The concept of survival and sexual reproduction all boils down to the process of natural
selection. This is when the environment determines which individuals live and die, based
on which traits are best suited for the environment. There are three kinds of natural
selection pressures that affect individuals of a species: directional, stabilizing, and
disruptive. In directional selection, only individuals with the most unusual features
survive to reproduce, resulting in a huge exaggeration of those features in the population.
Stabilizing selection is when the average traits are most beneficial, and those average
individuals will live to reproduce, causing little physical change within a species. In
disruptive selection, individuals with the most extreme features (on both sides) survive,
but at the expense of the average individuals. This leads to extinction of average
individuals and can even lead to the formation of different species from the same genetic
ancestor.
In order to pass on genetic information to the next generation, an individual needs to
reproduce. Most species in the world use sexual reproduction, but why has it become the
most widespread form of reproduction? Sexual reproduction is costly and inefficient,
especially when compared to cloning or other self-duplicating mechanisms. This has its
disadvantages, especially when the parent has a good set of genes, because the child can
end up being genetically worse off than the parent. So why do we still rely on sexual
reproduction? The main reason for this is in order to exchange and mix genes, resulting in
every individual being different from another, which means there are more genetic
combinations for natural selection to try out and determine which are most beneficial in
the current environment. The second reason is called the Red Queen Theory, which states
that sexual reproduction is a way of avoiding the affect of pathogens in the environment.
If our genetics are constantly changing, that means our immune systems are as well, just
fast enough to keep up with constantly changing viruses and bacteria that invade our
bodies every day. Those individuals with the best immune systems will survive to pass
those genes to their offspring, while other individuals with less than optimal immune
systems will die before reproduction.
There exists a continuum of sexual reproduction strategies, based on the environment in
which the individual lives. On one end of the spectrum is a strategy called the “r-selected
species,” that rely on having a large number of offspring because they live in a rapidly
changing environment. Only those offspring that possess the best combination of genes
will survive the environment and live to pass their genes to the next generation, and so
forth. On the other end of the spectrum are the “k-selected species,” that rely on having
fewer children who are better adapted for the slow-changing environment in which they
live. These children will all most likely survive, because their specialized traits will allow
them to thrive in a slow-changing environment and pass their genes to the next
generation. However, k-selected species are very similar to asexual species in the fact
that they do not have much variation between generations. This means that if their
environment began changing at a more rapid rate, their offspring would begin to die out,
and this is the case for many of these species. Most endangered species in the world are
of this k-selected species group, and are unable to replenish their populations fast enough
to outrun the changing environment.
There are two reproductive sexes only because two is the minimum requirement for
sexual reproduction. However, many microbial species have more than one genetic type,
and any combination of mating is possible, so two sexes is not the limit. The two sexes
are distinguished mainly by gamete size, where the female produces a large, expensive,
rare nutrient rich gamete (egg) while the male produces small, inexpensive, and
numerous nutrient poor gametes (sperm). This results in the fact that females invest more
reproductive effort in offspring, since it costs her more time and energy to make her
gamete than it does a male. This leads to a phenomenon called sexual selection, because
the female is more invested in offspring, wants them to survive, and will choose the best
genetic match to mate with as a result. Sexual selection occurs when a female of a species
decides what the best male traits are and only chooses to reproduce with those certain
males, ensuring that only his genes will survive another generation. This selection can
also lead to sexual dimorphism, or the differences between females and males, because
the more different (genetically) a male is from a female, the more chance their offspring
will have to survive.
Evolutionarily speaking, if sexual reproduction occurred for the benefit of the gene alone,
the sex ratio today would be about 99 females to one male, since males are able to
reproduce much more quickly than females are. The single male would be able to
produce offspring with all 99 females, and would guarantee that his genes would be
passed to the next generation. However, there is a reason why this is not the current sex
ratio. If the ratio was 99:1, each female in the group would want to have a son rather than
a daughter, because that son would be one of a very few number of males, ensuring that
her genes would be passed to the next generation as well. Because it would be most
beneficial to have a son, females would be spending more time and resources on their
sons and their daughters, allowing the sons to prosper while the daughters would die out.
This would eventually lead to equilibrium, where the sex ratio is about 50:50, and there
would be a relatively equal number of males as females in a population. However, at
birth the sex ratio is not 50:50; there are more males born than females. But in the process
of growing up to reach reproductive age, more males die than females, evening out the
population ratio. This can be argued that because males are more reproductively
expendable (less are required to produce offspring), they are the genetic “guinea pigs” of
natural selection. Combinations of genes are tested out in males and only the ones that
survive to reproduce are able to pass their genes.
Many parenting strategies affect which offspring survive to reproductive age. Parents that
are unable to provide for offspring after reproduction efforts will eventually die out, since
they are unable to ensure that their offspring will continue to maturity. Many species
engage in semelparity, in which the last thing the parents do is reproducing, then die. This
ensures that the parent will have enough energy to reproduce, but then the parent will not
be present to care for children after birth. In this model, a large number of offspring is
common, ensuring that some of the offspring will reach reproductive age. Most
mammalian species are iteroparous, which means they breed more than once. They
typically have few offspring because they need to be able to care for both the offspring
and for themselves, in order to be able to breed again. Many avian species will lay more
eggs than they can rear, and many times some of the hatchlings are left to die, as the
mother cannot take care of all of them. However, if she happens to be in very good health
or is capable of providing more food than average, she will be able to rear more offspring
than average, thereby increasing her genetic representation in the next generation. Even
after the birthing stage, many times a parent will favor one or two of its offspring more
than others, because the chance for survival is higher. More resources will be given to the
strongest offspring in the hopes that it will become the most likely to mate later in life,
and have the most offspring. All of these factors determine which offspring live to
reproduce, and how many offspring they will have in turn.
Many times offspring will suffer for the benefit of the parents, especially if the offspring
is not yet of reproductive age. A parent dying of hunger will more likely eat the food
herself than give it to an infant, because her reproductive value is higher at the time than
the infant’s. She would be more readily available for future reproduction than the child
and would therefore sacrifice the child to ensure future reproduction. However, it is also
the case where females who cannot have any more of their own offspring will aid those
who are fertile in caring for offspring, in order to benefit the species and ensure that the
current offspring survive. This can also be said for offspring that are infertile or unable to
have any offspring of their own, in which case they would help their mothers or other
females of the group rear their children so those fertile females can contribute more to the
species’ population. This is an example of the all-important principle of inclusive fitness.
An excellent example of this principle of inclusive fitness is exemplified today is
homosexuality, where it is argued that a homosexual man in a family can be viewed as a
sort of “insurance policy,” helping to ensure that all offspring survive. Rather than
producing their own children to devote their time and resources to, homosexuals can
devote that same time and resources to family members and some of their genes will still
be passed into the next generation. In addition to helping family members, they also
reduce competition between them and their siblings, as the siblings would not be
competing with the homosexual sibling for mates.
There are many factors that affect sex that we have no control over, and can occur before
we were even born, like in the instance of evolution. All of these factors help explain to
us why things are the way they are today, but they are not things that can be changed, or
that we would even want to change. Sexual reproduction has been molded into what it is
today by thousands of years of evolution, and today’s strategies work best in today’s
environment.