Introduction
On of the most important contributions made to the science of evolution by
Charles Darwin is the concept of natural selection. The idea that members
of a species compete with each other for resources and that individuals that are
better adapted to their lifestyle have a better chance of surviving to reproduce
revolutionized the field of evolution, though it was not accepted until several
decades after Darwin first proposed it. Today, natural selection forms the
basis for our understanding of how species change over time.
Natural selection may act to change a trait in many different ways. When
selection pressures favor the average form of the trait, selection is said to be
stabilizing. Directional selection occurs when selection pressures
favor one extreme of the trait distribution. Selection is disruptive when
the average form of the trait is selected against while either extreme is
unaffected.
In addition to natural selection, there are two other types of selection.
Sexual selection, which Darwin believed was distinct from natural selection,
involves the selection of traits based on their role in courtship and mating.
Artificial selection is the selective breeding of species by humans to
increase desirable traits, though the traits do not necessarily have to confer
greater fitness.
