>>Genetics and evolution are often used to address social issues. The practical application
here is tricky, as people have tried to use genetics and evolutionary explanations for
a wide range of noble and sometimes shady purposes. We've already mentioned that human
genetic variation is inconsistent with the idea of distinct genetic races, and that human
racial categories commonly used in the United States have no compelling genetic or evolutionary
justification. In other words, from the standpoint of evolutionary theory, this practice is impractical.
Although from a more cultural or social standpoint, ethnic identity may be more meaningful. Moreover,
we noted that trying to engineer humans into a super species is also largely impractical
because we simply cannot predict reliably what environmental factors should take priority.
One of the social topics that students frequently ask me in my class is how evolution can explain
homosexuality. If evolution favors reproductive fitness, how can homosexuality be fit? This
is certainly a fascinating subject. The bottom line is that at the moment, there's no compelling
evidence of a gay gene. That said, homosexuality and bisexuality behavior is common in nature.
Sexual diversity is arguably the norm in complex organisms.
Our close evolutionary cousins are a fabulous example. Bonobos, otherwise known as pygmy
chimpanzees, have few to no sex taboos. They don't discriminate based on age or sex, and
they don't form long-term sexual partnerships. When excited, group sex is common and seemingly
a celebratory activity. They use sex to say "hi," to end conflict, to reconcile differences,
or simply to pass the time.
Regardless of whether homosexuality is genetically encoded or not, there can be no doubt that
homosexuality and sexual diversity in general is a part of nature. It's natural. But why?
Why is it natural? Some feel that for something to be natural, it also needs to be favorable
by natural selection. In other words, it needs to be adaptive. We've already learned in my
class that this is not the case. Natural selection is only one of four forces of evolution. Others
argue that homosexual behavior can be adaptive when considering the mother's needs in a larger
family. In this argument, while there's no specific gay gene, homosexuality is instead
influenced by epigenetic markers over a series of genes in the mother.
Epigenetics refers to the biochemical events occurring above the level of genome. You can
think of epigenetic events as dimmer switches that up-regulate or down-regulate the expression
of certain genes. A current hypothesis is that epigenetic traits are shaping homosexual
behavior, and that the genetically encoded ability to make these epigenetic adjustments
is what's actually under natural selection and is adaptive. But has there been any evidence
that this is the case? Well, some, yes. This hypothesis initially stems from an observation
about fraternal birth order, the number and order of sons born in a family. For men, the
more biological older brothers you have, the greater your odds that you will have a homosexual
orientation. Importantly, please don't over interpret this. Most men with many brothers
are still happily straight, and there are plenty of happy gay men without brothers.
Nevertheless, there does appear to be this trend. The trend has also been observed to
have some epigenetic links that you could see in the genome, but the complete picture
is still very unresolved. One hypothesis is that this trend is adaptive and is connected
to the need for representation of maternal caregiving in families burdened by way too
many boys. That is, having at least one child assume a more maternal role in the family
is adaptive for the mother. It increases their chance of passing on their genes by providing
additional caregiving to the older siblings. But the reverse pattern has not been found
for gay women. And while an interesting hypothesis, it's still heavily burdened by much conjecture.
While the epigenetic evidence is compelling, the interpretation of why we see the epigenetic
evidence still needs refinement. I suspect most of the scientists that study this pattern
would agree with me. Thus, while sexual diversity is clearly natural, understanding why it's
natural still requires a fair amount of research.