in whether a given trait will be passed on as individuals survive and reproduce. Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when studies of the fossil record and the diversity of living organisms convinced most scientists that species changed over time. However, the mechanism driving these changes remained unclear until the theories of natural selection were independently discovered by Charles Darwin and Alfred Wallace. Darwin's landmark work On the Origin of Species of 1859 brought the new theories of evolution by natural selection to a wide audience. Darwin's work soon led to overwhelming acceptance of evolution among scientists.

In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, which connected the units of evolution (genes) and the mechanism of evolution (natural selection). This powerful explanatory and predictive theory directs research by constantly raising new questions, and it has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth. This text uses material from Wikipedia , licensed under CC BY-SA Population Genetics - Biology Encyclopedia - body, examples, human, process, different, chromosomes, DNA, blood Population Genetics - Biology Encyclopedia Population Genetics Photo by: Zoe The field of population genetics examines the amount of genetic variation within populations and the processes that influence this variation. A population is defined as a group of interbreeding individuals that exist together at the same time. Genetic variation refers to the degree of difference found among individuals, for instance in height, coat color, or other less observable traits.

The particular set of genes carried by an individual is known as his or her genotype, while all the genes in a population together comprise the "gene pool." Foundations The foundation for population genetics was laid in 1908, when Godfrey Hardy and Wilhelm Weinberg independently published what is now known as the Hardy-Weinberg equilibrium. The "equilibrium" is a simple prediction of genotype frequencies in any given generation, and the observation that the genotype frequencies are expected to remain constant from generation to generation as long as several simple assumptions are met. This description of stasis provides a counterpoint to studies of how populations change over time. The 1920s and 1930s witnessed the real development of population genetics, with important contributions by Ronald Fisher, Sewall Wright, and John B. S. Haldane. They, with many others, clearly established the basic processes which caused populations to change over time: selection, genetic drift, migration, and mutation. The change in the genetic makeup of a population over time, usually measured in terms of allele frequencies, is equivalent to evolutionary change.

For this reason, population genetics provides the groundwork for scientists' understanding of evolution, in particular microevolution, or changes within one or several populations over a limited time span. The questions addressed by population genetics are quite varied, but many fall within several broad categories. How much genetic variation is found in populations, and what processes govern this? How will a population change over time, and can a stable endpoint be determined? How much and why do populations of the same species differ? The answer is always cast in terms of selection, drift, mutation, migration, and the complex interplay among them. Of the four, selection and genetic drift are usually given credit as the major forces. Selection Simply put, selection occurs when some genotypes in the population are on average more successful in reproduction. These genotypes may survive better, produce more offspring, or be more successful in attracting mates; the alleles responsible for these traits are then passed on to offspring. There is broad theoretical consensus and abundant empirical data to suggest that selection can change populations radically and quickly. If one genetic variant, or allele, increases survivorship or fertility, selection will increase the frequency of the favored allele, and concurrently eliminate other alleles.

This type of selection, called directional selection, decreases the amount of genetic variation in populations. Alternatively, an individual carrying two different alleles for the same gene (a heterozygote) may have advantages, as exemplified by the well-known example of the sickle-cell allele in Africa, in which heterozygotes are more resistant to malaria. In this case, called overdominant selection, genetic variation is preserved in the population. Although a number of similar examples are known, directional selection is much more common than overdominant selection; this implies that the common action of selection is to decrease genetic variation within populations. It is equally clear that if different (initally similar) populations occupy different habitats, selection can create differences among populations by favoring different alleles in different areas. Genetic Drift Often overlooked by the layperson, genetic drift is given a place of importance in population genetics. While some analyses of genetic drift quickly become complicated, the basic process of drift is simple and involves random Cheetahs, which have very little genetic variation, are presumed to have gone through several genetic bottlenecks. changes in allele frequency. In sexual species, the frequency of alleles contained in the progeny may not perfectly match the frequency of the alleles contained in the parents.

As an analogy, consider flipping a coin twenty times. Although one might expect ten heads and ten tails, the actual outcome may be slightly different; in this example, the outcome (progeny) does not perfectly represent the relative frequency of heads and tails (the parents). What does this mean for populations? Start by considering neutral alleles, which have no impact on survival or reproduction. (An example is the presence or absence of a widow's peak hairline.) The frequency of a neutral allele may shift slightly between generations, sometimes increasing and sometimes decreasing. What outcomes are expected from this process? Suppose that a particular allele shifts frequency at random for a number of generations, eventually becoming very rare, with perhaps only one copy in the population. If the individual carrying this allele does not pass it on to any offspring or fails to have any offspring, the allele will be lost to the population. Once lost, the allele is gone from the population forever. In this light, drift causes the loss of genetic variation over time. All populations are subject to this process, with smaller populations more strongly affected than larger ones. Perhaps better known than the pervasive, general effects of genetic drift are special examples of drift associated with unusually small populations.

Genetic bottlenecks occur when a small number of individuals from a much larger population are the sole contributors to future generations; this occurs when a catastrophe kills most of the population, or when a few individuals start a new population in different area. Genetic bottlenecks reduce the genetic variation in the new or subsequent population relative to the old. Cheetahs, which have very little genetic variation, are presumed to have gone through several genetic bottlenecks. Occasionally, these new populations may have particular alleles that are much more common than in the original population, by chance alone. This is usually called the founder effect. Migration and Mutation Migration may also be important in shaping the genetic variation within populations and the differences among them. To geneticists, the word "migration" is synonymous with the term "gene flow." Immigration may change allele frequencies within a population if the immigrants differ genetically. The general effect of gene flow among populations is to make all of the populations of a species more similar. It can also restore alleles lost through genetic drift, or introduce new alleles formed by mutation in another population. Migration is often seen as the "glue" that binds the subpopulation of a species together.

