Encyclopedia of Evolution.pdf - Online Reading Center

eproductive systems
Scobell has investigated populations of the cactus Echinocereus
coccineus, in which populations that live along the Rocky
Mountain corridor are usually hermaphroditic, while populations
that live in desert regions at lower elevation are usually
dioecious. Her analysis indicates that the occurrence of dioecy
is related to the abundance of hummingbirds. Where there
are more hummingbirds (which pollinate the red flowers), the
hummingbirds promote out-crossing and the cactus populations
are usually hermaphroditic; where hummingbirds are
scarce, the cactus populations appear to compensate for this
with a dioecious reproductive system.
Plants have evolved numerous adaptations that facilitate
pollination. Plants that rely on the wind for pollination produce
huge amounts of pollen, sometimes enough to turn the
sky yellow (as in ponderosa pine forests in early summer).
Angiosperms such as cottonwoods and birches that rely on
wind pollination have flowers with small or no petals and
do not produce nectar. In contrast, plants that rely on animals
to carry their pollen produce flowers with colorful petals
and produce nectar that attracts and feeds the animals.
The structure of the flower not only permits pollination by
certain animals but excludes other animals. Long, tubular
flowers, for example, can be pollinated by animals with long
tongues; animals with short tongues cannot get down into
the tube. The correct pollinators are rewarded; the wrong
pollinators, which might carry the pollen to the wrong plant,
are excluded. In most cases, the plant rewards the animal
and the animal pollinates the plant. However, in some cases,
pollination is not mutually advantageous. The animal may
steal nectar without touching the stamens, therefore without
pollinating the flower; or the plant may trick the animal
into pollinating its flowers, without rewarding it, and maybe
even killing it. To a certain extent, the evolutionary history
of plants has been the increasingly successful adaptation to
new climatic conditions. But most of the astounding diversity
of plant species, therefore most of the story of plant evolution,
has been due to the coevolution of plants and their
pollinators.
Animal Adaptations That Promote Crossbreeding
Nearly all animals are either male or female. The few exceptions
include earthworms, which have both male and female
parts. Even they, however, breed with one another.
In some cases, as in certain mollusks and fishes, an animal
can change from one sex to another. In mollusks that
form clusters, an individual may adjust its sex according to
the other individuals around it. In fishes as in many other vertebrates,
dominant males often get to do all of the breeding.
Smaller individuals tend to be female, then when they grow
to be large, some of the females will transform into males.
Most animals, however, remain one sex all of their lives.
Because most animals are one sex or the other, crossbreeding
seems inevitable. But scientists seek evolutionary
reasons why most animals are male or female rather than
both. The general understanding is that the male function
requires the successful delivery of sperm to as many females
as possible, while the female function requires not just the
receipt of sperm but the production of eggs and, in many
cases, the care of the young. The male function may be more
efficiently performed by animals that specialize upon behaviors
that maximize their reproduction at the expense of other
males. The female function may be more efficiently performed
by animals that specialize upon behaviors that maximize
their care of the offspring. This occurs because a female
usually cannot increase reproductive output by having more
mates, while a male can—because sperm are cheap. There
would therefore be an advantage to an animal that specializes
upon just one sexual function and does it well. Plants seldom
specialize upon just one sex, because they are limited in
the kinds of behavior that they can have. A male cottonwood
tree can release pollen into the wind but cannot fight other
male cottonwood trees, or perform mating dances that allow
female cottonwood trees to choose among them.
Evolutionary biologists have traditionally believed that
males benefit from maximizing the number of mates, but
females do not, because while the female is pregnant she cannot
produce a greater number of offspring by mating again.
This is called the Bateman principle, after the geneticist A. J.
Bateman who first elaborated it. As a result, there is a continual
“battle of the sexes” in which the male wants more mates
and the female wants to keep the male from pursuing them.
This is expressed in the ditty variously attributed to George
Bernard Shaw, William James, Ogden Nash, or Dorothy
Parker:
Hoggamus, higgamus,
Men are polygamous,
Higgamus, hoggamus,
Women monogamous.
This pattern has not been supported by research in wild
populations. What matters is not male v. female, but which
sex invests more in each of the offspring. The number of
offspring produced as a function of the number of mates is
called a Bateman gradient. In most species, females provide
most of the investment in production of offspring. In these
cases, males have a steeper Bateman gradient than females
(see figure on page 343). In some species, males provide
most of the care of the offspring, and it is females that have
a steeper Bateman gradient. Moreover, offspring quality may
matter just as much as number of offspring. Females may
be able to enhance the quality of their offspring by having
additional mates, even if they cannot increase the number of
offspring.
Males in many animal societies compete with one another
for females and tend to be more violent than females, regardless
of the type of reproductive system. Sociobiologists (see
sociobiology) attribute human aggression, from individual
aggressions to full-scale wars, to a genetically based violent
behavior pattern in humans, although many other scientists
insist that these behaviors are learned and can be unlearned.
Males usually fight other males. In some cases, males are
violent toward females. More often, males treat females as
resources, and the violence is toward using rather than greatly
harming them. Even in humans, the primary historical pattern
is for conquering armies to kill the men and rape the women.