Biology Reference
In-Depth Information
of the concept of “population” as simply the “sample” being used (see also DiGangi and
Moore [Chapter 1], this volume).
Evolutionary theory
comes into play for understanding the context of genetic data
through time.
Evolution
simply refers to change over time in one or more inherited traits
found in populations of organisms, and evolutionary theory attempts to provide explanatory
mechanisms for that change. Four classic explanatory mechanisms
d
(1) mutation, (2) genetic
drift, (3) gene flow, and (4) natural selection
d
are often referred to as the “forces of
evolution.”
The Four Forces of Evolution
Although any of the four forces alone, or in combination, can act to change frequencies of
genetic variants in populations from one generation to the next,
mutation
is the ultimate
source of new genetic variation. Amutation is a change in the DNA sequence of an organism,
and is only of potential evolutionary importance if it is inherited by one or more offspring.
This means that even though an individual may develop a mutation, and is somehow
affected by it (for better or for worse), if it is not passed on to future generations, it will
not have an impact on the population.
Genetic drift
refers to changes in the frequency of gene variants, or
alleles
, in a population
due to random sampling. Random sampling of alleles in a population context can occur in
multiple ways. For example, alleles present in offspring are a random sample of those in
the parents. Whether a given individual survives long enough to reproduce could be deter-
mined by chance events, such as a fatal car crash or a natural disaster. From one generation to
the next, the combined and accumulated effect of these random processes may be to drive
one or more alleles to the point of disappearing entirely from a population, thereby reducing
overall population genetic variation.
Mutational changes can counteract the variation-reducing effect of genetic drift in a pop-
ulation by introducing new variation, as can
gene flow
. Gene flow is simply the movement of
alleles from one population to another via people mating with individuals outside of their
own group. Even though genetic drift may be culling variants from a population over
time, gene flow provides a constant replenishment of alleles, depending on its rate (i.e.
how many individuals, along with their gene variants, mate between populations every
generation). On the whole, gene flow introduces new genetic variation into a population at
a much faster rate than does mutation.
The last evolutionary force is
natural selection
. Natural selection is the process by which
alleles become more or less common in a population because of factors impinging on the
survival and reproduction of their bearers. Natural selection acts on the expressed character-
istics (i.e., the
phenotype
) of an organism, such as all the factors that help a hare outrun
a hungry cheetah, including long legs, fast twitch muscle fibers in the right places, and small
size for hiding in hard-to-reach places. Only those heritable aspects of any phenotype (i.e.,
the
genotype
) that confer advantages in survival and reproduction will ultimately become
more common in a population. Over time, this process can result in adaptations that
specialize populations for particular environments (be they ecological or social) and may
eventually result in the development of a new species.
Box 16.1
gives some examples of how each of these four forces has impacted human
evolution and variation.