Emigration is not expected to change populations unless the migrants are genetically different from those that remain; this is rarely observed, so emigration is often ignored. The last important process is mutation. Mutation is now understood in great detail at the molecular level, and consists of any change in the deoxyribonucleic acid (DNA) sequence of an organism. These mutations range from single base substitutions to the deletion or addition of tens or hundreds of bases to the duplication or reorganization of entire chromosomes . Mutation is most important as the sole source of all new genetic variation, which can then be spread from the population of origin by migration. This importance should not be undervalued, although the impact of mutation on most populations is negligible at any given time. This is because mutation rates are typically very low. Questions and Contributions The real challenge of population genetics has been in understanding how the four processes work together to produce the observable patterns. For instance, genetic drift eliminates variation from populations, as do the most common modes of natural selection. How then can the abundance of genetic variation in the world be explained?

This question has many complicated answers, but some cases, such as the observation of deleterious alleles in humans (for example, alleles for phenylketonuria, a genetic disease), might be explained in terms of mutation and selection. Mutation adds these alleles to a population, and selection removes them; although the rate of mutation is likely to be nearly constant, the rate at which selection removes them increases as the abundance of the allele increases. This is certainly true for recessive alleles, which are only expressed when an individual has two copies. With only one, the allele remains unexpressed and therefore not selected. At some point, predictable from the mutation rate and physical consequences of the disease, the two opposing forces balance, producing the stable persistence of the disease allele at low frequency. As a discipline, population genetics has contributed greatly to scientists' understanding of many disparate topics, including the development of resistance of insects to insecticides and of pathogenic bacteria to antibiotics, an explanation of human genetic variation like the alleles for sickle-cell anemia and blood groups, the evolutionary relationships among species, and many others. Of particular interest is the use of genetic data in conservation biology.

By definition, endangered and threatened species have reduced population sizes, making them subject to the vagaries of genetic drift and also to inbreeding. Inbreeding is mating between genetically related individuals, and often leads to inbreeding depression, a reduction of health, vigor, and fertility. Genetic drift leads to a loss of genetic variation, which limits what selection can do to produce adaptations if the environment changes. Keeping these two issues in mind, greatly reduced populations may be at increasingly greater risk for genetic reasons, leading to further declines. SEE ALSO General Biology/Evolution of Life - Wikibooks, open books for an open world General Biology/Evolution of Life Key Terms[ edit ] Evolution: Any change in allele frequency in a population, often the result of natural selection. Natural selection: differential reproduction of genotype within a population; one genotype reproduces more successfully than another and donates more copies of itself to the next generation. This means that allele frequencies change within a population. Natural selection is the only mechanism known to produce complex adaptations in nature. Natural selection can occur in any population that has heritable fitness differences. Fitness: the ability of an individual to contribute its genes to the next generation. Differences in fitness are central to natural selection.

Relative fitness: fitness of an individual compared to others in its species. Hardy–Weinberg principle states that both allele and genotype frequencies in a population remain constant or are in equilibrium from generation to generation unless specific disturbing influences are introduced. Natural Selection[ edit ] The tree diagram used to show the divergence of species. It also is the only illustration in the Origin of Species. Both Charles Darwin and Alfred Russell Wallace proposed natural selection. Wallace went to Darwin for help getting published and the result was that the two presented their papers together. Natural selection is the result of violation of Hardy-Weinberg equilibrium, a state of stability in a population where allele frequencies do not change. A population stays in Hardy-Weinberg equilibrium when five assumptions (or prerequisites) are maintained: Large population size No differential immigration or emigration of alleles (no gene flow in or out of the population) No mutation No natural selection Random mating (with respect to genotype), that is, individuals in species do not choose mating partners based on their genotype Results of Hardy-Weinberg equilibrium: genotype frequencies are in equilibrium equilibrium reached in one generation of random mating Q: What happens when we violate one of the five assumptions of Hardy-Weinberg equilibrium? A: Allele frequencies change.

The Hardy-Weinberg principle is named after English mathematician G. H. Hardy and German physician G. Weinberg, who independently came to similar conclusions in 1908. Note: In our lab computer simulation lab experiment, we relax the H-W restriction against natural selection, resulting in simulated evolution. When not all phenotype fitnesses are equal, selection occurs, allele frequencies change, and evolution results. Q: Will a dominant allele take over a recessive allele in frequency? A: No. The Hardy-Weinberg principal shows that any allele that confers greater fitness will become more frequent in the population than an allele that confers lesser fitness, and disproves the easy assumption that a dominant allele would overtake a population over time. Allele frequency is based on fitness, not whether it is dominant or recessive. Hardy-Weinberg equation: s {\displaystyle 1-s} Examples of adaptation due to natural selection Skin color of the Australian Death adder: brilliant orange skin very unusual in an animal but quite similar to the local soil color. Likely that the snake moved towards this color over time as snakes with brownish and then orange coloring experienced higher survivorship than other-colored snakes.

Pesticide resistance in insects: Pesticide DDT (developed in ‘30s to combat malaria by knocking out its vector, malaria-carrying mosquitoes) had an initially high kill rate that diminished over time until it became essentially ineffective against insects. Hundreds of similar pesticide resistances have been developed in other insect species. Antibiotic resistance in bacteria: disease-causing bacteria have followed a similar adaptive path to the insects. This is caused by incorrect use of antibiot-ics for disease treatment. An incomplete application of antibiotics kills most of the disease-carrying microbes in a patient’s body. The most antibiotic-resistant germs have then been selected. These multiply and can become increasingly uncontrollable by further antibiotic treatment. Public health officials fear that this process when repeated over time will create super-germs that will be re-sistant to all drugs. Tuberculosis (TB) is a disease currently following this path of increasing antibiotic resistance. There are various mechanisms of acquired chemical resistance: Behavioral change Decreased sensitivity to chemical at target site Degradation of the target Industrial melanism in English pepper moth (Biston betularia). The moth has two naturally occurring morphs, or varieties: typical (light-colored) and carbonized (dark-colored).

