Every animal has unique physical characteristics that are selected for over time, which contribute
to its survival and therefore reproduction. However, it is hard to believe that every
trait is adaptive in the same way. It is easy to understand how running faster, jumping
higher, and biting harder can help an organism survive. But other features are not so obvious.
For example, we have probably all seen the male peacock flash its beautiful tail feathers,
more than likely at a zoo. In the wild, such a tail would be quite cumbersome and might
make the bird more vulnerable to prey. Why then, you might ask, would a peacock’s tail
evolve? Recall that natural selection is ultimately
about organisms passing their genes along. Being better equipped for survival ensures
a greater probability of surviving long enough to reproduce, but we can also look at the
reproductive act itself, outside of the context of survival. The traits that aid in an animal’s
ability to reproduce evolve through a process called sexual selection. Sexual selection
is a driving force of evolution that can lead to specialized characteristics that, while
not adaptive, lead to increased reproductive success. So how does it work? It essentially
comes down to availability over mates. With sexually reproducing species, females produce
relatively few gametes, which we know of as eggs, while males produce an excess of gametes,
which are called sperm. This imbalance contributes to Bateman’s Principle, where female reproductive
success is limited by access to resources, while male reproductive success is limited
by access to females. More powerful males can potentially have many offspring, taking
away opportunities from other males and leading to a large variation in reproductive success
amongst the males in a population. However, a successful female is not taking away reproductive
opportunities from other females, leading to lower variation in reproductive success
among females. If the variance in reproductive success is high in one sex, then the effects
of sexual selection will also be high in that sex. To stand out above the rest, these individuals
develop secondary sex characteristics, such as a male peacock’s tail, to become the
best of the competition and successfully reproduce. However, it’s not just about the particular
sex. In species where parental care is required, reproductive success is affected by the contribution
of each sex to raising the offspring. If investment in the offspring is high, the sex caring for
the offspring has even more limited reproductive availability, forcing the opposite sex to
wait longer for access to mates and increasing their reproductive variance. This steepens
the competition for access to mates, leading to stronger sexual selection.
If both sexes are involved in parental care, then there are fewer mating opportunities
available for both, and therefore lower variation in reproductive success among the sexes, lessening
the effects of sexual selection. When parents are busy taking care of offspring, there is
less time for copulation. Species where males and females look alike, such as penguins,
are called sexually monomorphic, whereas if males and females have significant anatomical
differences, such as with humans and many other mammals, these species are called sexually
dimorphic. As we touched upon when first introducing
natural selecton, there are two types of sexual selection, those being intrasexual and intersexual.
With intrasexual selection, individuals within the same sex, often males, compete with each
other to fight off potential competitors for a female. This can often lead to the development
of antlers, horns, or large body sizes. For example, male elephant seals compete over
control of a harem, where the largest male gains exclusive access to many females.
Intersexual selection, on the other hand, is more like a job interview, where the hiring
manager has many choices among potential candidates. Most of the time, females are choosing among
males, as females tend to have the highest investment in reproduction and males have
the highest variability in reproductive success. Females choose traits that will lead to increased
reproductive success for their offspring. This can lead to males competing amongst each
other for female attention, such as the case with animals who lek, which means to take
part in a communal breeding display. Males aggregate in arenas to perform mating displays
for the female, who chooses the best performer as her mate. Frequently, males can spend most
of their time at the lek when not foraging for food.
While it makes sense for females to choose males that offer increased access to food,
protection, or those that are free from parasites, why would females choose between males that
don’t display any of these obvious benefits? One explanation is that secondary sex characteristics
signal increased fitness, that is, capacity for survival and reproductive success. Individuals
with prominent secondary sex characteristics are displaying their quality as a mate, because
these features indicate the presence of genes that will somehow enhance the survival and
reproductive success of potential offspring. This is according to the good genes hypothesis.
The fact that these individuals with unnecessary and potentially hazardous secondary sex characteristics,
such as elaborate plumage, can survive at all, demonstrates the influence of these characteristics
on mate selection. This hypothesis is in line with natural selection, since the choosing
of these mates would lead to higher reproductive success for the genes associated with these
characteristics. However, many scientists believe it may be
a bit more arbitrary. R. A. Fisher proposed that females can evolve random preferences
for arbitrary traits, and that the advantage of the trait is only apparent if females choose
males with the trait, and that offspring with that trait are more sexually attractive to
females with that preference. If enough females in a population have that preference, then
selection on that trait increases so much that it can lead to runaway sexual selection,
a self-reinforcing cycle that leads to the evolution of some arbitrary trait. Like a
train that’s been derailed, the runaway sexual selection phenomenon can become so
extreme that preference for disadvantageous traits, like the large peacock’s tail, can
become the norm. Sexual selection often occurs in the steps
associated with gaining opportunities for reproduction, such as through the attraction
and retention of mates. However, sex doesn’t always lead to reproductive success. Post-copulatory
selection describes the processes that happen during and after sex. For example, there is
sperm competition, where a female has access to two or more males and the sperm compete
for access to the egg. This has led to the evolution of traits that increase the likelihood
of fertilization, such as longer and larger ejaculates. Females can also choose the sperm
from a particular male to fertilize their eggs, exerting biological or chemical means
such as destroying sperm, storing sperm, or investing less in a particular offspring.
In any case, between intersexual and intrasexual selection, we can begin to understand precisely
why traits like sexual dimorphism exist all over the animal kingdom, everywhere from birds
to humans, and we can even begin to better understand human psychology when it comes
to mate selection.