Evolution ❮ 141These three processes describe the way in which allele frequencies can change as a result of
the forces of natural selection. It is also important to remember two other types of selection
that complement natural selection: sexual selection and artificial selection.
Sexual selectionoccurs because individuals differ in mating success. In other words,
because not all individuals will have the maximum number of possible offspring, there must
be some reason why some individuals have greater reproductive success than others. Think
about how this is different from natural selection, which includes both reproduction and
survival. Sexual selection is purely about access to mating opportunities.
Sexual selection occurs by two primary processes: within-sex competitionandchoice.
In mammals and many nonmammalian species, females are limited in the number of off-
spring they can produce in their lifetimes (because of internal gestation), while males are not
(because sperm are cheap to produce and few males participate in offspring care). Which sex
do you think will compete, and which sex will be choosier? In most mammals for instance,
males compete and females choose. It makes sense that males have to compete because
females are a limiting resource, and it makes sense that females are choosy because they invest
a lot in each reproductive effort. This leads to the evolution of characters that are designed
for two main functions: (1) as weaponry or other tools for male competition (e.g., large testes
for sperm competition) and (2) as traits that increase mating opportunities because females
prefer to mate with males who have them (e.g., colorful feathers in many birds).
On what do females base their choices? While you need not become an expert on this
matter, it is important to remember that female mate choice for certain characters is not
random. One hypothesis for why females choose males with colorful feathers, for example,
is that colorful feathers indicate good genes, which is important for a female’s offspring.
Bright colors are costly, so a male with brightly colored feathers is probably healthy (which
may, in turn, indicate an ability to reduce parasite load, for example). We call such sexu-
ally selected traits that are the result of female choice honest indicators.Keep in mind that
selecting a mate for particular features does not necessarily involve conscious thought, and
in most animals never does; the female does not think, “Oh! What nice feathers. He must
come from good genes.” Rather, females who choose males that display honest indicators
have more surviving offspring than do females who don’t, and as a result, the “choosing
males with colorful feathers” trait increases in the population.Selection
Original
New(a) (b) (c)Figure 12.1 Three types of selection: (a) directional; (b) stabilizing; (c) disruptive.http://www.ebook3000.com