The more common morph has historically been the typical, but a change in its habitat led to an increase in frequency of the carbonized. (Air pollution killed off light-colored lichen on tree trunks, leaving moths exposed on dark bark instead of lighter lichen. Predatory birds acted as an agent against the more visible morph of the moth, which responded to the natural selection by increasing the frequency of the less visible morph. Monkeys who kill others’ young: Observed by Sarah Hrdy. Langur monkeys live in social troops with one male presiding over several females and their young. Female young stay within the troop but male young are kicked out to roam the fringes of the troop and try to take over other troops. Incoming male leaders kill suckling young, as well as the babies of females with whom he hasn’t mated. Q: Why? A: Because lactating females are in irregular estrus cycle and unavailable to become pregnant. Killing their young frees them up to become pregnant sooner with his babies. The counter adaptation in females: they mate with the new male even if already pregnant, as the new leader will not then not later kill their baby. (Similar situations have since been observed in other species, such as mice).

Selfless turkey problem: 1st generation male descendants of one female turkey set up a brotherhood, a tiny social hierarchy headed by the alpha male. The brotherhood courts females as a group. A female selects one brotherhood for mating, usually a larger one, then mates with the alpha male only. Q: Did natural selection shape the behavior of these brother turkeys? A: No answer given, but an implied “yes”. Q: Has natural selection tuned all characteristics of every organism? A: No. Example: The Indian rhino has 1 horn, and the African rhino has 2 horns. This difference is likely a historical accident, not an adaptation, as two horns don’t seem to give an advantage over one. Five constraints on evolution: Pleiotropic constraints Historical constraints: “present variation biases future possibilities”. Variation comes on top of past history. Formal constraints: Variation can’t defy laws of physics. Ex: pigs don’t fly, and insects are limited in size by their exoskeletons. You-can’t-get-there-from-here constraints: An advantageous end result must follow many tiny advantageous steps.

Ex: There are no live-bearing birds, possibly because the thin eggshell necessary for gas transfer if incubated inside the bird is so disadvantageous for a egg development outside the bird. However, flight feathers evolved from reptilian scales because each tiny change to the scales over time was advantageous, and afterwards allowed the evolution of flight. Time/variation constraints: new alleles formed thru random mutation, which needs time and is ultimately pushed by mathematic probabilities rather than pulled to a specific end. In limited time, limited results. Ex: Human heart disease the possible result of recent changes in human diet (last 100s of years as opposed to last 100,000s of years of human evolution). Pleiotropic constraints: (pleiotropy: when one gene has multiple phenotypic expressions) one allele may have several effects, some good & some bad. If the sum of parts is positive, the allele is favored (even if it carries along some unfavorable baggage). Ex: Cystic fibrosis allele may have been advantageous against cholera in pre-industrial Europe. Mutation: a random process that is the “ultimate” source for natural selection. Little mutation results in advantageous change, and so is very inefficient.

Q: Why does it work at all? A: Because: It has lots of time to work Each individual divides burden as a source of potential mutation; lots of chances for something to go well Sampling error: the difference between a sample and the actual population. Greater for smaller sample sizes. Unrelated to population size, based on abso-lute size of sample. Q: What happens when the population size is limited, restricting Hardy-Weinberg equilibrium? A: In small populations, chance events can dramatically change allele frequencies. Example: “A” and “a” alleles are in equal frequencies in a population: p 1.0 {\displaystyle q\prime =f(A)=1.0} One allele is completely eliminated! (If the population size is 40, the chances of this are 1/500,000). In this way, alleles can be eliminated from a real population. Q: How can small sample sizes occur in real populations?

Genetic bottleneck: population experiences a crash in numbers (this likely occurred recently in cheetahs, who have very little genetic variation) Founder effect: small group of “colonists” forms a new population Genetic drift: population lingers in low numbers, as in an endangered species. Virtual small population size: a small proportion of individuals in population cover reproductive resposibilites. Usually occurs with males, sometimes females. Ex: Mammals such as seals, where one giant male guards over and mates with harem of many females. This is because the females go to an isolated spot to birth to avoid predators, then go into heat just days after giving birth. A large male fights other males for the privilege of guarding these females and their offspring as “beach master”. The females intermittently go off to feed, coming back to feed their calves. Result in males is that they are much larger than the females; they store lots of body fat so they can stay on the beach a long time (as soon as they leave another male will come take their spot). Effective breeding population for one generation is therefore much smaller than actual number of living members.

Another example: in wolves, only the alpha male and alpha female breed, and the others just help raise their young. So a wolf population of sixty may have a breeding population of just six. Immigration / emigration: “gene flow” – individual moves in from a different area & brings new allele frequencies. “A little bit can do a lot”. Ex: interracial marriage. Mutation: produces variation in gene pool. Rates are low and work very slowly: just 1/100,000 to 1/1,000,000 mutations occur per locus per gamete per generation. Mutation alone is not enough to drive an allele to a higher frequency. A general rule: any allele with a frequency of 1% or more of the total population was not driven there by mutation. Non-random mating: individuals choose mates based on genotype. (Positive) assortative mating: choosing a mate with a genotype similar to your own, leading to homozygotic offspring. Dissortative mating: choosing for dissimilar genotype, leading to heterozygotes.

Inbreeding: mating with close relatives: a way to mate with your own genotype (and produce homozygotic offspring) Note: the average human has about thirty lethal recessive allele loci in his total chromatin. Mating with close kin increases the possibility of an offspring with double recessive lethal alleles. Many species have outbreeding behaviors to discourage inbreeding. Ex: langur society: females born in a troop remain in their troop but males are booted out, discouraging brothers mating with sisters. Non-random mating can change genotype frequencies but NOT allele frequencies by itself, therefore not responsible for evolution. But it can expose certain alleles to selection by making them homo or heterozygous in the genotype. Aging senescence: decline in performance in the general body of an organism with increasing age. This is a selectable trait and is not present in all organisms. Tissue does not “need” to be senescent. Some protists basically “live forever”; fruit flies can be bred for longevity. Q: Why is this? A: Gibson does not say why, but suggests that it is a selectively favored trait.

