Encyclopedia of Evolution.pdf - Online Reading Center

sexual selection
Because, in these instances, the male reproductive investment
may be completed upon successful copulation, the male
may be expendable. In many spiders and in praying mantises
(Mantis religiosa), females often eat the males—in the case of
the mantis, even while copulation is occurring.
There are cases in which the males have the larger reproductive
investment, and females choose among them. Females
may provide eggs to males, who take care of them; often,
males receive eggs from more than one female:
• In the rhea (Rhea americana), a South American bird similar
to the ostrich, the male incubates the eggs and takes
care of the hatchlings.
• In some wading birds, such as the red-necked phalarope
(Phalaropus lobatus), it is the males that raise the young.
The red-necked phalarope is also one of the few bird species
in which the females are more brightly colored than
the males.
• In some stickleback fishes such as Gasterosteus aculeatus,
females prefer to provide eggs to males who already have
eggs in the nests they have built. This has led to the evolution
of stealing behavior, in which males will steal eggs
from the nests built by other males.
• Males of the fish species Cyrtocara eucinostomus, though
only 10 centimeters in length, build enormous sand castles
a meter wide at the base, one mouthful of sand at a time.
The female chooses the male that constructs the best castle.
• In some frogs, and fishes such as the sea horse, the female
lays the eggs and then the male scoops them up into his
mouth to protect them while they hatch, a process called
mouth-brooding. This activity benefits the male in that he
can be certain the offspring are his. However, during this
prolonged period of time, the male cannot fertilize any
other eggs. It therefore represents a considerable expense
for the male. Since it typically takes more than one male
to mouth-brood the eggs produced by one female, it is
the females that compete with one another for access to
mouth-brooding males.
Therefore, while sperm are individually cheap, male reproductive
expenditure can be expensive.
In some instances, the females may be so widely dispersed
that a male is lucky to ever find one, and a female is
unlikely to be found by more than one male. In these cases,
there is no advantage for a male to be large. In some species
of invertebrates (such as some barnacles), as well as some
fishes, the males are small and even merge into the flesh of
the females so that they resemble an organ or a lump on the
female.
Sexual selection has operated strongly during human
evolution. Men have evolved numerous fitness indicators to
influence female choice. Some of these fitness indicators are
visible characteristics such as muscularity; sexual selection
may have caused men to become more muscular than they
need to be as a result. Most male fitness indicators have less
to do with appearance than with function. The human penis
is larger than that of some other primates. There may be two
reasons for this. One reason is, as noted above, large penes
are associated with promiscuous mating systems, as in chimpanzees,
while small penes are associated with harem mating
systems, as in gorillas; humans are intermediate. Another reason
is that large penis size may have been selected because
of tactile pleasure that it provides to the females, who would
then choose to keep company with the man who could provide
this pleasure. This explanation would make no sense
if pregnancy often occurred after a single copulation (as in
cats), because by the time the female determined that a certain
male gave less pleasure than others it would be too late
to reject him. However, because numerous copulations are
typically necessary for pregnancy to occur in humans, pleasure
is a useful piece of information on which a woman could
partly base her decision. Therefore female choice that maintains
a relationship, as well as the choice that initiates it, has
been important in human evolution.
As noted above, males sometimes select among competing
females. This is especially the case in animal species that
have expensive offspring, such as humans. Human reproduction
is so expensive that there is a definite survival benefit
to the offspring if both parents help to raise them (see life
history, evolution of). The genetic fitness of the men, as
well as the women, would be enhanced if the man stays with
the woman and helps with the provision and protection of
the offspring. If the woman has fitness indicators that will
attract the man and entice him to stay, her reproductive success
may be enhanced, as well as his. The man, in turn, may
want his genes to be carried by a woman who is most likely
to raise the children to adulthood. Women have, therefore,
also evolved fitness indicators that show that they are healthy
and well-nourished. A primary indicator of health is the face.
A face that is symmetrical and unblemished is an indicator
of past and current health. Another indicator is fat accumulation.
Fat accumulation serves not only as a fitness indicator
but as a direct contributor to the ability to raise children,
through nursing and later the ability to survive food shortages
during which feeding the child or children receives top
priority. Fat accumulation throughout the body would be
less obvious than fat accumulation in visible places such as
breasts and buttocks. A fitness indicator, in order to be effective,
must be expensive and show variation among individuals.
Faces, breasts, and buttocks not only indicate health and
nutrition but also age. If skin did not wrinkle and breasts did
not sag, with advancing age, they would not have served as
reliable indicators of female fitness.
Evolutionary scientists have been slow to catch on to
Darwin’s insight that sexual selection has been paramount
in human evolution. They have instead focused on the more
practical survival aspects of life as paramount in human evolution,
for example language as a means of communicating
the locations of resources or sorting out social hierarchies;
tools as means of obtaining resources, or as weapons against
competing tribes; and the relative hairlessness of the human
body as an adaptation that allows warm-blooded mammals
to cool off in the hot African savannas. In general, all of the
many adaptations that are collectively called intelligence
have been attributed to natural selection for the serious busi