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The evolutionary process has three basic
components: variation, inheritance and
differential survival or reproduction.
If these components are present in populations, then adaptive evolution
will occur. First, nearly every
characteristic of organisms varies slightly among individuals within
populations. This variation is clear if
we take any group of people, dogs, fish, plants or any other group of
organisms. Looking closely, we will see
that aspects of size, shape, behavior and other traits vary among individuals. Some of this variation is due to the
environment that an individual has experienced. For example, getting a bit more food might make an organism slightly larger than others in its
population. This kind of variation,
caused by differences in the environment experienced by organisms, is not
passed on to offspring and does not contribute to the evolutionary
process. However, many differences
among individuals are due to genetic differences, and this does contribute to evolution. For
example, taller than average parents tend to have taller than average offspring
because they pass on their genes for large size. Finally, when the differences among individuals affect their
ability to survive and reproduce, then the individuals with the favorable
characters will leave more offspring than others. This differential survival or reproduction of individuals with
different characteristics is natural selection. If the favorable trait is inherited, then more of the next
generation will have that trait, having inherited it from their successful
parents.
This is the basis of evolution by natural
selection: variation that is inherited
(genetic variability) confers an advantage in survival or reproduction (natural
selection) causes change over time and descent with modification. The traits that confer success are called
adaptations, and these spread through populations because the individuals that
bear them leave more offspring than do individuals with other
characteristics. When inherited
variation affects survival or reproduction, then evolution is inevitable.
Contributed by: Dr. Sara Via
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