After class the professors seem to say that senescence is not necessarily an evolutionarily favored trait as much as a byproduct of the processes that led to higher development. In order to achieve delayed reproduction, more resources were put towards early survivorship at the expense of later survivorship. A related example is the difference between salmon in the Atlantic and Pacific oceans. Atlantic salmon live through multiple spawning seasons but Pacific salmon have been selected to put all their energy into one, difficult spawning cycle, where they die right after. Types of selection: Directional selection Disruptive selection Stabilizing selection: Selection that moves organisms toward the center of their range of possible traits. Ex: human birth weight; especially large and small babies suffer greater infant mortality, favoring babies of intermediate weight. A stabilizing environment results in fossil records that are unchanged for millions of years, such as for the body plans of sharks and horseshoe crabs. Directional selection: Selection that acts to eliminate an extreme from an array of phenotypes. Ex: Metals such as copper are usually almost lethal to some plants. A strain of copper-resistant grass has developed over many generations of growing in contaminated high-Cu soil. Disruptive selection: Selection that tends to eliminate intermediate type.

Ex: African seedcracker: has two bill sizes, one large and one small, each one best suited for a different kind of locally-abundant seed. Intermediate bill types are unfavored by selection because they are poorly suited for either kind of seed. Here, the homozygote that results in one bill type or the other is favored over the heterozygote, which produces the intermediate bill type. Consider a population that has the following genotypic frequencies for a given locus having two alleles, “A” and “a”: Genotype Aa' = .48 aa' = .16 We see that the genotype frequencies change from their initial values to their final vales. Therefore the population was not in Hardy-Weinberg equilibrium. Q: If the numbers are only somewhat dissimilar how do we tell if they should be considered similar or dissimilar? A: This is determined with statistical and mathematical analyses beyond the scope of this class. Tests have been done on breeding many organisms for a certain trait by divid-ing successive generations into groups based on phenotype. Ex: Fruit flies (Drosophila) artificially selected into two groups: one with many abdominal bristles, one with few bristles.

We find that neither of the two resulting groups even overlaps the bristle range found in the starting generation. This mimics the results of natural selection on a wild population under certain circumstances. This is similar to the process used for thousands of years by plant and animal breeders. Heterozygote advantage: when heterozygote genotype confers greater fitness than a homozygote. A classic example of this is sickle-cell anemia. This human disease occurs when a person is homozygotic for the recessive allele. The recessive allele increases survivorship against malaria in the herozygotic carrier. Natural selection works in high malaria areas to increase the frequency of this allele in humans. The allele frequency is highest in central Africa, where malaria has been a big killer. Genotype Speciation: Q: How do we get a new species? Q: What is a species? A: There are different definitions, but only one that we will go over in depth. Biological species concept (BSC): of Ernst Mayer. A species defined as a reproductively isolated population. If two organisms can interbreed, they are one species; if they cannot, they are not of the same species.

Not a perfect definition as lions and tigers can interbreed in captivity, and this works only with sexually reproducing organisms (dandelions reproduce asexually). Phenotypic definition: species defined by phenotypic gaps in a population. Cladistic definition: populations or population groups which are members a single clade Clade: a branch of the evolutionary tree. Two ways to name a new species: Anagenesis: an arbitrary / convenient way to name a new species after it has undergone enough change through a single evolutionary line Cladogenesis: the creation of clades Cladogenesis has three splitting models / hypotheses based on patra, “homeland” Allopatric speciation (allos: “other”; aka geographic isolation model, common) Allopatric species: don’t occur together. Sympatric speciation (sym: “together with”; common in plants) Parapatric speciation (para: “beside”; uncommon) Parapatric speciation: perhaps the only example we have of this is found in the copper-resistant mine grass previously discussed. Selection strongly favors the genotype homozygous for the Cu-resistant allele.

This has resulted in an adaptation in the pollination time to make it different than the pollination time for the surrounding grasses, avoiding cross-fertilization. Here is speciation without geographic isolation caused by the intense selection. Sympatric speciation: (is this info right? Book seems to conflict with lecture) a common source of new species in plants but uncommon in animals, this occurs when gametes from two different species cross and create a viable new species. This believed to be the source of ½ of the flowering plants. Polyploidization: the most important model of sympatric speciation. Or, the doubling of chromosomes that can lead to sympatric speciation. Allopolyploid: polyploidization triggered by an interspecific hybridization event. Species A n=2 n=5 The resulting zygote may be unviable or infertile. If viable, it may later develop fertility. This is due to the differences between cell divisions in mitosis and meiosis (and we thought that we had escaped). In mitosis, chromatids self-replicate. In meiosis, each chromatid has to “find” its homologue.

If no homologue is present, the result is viable but infertile. However, with an abnormal mitotic event, cytokinesis (division of the cytoplasm after nuclear division) does not occur, resulting in a cell with double the normal number of chromosomes. If this cell enters meiosis, it can be fertile. (Many examples of this). Ex: Bread wheat is the result of two successive grass hybridization events. Grass went from having 14 > 28 > 42 chromosomes. Geographic speciation Geographic variation: Differences in a species based on geographic location, usually genetically based. Found in humans, where genotypes from one place are different than those from another. More common in less mobile populations as it is opposed by gene flow. Ex: yarrow, a plant which is found to be shorter at higher altitudes. Q: Is this variation due to genes or to environment? A: a “common garden experiment” finds the answer. Seeds of different origins are grown in the same environment. Result is that seeds from higher altitudes grow into shorter yarrow plants, revealing an underlying genetic factor.

Ex: common garter snake, Thamnophis surtalis, common throughout the US and Canada, is divided into various local sub species with local colorations. As garter snakes are not highly mobile, the populations can become locally adapted. In mallard ducks this geographic variation does not occur as each generation a male hooks up with a female in the southern “win-tering grounds”, then follows her up to her northern home, which may be very distant from his other home. This results in a genetic shuffle each generation independent of location. One population can be broken into different, isolated populations for a time, al-lowing two distinct evolutionary pathways. Ex: a glacier advances over North America and splits a population into SW and SE groups. When the barrier is removed (the glacier retreats) the populations can come into contact again, called secondary contact. Q: Then what happens? A: If inbreeding results in fertile offspring, they are still of the same species. If not, they are now new species. Isolating mechanisms: anything that acts to prevent the production of viable zygotes between two organisms.

