Evolution: Frequently Asked Questions

Biological evolution refers to the cumulative changes that occur in a population over time.
These changes are produced at the genetic level as organisms' genes mutate and/or recombine in different ways during
reproduction and are passed on to future generations. Sometimes, individuals inherit new characteristics that give them a
survival and reproductive advantage in their local environments; these characteristics tend to increase in
frequency in the population, while those that are disadvantageous decrease in frequency. This process of differential
survival and reproduction is known as natural selection. Non-genetic changes that occur during an organism's life span,
such as increases in muscle mass due to exercise and diet, cannot be passed on to the next generation and are not examples
of evolution.
In science, a theory is a rigorously tested statement of general principles that explains
observable and recorded aspects of the world. A scientific theory therefore describes a higher level of understanding
that ties "facts" together. A scientific theory stands until proven wrong -- it is never proven correct. The Darwinian
theory of evolution has withstood the test of time and thousands of scientific experiments; nothing has disproved it
since Darwin first proposed it more than 150 years ago. Indeed, many scientific advances, in a range of scientific
disciplines including physics, geology, chemistry, and molecular biology, have supported, refined, and expanded
evolutionary theory far beyond anything Darwin could have imagined.
Yes. Just as the tree of life illustrates, all organisms, both living and extinct, are related.
Every branch of the tree represents a species, and every fork separating one species from another represents
the common ancestor shared by these species. While the tree's countless forks and far-reaching branches clearly
show that relatedness among species varies greatly, it is also easy to see that every pair of species share a common
ancestor from some point in evolutionary history. For example, scientists estimate that the common ancestor shared by
humans and chimpanzees lived some 5 to 8 million years ago. Humans and bacteria obviously share a much more distant common ancestor,
but our relationship to these single-celled organisms is no less real. Indeed, DNA analyses show that although
humans share far more genetic material with our fellow primates than we do with single-celled organisms,
we still have more than 200 genes in common with bacteria.
It is important to realize that describing organisms as relatives does not mean that one of those
organisms is an ancestor of the other, or, for that matter, that any living species is the ancestor of
any other living species. A person may be related to blood relatives, such as cousins, aunts, and uncles,
because she shares with them one or more common ancestors, such as a grandparent, or great-grandparent.
But those cousins, aunts, and uncles are not her ancestors. In the same way, humans and other living primates
are related, but none of these living relatives is a human ancestor.
Members of one species do not normally interbreed with members of other
species in nature. Sometimes, members of different species, such as lions and tigers, can interbreed if
kept together in captivity. But in nature, geographic isolation and differences in behavior, such as choice
of habitat, keep these sorts of closely related animal species apart. Similarly, closely related species of
plants can sometimes be hybridized by horticulturists, but these hybrids are rarely found in nature. A species,
then, is defined by science as a group of interbreeding or potentially interbreeding populations that is
reproductively isolated from other such groups.
Genes are the portions of an organism's DNA that carry the code responsible for building
that organism in a very specific way. Genes -- and, thus, the traits they code for -- are passed
from parent to offspring. From generation to generation, well-understood molecular mechanisms reshuffle,
duplicate, and alter genes in a way that produces genetic variation. This variation is the raw material for evolution.
Sexual reproduction allows an organism to combine half of its genes with half
of another individual's genes, which means new combinations of genes are
produced every generation. In addition, when eggs and sperm are produced,
genetic material is shuffled and recombined in ways that produce new combinations
of genes. Sexual reproduction thus increases genetic variation, which increases the raw
material on which natural selection operates. Genetic variation within a species -- also
known as genetic diversity -- increases a species' opportunity for change over successive generations.
Evolution is not a random process. The genetic variation on which natural selection acts
may occur randomly, but natural selection itself is not random at all. The survival and
reproductive success of an individual is directly related to the ways its inherited traits
function in the context of its local environment. Whether or not an individual survives and
reproduces depends on whether it has genes that produce traits that are well adapted to its environment.
Evolution and "survival of the fittest" are not the same thing. Evolution refers to the
cumulative changes in a population or species through time. "Survival of the fittest" is a
popular term that refers to the process of natural selection, a mechanism that drives evolutionary change.
Natural selection works by giving individuals who are better adapted to a given set of environmental
conditions an advantage over those that are not as well adapted. Survival of the fittest usually makes
one think of the biggest, strongest, or smartest individuals being the winners, but in a biological sense,
evolutionary fitness refers to the ability to survive and reproduce in a particular environment. Popular
interpretations of "survival of the fittest" typically ignore the importance of both reproduction and cooperation.
To survive but not pass on one's genes to the next generation is to be biologically unfit. And many organisms
are the "fittest" because they cooperate with other organisms, rather than competing with them.
In the process of natural selection, individuals in a population who are well-adapted
to a particular set of environmental conditions have an advantage over those who are
not so well adapted. The advantage comes in the form of survival and reproductive success.
For example, those individuals who are better able to find and use a food resource will, on
average, live longer and produce more offspring than those who are less successful at finding food.
Inherited traits that increase individuals' fitness are then passed to their offspring, thus giving the
offspring the same advantages.
Individual organisms don't evolve. Populations evolve. Because individuals in a population
vary, some in the population are better able to survive and reproduce given a particular set
of environmental conditions. These individuals generally survive and produce more offspring,
thus passing their advantageous traits on to the next generation. Over time, the population changes.
No. Many people, from evolutionary biologists to important religious figures like Pope
John Paul II, contend that the time-tested theory of evolution does not refute the presence
of God. They acknowledge that evolution is the description of a process that governs the
development of life on Earth. Like other scientific theories, including Copernican theory, atomic
theory, and the germ theory of disease, evolution deals only with objects, events, and processes
in the material world. Science has nothing to say one way or the other about the existence of God
or about people's spiritual beliefs.