attracting mates. Most of these traits have been changed through natural selection so they
allow a plant, animal, or bacteria to survive and reproduce relatively well in their environ-
ments. These traits are called adaptations. As environments have changed considerably over
time, organisms must constantly adapt to those environments. It is the great diversity of
species that increases the chance that at least some organisms adapt and survive any major
changes in the environment.
Imagine how in winter dark fur makes a rabbit easy for fox to spot and catch in the snow.
Natural selection suggests that white-fur is an advantageous trait that improves the chance
that a rabbit will survive, reproduce and pass the trait of white fur on to future generations
(Figure7.12). Dark fur rabbits will become uncommon.
Figure 7.12: In winter, the fur of arctic hares turns white. The camouflage may make it
more difficult for fox and other predators to locate hares against the white snow. ( 27 )
Polygenic Inheritance and Natural Selection
But natural selection leading to evolution does not just select for certain individuals, it
selects for groups. More than one individual must adapt to the environment to maintain a
population. Natural selection determines which groups of organisms survive, based on their
traits, and which do not, that is, natural selection determines the differential survival of
groups of organisms.
Although some traits are determined by a single gene, many are influenced by more than one
gene (polygenic). The result of polygenic inheritance is a continuous spectrum of phenotypic
values which often show a bell curve pattern of variation.
Given this pattern of phenotypic variability, natural selection can take three forms (Figure
7.14). We will use the hypothetical color distribution in this figure to illustrate the three