Classified into two groups: prezygotic post zygotic The prezygotic mechanisms prevent the fertilization of an egg by the sperm, and hence the production of a zygote, commonly when male courtship is unrecognized by the female. Postzygotic isolating mechanisms result in a zygote which isn’t viable (dies), or cannot reproduce. Ex: A cross between a donkey and a horse produces a mule, which is sterile. Geographic separation can result in two quite morphologically different popula-tions that interbreed on secondary contact and are therefore one biological species. Like, they can look all different and stuff but still breed with each other. And sometimes, a seemingly slight difference prevents interbreeding. The point: the “amount” of differences that arise during the isolation of two populations doesn’t necessarily determine whether they will successfully interbreed upon secondary contact and therefore be considered one or two species. Ex: Eastern and western meadow larks are almost identical in appearance (morphologically similar) but are distinct biological species. This is possibly the result changes which occurred during isolation caused by glacial separation.

On secondary contact these two populations apparently did not respond to each other for mating, becoming two biological (reproductive) species. (There are various examples like this one). Peripheral isolates: a relatively common version of geographic isolation. At the edge of a species range an organism lives at the limit of its survivability. When these areas become separated from each other, allopatric speciation can occur, high rates of selection can push allele frequencies. Habitat islands: areas where a habitat favorable to a species is surrounded by an area where the species does not survive. Ex: a volcanic island in an ocean. In this case, founder populations arrive and find ecological vacuums, or different conditions, that influence adaptation. Adaptive radiation, a series of adaptation events, occurs as various species interact and adjust through evolution. Ex: the variation found in Darwin’s finches was caused when a founder population of finches adapted to the unique conditions of each island). Note: even greater variation is found amongst the finches of the Hawaiian Islands. Hawaii is also the home to countless species of Drosophila (fruit flies).

Binomial nomenclature: “two-named naming” (bi: two, nomin: name) originated by Carolus Linnaeus (Carl von Linné) of Sweden. Each species of organism has its own unique two-part name, made up of Genus name, which is capitalized, and denotes a group of species Species epithet, not capitalized, describes one species within the genus. (Epithet: “descriptor”, or according to “a characterizing word or phrase accompanying or occurring in place of the name of a person or thing”.) Note that not all species epithets by themselves are unique to a species; in fact some are reused frequently, such as vulgaris, used in Pinguicula vulgaris and in Sturnus vulgaris, the scientific names for the common butterwort and the European starling (vulgaris: “common”). Species epithets commonly refer to such things as color. Homo sapiens is the binomial name for the modern human. Homo is a genus of related primates (all but mankind are now extinct) and sapiens is a descrip-tor, “wise”. When typed, this two-part name is customarily italicized, and when handwritten, underlined.

The naming system is hierarchical, with succeeding levels dividing organisms into more specific taxonomic groups. The broadest division is that of kingdom (or domain, a more basic designation now accepted by many biologists. Here is the full list from most general to most specific. (Domain) >> Kingdom >> Phylum >> Class >> Order >> Family >> Genus >> Species. There are subdivisions that are sometimes used between these levels (for example, subphylum) but we are not responsible for knowing them. The higher the level, the greater the difference between groups, reflecting pro-gressively earlier speciation events. Q: Why does Linnaus’ naming system neatly fit the evolutionary tree, as he invented it outside an evolutionary context? A: The organisms are related in a genealogical way with inherently hierarchical relationships. Taxon: a given group of organisms at some given rank. Ex: the taxon “Canis”, a group of dogs at level genus, vs. the taxon “canidae”, at level family. (Taxonomy: orderly classification of plants and animals according to their presumed natural relationships.) Monophyletic taxon: a species set, or grouping of species, made up of a common ancestor all its descended species.

Ex: great apes Paraphyletic taxon: a grouping of species just like the monophyletic taxon except that it excludes species that have diverged farthest from the common ancestor. Ex: Great apes, minus humans. Polyophyletic taxon: grouping of species that excludes the most recent common ancestor. Ex: A grouping of whales and sharks, without their common primitive ancestor (whatever that might be). There are two taxonomic schools of thought: Traditional (also poorly called “evolutionary”, poorly because the alternate is evolutionarily based as well) Cladistic (or phylogenetic, or Hennigian, after founder Wm Hennig) Both schools agree on how to draw the evolutionary tree, as well as that: the best way to group is in monophyletic groups, and the worst way to group is in polyphyletic groups. However they disagree in the treatment of the middle ground, paraphyletic taxonomic groups. The traditionalists say: These groups are OK, as they reflect important evolutionary change. The cladists say, paraphyletic groups are NOT OK, as the identification of “important” changes is based on people’s opinions. Cladism prefers greater objectivity.

Q: How do we estimate evolutionary sequences? A: Derived homology, or transition from [ a -> a’ ], ex: a species’ loss of a digit thru time. Synapomorphy: a “shared derived similarity” derived character state, shared under two or more taxa under consideration used to infer common ancestry Gibson says that synapomorphy gives the only real idea of branching sequence symplesiomorphy: an ancestral character state inherited from the common ancestor of the taxa Evolutionary branching Q: How can we generate hypotheses about evolutionary branching? (How can we figure out which branching sequence is most likely the historical one?) A: There are various ways to do this. Generally, we start by considering certain characters, then arrange them into possible trees. Characters are found in either a more ancestral or more derived state, and sometimes the trait can disappear completely or even reappear. When a species goes from having one version of a trait to another version, it is called a trait change. One way to determine the most likely branching sequences is by creating examples of all the possible sequences. Although this is a simple process, it is not feasible in many cases because the number of possible branching sequences is so great.

For example, a group of just 10 species has over 34 million possible evolutionary trees. (Certain methods are used to eliminate many of these options and make the process simpler). The simplest sequence is the one that has the smallest number of trait changes, and is preferred. They are then used as a basis for further refinement that comes with more data. Punctuated equilibrium: a hypothesis about the tempo and mode of evolution, presented by Stephen Gould and Niles Eldridge as a challenge to the traditional view known as phyletic gradualism. Tempo Long periods of no change (stasis) “punctuated” by rare events of rapid change Largely restricted to speci-ation events Current thought sees evolutionary change as neither of these two extremes but rather somewhere in between, or sometimes more like the one and other times more like the other. Evolutionary “novelties” Q: How can we explain macroevolution? Can the incremental changes of microevolution lead to the big changes of macroevolution, such as flight in birds? What other processes can lead to major changes in species?

