Evolution

Biological evolution is a concept first proposed by Charles Darwin 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.
== evidence ==
the evidence for evolution comes from many fields of biology:
* anatomy:Species may share similar physical features because the feature was present in a common ancestor (homologous structures).
* molecular biology:DNA and the genetic code reflect the shared ancestry of life. DNA comparisons can show how related species are.
* biogeography:The global distribution of organisms and the unique features of island species reflect evolution and geological change.
* Fossils:Fossils document the existence of now-extinct past species that are related to present-day species.
* Direct observation:We can directly observe small-scale evolution in organisms with short lifecycles (e.g., pesticide-resistant insects).
== Evolutionpens on large and small scales ==
Biologists sometimes define two types of evolution based on scales
* Macroevolution: large-scale changes that occur over extended time periods, such as the formation of new species and groups.
* Microevolution: small-scale changes that affect just one or a few genes and happen in populations over shorter timescales.
Microevolution and macroevolution aren’t really two different processes. They’re the same process – evolution – occurring on different timescales. Microevolutionary processes occurring over thousands or millions of years can add up to large-scale changes that define new species or groups.
== Determining relationships from similar features ==
In general, biologists don't draw conclusions about how species are related on the basis of any single feature they think is homologous. Instead, they study a large collection of features (often, both physical features and DNA sequences) and draw conclusions about relatedness based on these features as a group. We will explore this idea further when we examine phylogenetic trees.
== Molecular biology ==
Like structural homologies, similarities between biological molecules can reflect shared evolutionary ancestry. 
* At the most basic level, all living organisms share:The same genetic material (DNA)
* The same, or highly similar, genetic codes
* The same basic process of gene expression (transcription and translation)
* The same molecular building blocks, such as amino acids
These shared features suggest that all living things are descended from a common ancestor, and that this ancestor had DNA as its genetic material, used the genetic code, and expressed its genes by transcription and translation. Present-day organisms all share these features because they were "inherited" from the ancestor (and because any big changes in this basic machinery would have broken the basic functionality of cells).
Although they're great for establishing the common origins of life, features like having DNA or carrying out transcription and translation are not so useful for figuring out how related particular organisms are. If we want to determine which organisms in a group are most closely related, we need to use different types of molecular features, such as the nucleotide sequences of genes.
== Direct observation of microevolution ==
Important modern-day examples of evolution include the emergence of drug-resistant bacteria and pesticide-resistant insects.
For example, in the 1950s, there was a worldwide effort to eradicate malaria by eliminating its carriers (certain types of mosquitos). The pesticide DDT was sprayed broadly in areas where the mosquitoes lived, and at first, the DDT was highly effective at killing the mosquitos. However, over time, the DDT became less and less effective, and more and more mosquitoes survived. This was because the mosquito population evolved resistance to the pesticide.
Emergence of DDT resistance is an example of evolution by natural selection7start superscript, 7, end superscript.
* Before DDT was applied, a tiny fraction of mosquitos in the population would have had naturally occurring gene versions (alleles) that made them resistant to DDT. These versions would have appeared through random mutation, or changes in DNA sequence. Without DDT around, the resistant alleles would not have helped mosquitoes survive or reproduce (and might even have been harmful), so they would have remained rare.
* When DDT spraying began, most of the mosquitos would have been killed by the pesticide. Which mosquitos would have survived? For the most part, only the rare individuals that happened to have DDT resistance alleles (and thus survived being sprayed with DDT). These surviving mosquitoes would have been able to reproduce and leave offspring.
* Over generations, more and more DDT-resistant mosquitoes would have been born into the population. That's because resistant parents would have been consistently more likely to survive and reproduce than non-resistant parents, and would have passed their DDT resistance alleles (and thus, the capacity to survive DDT) on to their offspring. Eventually, the mosquito populations would have bounced back to high numbers, but would have been composed largely of DDT-resistant individuals.
In parts of the world where DDT has been used extensively in the past, many of the mosquitoes are now resistant. DDT can no longer be used to control the mosquito populations (and reduce malaria) in these regions.
Category:Biology