Encyclopedia of Evolution.pdf - Online Reading Center

sexual selection
more offspring, than individuals in which the characteristic is
absent or less well developed.
Sexual selection is similar to natural selection, in that
a characteristic evolves because it causes the individuals that
possess the trait, or that possess it in a more highly developed
state, to leave more offspring than individuals that do not
possess it. The difference is that sexual selection favors characteristics
that directly enhance reproductive success rather
than characteristics that enhance it indirectly through survival
or obtaining resources. Charles Darwin (see Darwin,
Charles) proposed natural selection as the major mechanism
of evolution in his world-changing 1859 book On the Origin
of Species By Means of Natural Selection. He also proposed
sexual selection in his 1871 book The Descent of Man and
Selection in Relation to Sex (see origin of species [book];
Descent of man [book]).
Sexual Selection in Animals
In most animal species, males can produce more offspring
than can females (see reproductive systems). This is partly
because sperm cells are small and cheap, while egg cells are
large and expensive. In animals with external fertilization,
such as most fishes, this may be almost the entire difference in
reproductive cost between males and females. In species with
internal fertilization (such as most terrestrial vertebrates), at
least part of the embryonic development occurs within the
body of the female. Bird and reptile eggs are relatively large
by the time the female lays them. In terrestrial species with
live birth (vivipary), it is the female in which embryonic
development occurs. In most animal species in which newborns
receive parental care, it is the female that provides a
disproportionately large amount of that care.
Sexual selection can operate at the level of the reproductive
cell, or of the reproductive individual:
Sexual selection upon sperm. In many animal species,
males produce millions of sperm while females produce far
fewer eggs. This can lead to an intense selection process
occurring upon the sperm themselves. In humans, very few
of the many millions of sperm reach the egg. This is why a
sperm count of 50 million, although that sounds like a large
number, may indicate infertility.
• The acidic environment of the vagina kills many sperm.
• The sperm must also be strong swimmers, to climb up
through the cervix into the uterus.
• Since the sperm are genetically different from the cells of
the woman’s body, the woman’s immune system begins to
launch an attack on the sperm.
Selection of strong sperm may also result in the selection
of strong offspring, because many of the metabolic genes
that promote sperm success also promote the vigor of the offspring.
Sexual selection has not only favored the production
of strong sperm but also the production of chemicals in the
semen that partially suppress the woman’s immune response.
Selection for successful sperm is a very important part of
reproductive success. Each sperm is the product of a long
series of cell divisions, each one an opportunity for mutations.
There are therefore many mutant sperm that need
to be “weeded out.” This explanation seems inadequate to
explain all of the characteristics of sperm cells in all animal
species. No convincing explanation has yet been offered for
why the sperm of Drosophila bifurca, a species of fruit fly
that is about one-twentieth of an inch (1 mm) long, can have
tails almost two inches (6 cm) long.
Sexual selection at the individual level. Since one male
can produce so many sperm, and since in many animal species
the act of mating is the male’s only contribution to the
success of the offspring, a female animal cannot typically
increase her reproductive success by mating more often.
Therefore it is usually males that compete with other males
for access to females; and females that choose which males
they allow to fertilize their eggs.
Females may choose among males on the basis of genetically
based characteristics that have no connection with the
quality of the male. According to the “sexy sons” hypothesis,
females in a species may choose a male with any characteristic,
perhaps an outlandish color pattern. The sons of these
females will also have this characteristic, which will enhance
their chances of being chosen by females in the next generation.
Evolutionary theorist R. A. Fisher (see Fisher, R. A.)
first pointed out that this could result in a positive feedback
loop of runaway sexual selection.
Females may choose among males on the basis of genetically
based characteristics that indicate the quality of the male:
• The best genes may be those that allow males to obtain
resources most effectively.
• The best genes may be those that allow resistance to parasites
and diseases. Because parasites can evolve so much
more rapidly than their hosts, most animal species are in
a continual race to keep ahead of their parasites (see red
queen hypothesis).
There are many fitness indicators that females can use to
choose the best males. A fitness indicator is any trait that reliably
displays the strength of the state of health of the male.
A healthy male is likely to be superior in resource acquisition
and in resistance to parasites and therefore likely to have
higher potential fitness. A fitness indicator must, therefore, be
expensive—and the more expensive it is, the better a fitness
indicator it is. This is the “handicap principle” proposed by
evolutionary biologist Amotz Zahavi. Evolutionary biologists
William Hamilton and Marlene Zuk proposed that the most
valuable fitness indicators are those that show the male to be
free from parasites. Fitness indicators include:
• A male that defends a large territory in prime quality habitat
not only has more resources to offer to the female but is
probably healthier and therefore able to drive away other
males. It is not only expensive for the male to obtain and
defend the territory but to advertise it to females. A male
mockingbird may sing all day for several weeks, which uses
a lot of energy and makes him noticeable to predators.
• Another fitness indicator is bodily ornamentation. Consider
a bird species that uses colored feathers as fitness indicators.
Any male, even an inferior one, can (potentially)
produce a few small colored feathers, but only a healthy