Preaptation (co-opting) A structure that serves one function and then is co-opted to serve another function Ex: flight feathers from reptilian scales Ex: Mammals’ mammary glands from… modified sweat glands?

Allometric growth A change in proportions which comes with increasing size, or different parts grow at different rates Selection for one body part may lead to development in another Ex: antler size in Irish elk, where huge antlers came as a result of oversize elk (antlers were still in proportion to normal deer antlers) Paedomorhosis (paidos: child) retention of juvenile characteristics in adult, or in extreme, reproduction in larval state Ex: gills retained from juvenile aquatic salamanders to normally land-habiting adult, and reproduction at aquatic, juvenile stage Adult chimps have greater change in skull shape from infancy than humans experience: chimp and human babies have similar skull shape, which the human more closely maintains to adulthood than the chimp Isometric growth: all parts grow at the same rate and stay in proportion, as in a growing salamander and in most organisms Algometric growth: different parts grow at different rates, resulting in changing proportions thru growth cycle, as in human, and in the godwit, a bird whose beak grows exponentially faster than its head does Analogous structures: two structures that serve a similar function but have different origins.

Examples: wings of a butterfly and a bird, leglessness in snakes and in certain lizards, white eggs in various diverse bird species Antennapedia: mutant fruitfly has small legs on head in place of normal antennae, caused by inappropriate development of cells into homologous part Bithorax: ancestors to modern insects had two full pairs of wings but today some of these have evolved the second pair into small balancing structures called halteres. (Some extant insects, such as the dragonfly, retain all four wings, one pair on the second and third segments of the thorax). Insects with only two wings are called dipthera, meaning “two wings”. In the bithorax mutant, halteres develop into a second full set of wings. Homeodomain: series of 60 amino acids (180 nucleotides) which encodes development and has remained similar throughout the evolution of very different organisms, spanning the animals and apparently extending into parts of the plant kingdom as well. Reveals the strong conservation of genes thru evolution. Biological Diversity 9 Ecology | Back to Top In previous chapters/units we have concentrated on the biology of the individual cell, tissue, and organism.

There are levels of organization above the individual organism that will be the subject of this unit. Individual organisms are grouped into populations, which in turn form communities, which form ecosystems. Ecosystems make up the biosphere, which includes all life on Earth. If there is life on other planets, will we need another level of organization? Biosphere : The sum of all living things taken in conjunction with their environment. In essence, where life occurs, from the upper reaches of the atmosphere to the top few meters of soil, to the bottoms of the oceans. We divide the earth into atmosphere (air), lithosphere (earth), hydrosphere (water), and biosphere (life). Ecosystem : The relationships of a smaller groups of organisms with each other and their environment. Scientists often speak of the interrelatedness of living things. Since, according to Darwin's theory, organisms adapt to their environment, they must also adapt to other organisms in that environment. We can discuss the flow of energy through an ecosystem from photosynthetic autotrophs to herbivores to carnivores. Community : The relationships between groups of different species.

For example, the desert communities consist of rabbits, coyotes, snakes, birds, mice and such plants as sahuaro cactus (Carnegia gigantea), Ocotillo, creosote bush, etc. Community structure can be disturbed by such things as fire, human activity, and over-population. Species : Groups of similar individuals who tend to mate and produce viable, fertile offspring. We often find species described not by their reproduction (a biological species) but rather by their form (anatomical or form species). Populations : Groups of similar individuals who tend to mate with each other in a limited geographic area. This can be as simple as a field of flowers, which is separated from another field by a hill or other area where none of these flowers occur. Individuals: One or more cells characterized by a unique arrangement of DNA "information". These can be unicellular or multicellular. The multicellular individual exhibits specialization of cell types and division of labor into tissues, organs, and organ systems. Organ System : (in multicellular organisms). A group of cells, tissues, and organs that perform a specific major function. For example: the cardiovascular system functions in circulation of blood. Organ : (in multicellular organisms).

A group of cells or tissues performing an overall function. For example: the heart is an organ that pumps blood within the cardiovascular system. Tissue : (in multicellular organisms). A group of cells performing a specific function. For example heart muscle tissue is found in the heart and its unique contraction properties aid the heart's functioning as a pump. . Cell : The fundamental unit of living things. Each cell has some sort of hereditary material (either DNA or more rarely RNA), energy acquiring chemicals, structures, etc. Living things, by definition, must have the metabolic chemicals plus a nucleic acid hereditary information molecule. Organelle : A subunit of a cell, an organelle is involved in a specific subcellular function, for example the ribosome (the site of protein synthesis) or mitochondrion (the site of ATP generation in eukaryotes). Molecules, atoms, and subatomic particles: The fundamental functional levels of biochemistry . Organization levels of life, in a graphic format. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

It is thus possible to study biology at many levels, from collections of organisms ( communities ), to the inner workings of a cell ( organelle ). Ecology is the study how organisms interact with each other and their physical environment. These interactions are often quite complex. Human activity frequently disturbs living systems and affects these interactions. Ecological predictions are, of a consequence, often more general than we would like. Population Growth | Back to Top A population is a group of individuals of the same species living in the same geographic area. The study of factors that affect growth, stability, and decline of populations is population dynamics. All populations undergo three distinct phases of their life cycle: growth stability decline Population growth occurs when available resources exceed the number of individuals able to exploit them. Reproduction is rapid, and death rates are low, producing a net increase in the population size. Population stability is often proceeded by a "crash" since the growing population eventually outstrips its available resources. Stability is usually the longest phase of a population's life cycle. Decline is the decrease in the number of individuals in a population, and eventually leads to population extinction . Factors Influencing Population Growth Nearly all populations will tend to grow exponentially as long as there are resources available.

Most populations have the potential to expand at an exponential rate, since reproduction is generally a multiplicative process. Two of the most basic factors that affect the rate of population growth are the birth rate , and the death rate . The intrinsic rate of increase is the birth rate minus the death rate. Two modes of population growth. The Exponential curve (also known as a J-curve) occurs when there is no limit to population size. The Logistic curve (also known as an S-curve) shows the effect of a limiting factor (in this case the carrying capacity of the environment). Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Population Growth Potential Is Related to Life History The age within it's individual life cycle at which an organism reproduces affects the rate of population increase. Life history refers to the age of sexual maturity, age of death, and other events in that individual's lifetime that influence reproductive traits. Some organisms grow fast, reproduce quickly, and have abundant offspring each reproductive cycle. Other organisms grow slowly, reproduce at a late age, and have few offspring per cycle. Most organisms are intermediate to these two extremes. Population curves.

a) three hypothetical populations (labelled I, II, and III); b, c, and d) three real populations. Note that the real curves approximate one of the three hypotheticals. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Age structure refers to the relative proportion of individuals in each age group of a population. Populations with more individuals aged at or before reproductive age have a pyramid-shaped age structure graph, and can expand rapidly as the young mature and breed. Stable populations have relatively the same numbers in each of the age classes. Comparison of the population age structuire in the United States and Mexico. Note the deographic bulge in the Mexican population. The effects of this buldge will be felt for generations. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The Baby Boomers and Gen X. As the population bulge, the baby Boomers born after World War II, aged and began to have children of their own this created a secondary bulge termed Generation X.

What happens when the Generation X members begin to have their own children? Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Human populations are in a growth phase. Since evolving about 200,000 years ago, our species has proliferated and spread over the Earth. Beginning in 1650, the slow population increases of our species exponentially increased. New technologies for hunting and farming have enabled this expansion. It took 1800 years to reach a total population of 1 billion, but only 130 years to reach 2 billion, and a mere 45 years to reach 4 billion. Despite technological advances, factors influencing population growth will eventually limit expansion of human population. These will involve limitation of physical and biological resources as world population increased to over six billion in 1999. The 1987 population was estimated at a puny 5 billion. Human population growth over the past 10,000 years. Note the effects of worldwide disease (the Black death) and technological advances on the populatiuon size.

Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Populations Transition Between Growth and Stability Limits on population growth can include food supply, space, and complex interactions with other physical and biological factors (including other species). After an initial period of exponential growth, a population will encounter a limiting factor that will cause the exponential growth to stop. The population enters a slower growth phase and may eventually stabilize at a fairly constant population size within some range of fluctuation. This model fits the logistic growth model . The carrying capacity is the point where population size levels off. Relationship between carrying capacity (K) and the population density over time. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Several Basic Controls Govern Population Size The environment is the ultimate cause of population stabilization. Two categories of factors are commonly used: physical environment and biological environment. Three subdivisions of the biological environment are competition , predation, and symbiosis. Physical environment factors include food, shelter, water supply, space availability, and (for plants) soil and light.

One of these factors may severely limit population size, even if the others are not as constrained. The Law of the Minimum states that population growth is limited by the resource in the shortest supply. The biological role played by a species in the environment is called a niche . Organisms/populations in competition have a niche overlap of a scarce resource for which they compete. Competitive exclusion occurs between two species when competition is so intense that one species completely eliminates the second species from an area. In nature this is rather rare. While owls and foxes may compete for a common food source, there are alternate sources of food available. Niche overlap is said to be minimal. Paramecium aurelia has a population nearly twice as large when it does not have to share its food source with a competing species. Competitive release occurs when the competing species is no longer present and its constraint on the winner's population size is removed. Graphs showing competition between two species of Paramecium. Since each population alone prospers (yop two graphs), when they are in a competition situation one species will win, the other will lose (bottom graph).

Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.. Predators kill and consume other organisms. Carnivores prey on animals, herbivores consume plants. Predators usually limit the prey population, although in extreme cases they can drive the prey to extinction. There are three major reasons why predators rarely kill and eat all the prey: Prey species often evolve protective mechanisms such as camouflage, poisons, spines, or large size to deter predation. Prey species often have refuges where the predators cannot reach them. Often the predator will switch its prey as the prey species becomes lower in abundance: prey switching. Fluctuations in predator (wolf) and prey (moose) populations over a 40-year span. Note the effects of declines in the wolf population in the late 1960s and again in the early 1980s on the moose population. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

Symbiosis has come to include all species interactions besides predation and competition. Mutualism is a symbiosis where both parties benefit, for example algae (zooxanthellae) inside reef-building coral. Parasitism is a symbiosis where one species benefits while harming the other. Parasites act more slowly than predators and often do not kill their host. Commensalism is a symbiosis where one species benefits and the other is neither harmed nor gains a benefit: Spanish moss on trees, barnacles on crab shells. Amensalism is a symbiosis where members of one population inhibit the growth of another while being unaffected themselves. The Real World Has a Complex Interaction of Population Controls Natural populations are not governed by a single control, but rather have the combined effects of many controls simultaneously playing roles in determining population size. If two beetle species interact in the laboratory, one result occurs; if a third species is introduced, a different outcome develops. The latter situation is more like nature, and changes in one population may have a domino effect on others. Which factors, if either, is more important in controlling population growth: physical or biological?

Physical factors may play a dominant role, and are called density independent regulation, since population density is not a factor The other extreme has biological factors dominant, and is referred to as density dependent regulation, since population density is a factor. It seems likely that one or the other extreme may dominate in some environments, with most environments having a combination control. Population Decline and Extinction Extinction is the elimination of all individuals in a group. Local extinction is the loss of all individuals in a population. Species extinction occurs when all members of a species and its component populations go extinct. Scientists estimate that 99% of all species that ever existed are now extinct. The ultimate cause of decline and extinction is environmental change. Changes in one of the physical factors of the environment may cause the decline and extinction; likewise the fossil record indicates that some extinctions are caused by migration of a competitor. Dramatic declines in human population happen periodically in response to an infectious disease. Bubonic plague infections killed half of Europe's population between 1346 and 1350, later plagues until 1700 killed one quarter of the European populace. Smallpox and other diseases decimated indigenous populations in North and South America.

Human Impact Human populations have continued to increase, due to use of technology that has disrupted natural populations. Destabilization of populations leads to possible outcomes: population growth as previous limits are removed population decline as new limits are imposed Agriculture and animal domestication are examples of population increase of favored organisms. In England alone more than 300,000 cats are put to sleep per year, yet before their domestication, the wild cat ancestors were rare and probably occupied only a small area in the Middle East. Pollution Pollutants generally are (unplanned?) releases of substances into the air and water. Many lakes often have nitrogen and phosphorous as limiting nutrients for aquatic and terrestrial plants. Runoff from agricultural fertilizers increases these nutrients, leading to runaway plant growth, or eutrophication. Increased plant populations eventually lead to increased bacterial populations that reduce oxygen levels in the water, causing fish and other organisms to suffocate. Pesticides and Competition Removal of a competing species can cause the ecological release of a population explosion in that species competitor. Pesticides sprayed on wheat fields often result in a secondary pest outbreak as more-tolerant-to-pesticide species expand once less tolerant competitors are removed.

Removal of Predators Predator release is common where humans hunt, trap, or otherwise reduce predator populations, allowing the prey population to increase. Elimination of wolves and panthers have led to increase in their natural prey: deer. There are more deer estimated in the United States than there were when Europeans arrived. Large deer populations often cause over grazing that in turn leads to starvation of the deer. Introduction of New Species Introduction of exotic or alien non-native species into new areas is perhaps the greatest single factor to affect natural populations. More than 1500 exotic insect species and more than 25 families of alien fish have been introduced into North America; in excess of 3000 plant species have also been introduced. The majority of accidental introductions may fail, however, once an introduced species becomes established, its population growth is explosive. Kudzu, a plant introduced to the American south from Japan, has taken over large areas of the countryside. Kudzu covering a building (left) and closeup of the flowers and leaves (right). Images from ~janthony/kudzu/ , photographs by Jack Anthony, used with permission.

Altering Population Growth | Back to Top Humans can remove or alter the constraints on population sizes, with both good and bad consequences. On the negative side, about 17% of the 1500 introduced insect species require the use of pesticides to control them. For example, African killer bees are expanding their population and migrating from northward from South America. These killer bees are much more agressive than the natives, and destroy native honeybee populations. On a positive note, human-induced population explosions can provide needed resources for growing human populations. Agriculture now produces more food per acre, allowing and sustaining increased human population size. Human action is causing the extinction of species at thousands of times the natural rate. Extinction is caused by alteration of a population's environment in a harmful way. Habitat disruption is the disturbance of the physical environment of a species, for example cutting a forest or draining wetlands. Habitat disruption in currently the leading cause of extinction. Changes in the biological environment occur in three ways. Species introduction: An exotic species is introduced into an area where it may have no predfators to control its population size, or where it can gratly out compete native organisms.

Examples include zebra mussels introduced into Lake Erie, and lake trout released into Yellowstone Lake where they are threatening the native cutthroat trout populations. Overhunting: When a predator population increases or becomes more efficient at killing the prey, the prey population may decline or go extinct. Examples today include big game hunting, which has in many places reduced the predator (or in this case prey) population. In human prehistory we may have caused the extinction of the mammoths and mastodons due to increased human hunting skill. Secondary extinction: Loss of food species can cause migration or extinction of any species that depends largely or solely on that species as a food source. Overkill is the shooting, trapping, or hunting of a species usually for sport or economic reasons. Unfortunately, this cannot eliminate "pest" species like cockroaches and mice due to their large population sizes and capacity to reproduce more rapidly than we can eliminate them. However, many large animals have been eliminated or had their populations drastically reduced (such as tigers, elephants, and leopards). The death of one species or population can cause the decline or elimination of others, a process known as secondary extinction.

Destruction of bamboo forests in China, the food for the giant panda, may cause the extinction of the panda. The extinction of the dodo bird has caused the Calviera tree to become unable to reproduce since the dodo ate the fruit and processed the seeds of that tree. Giant pandas eat an estimated 10,000 pounds of bamboo per panda per year. Image of a giant panda eating bamboo from #Giant Panda . Populations Have a Minimum Viable Size Even if a number of individuals survive, the population size may become too small for the species to continue. Small populations may have breeding problems. They are susceptible to random environmental fluctuations and genetic drift to a greater degree than are larger populations. The chance of extinction increases exponentially with decreasing population size. The minimum viable population (MVP) is the smallest population size that can avoid extinction by the two reasons listed above. If no severe environmental fluxes develop for a long enough time, a small population will recover. The MVP depends heavily on reproductive rates of the species. Range and Density | Back to Top Populations tend to have a maximum density near the center of their geographic range. Geographic range is the total area occupied by the species.

Outlying zones, where conditions are less optimal, include the zone of physiological stress (where individuals are rare), and eventually the zone of intolerance (where individuals are not found). The environment is usually never uniform enough to support uniform distribution of a species. Species thus have a dispersion pattern. Three patterns found include uniform, clumped, and random. Geographic ranges of species are dynamic, over time they can contract or expand due to environmental change or human activity. Often a species will require another species' presence, for example Drosophila in Hawaii. Species ranges can also expand due to human actions: brown trout are now found worldwide because of the sprerad of trout fishing. Links | Back to Top Tracking the genetic effects of global warming: Drosophila and other model systems This online article looks at the spread and microevolution withing the common fruit fly, Drosphilia and possible relationahips of this speciation and global warming. A bit technical in a few places, but an interesting read. Kudzu, the Vine Learn about this oriental import that has taken over large parts of the souther United States.

I remeber my first views of kudzu when I moved to central Georgia...green vines covering trees, houses, phone lines. Invaders Learn about many of the invaders that have already caused economic harm and ecological damage to the areas around New Orleans, Louisiana, USA.

Question: In biology, what word is given to the change in the genetic material of a population of organisms from one generation to the next?
Answer: Evolution