Gay people, they’re falling in love, getting married, living happily ever after
or having kids.
Well not having kids in the conventional way,
they might adopt or make use of a surrogate or sperm donor
but their intimate behavior isn’t conducive to reproduction.
The fact that homosexual sex doesn’t lead to reproduction implies a cost on an individual’s
fitness, defined in an evolutionary biology context
as the individual’s ability to reproduce
and this presents an ‘evolutionary paradox
So why does natural selection not drive homosexuality to extinction?
First we must realize is that being homosexual necessarily doesn’t mean that you won’t
have any children and pass on your genes.
Throughout history, and even today,
in many societies homosexual activity was fiercely discouraged.
Countless individuals with a preference for members of their own sex have entered
into heterosexual marriages, keeping their homosexuality
and associated activities, a secret
all while raising a family, passing their genes on to the next generation.
But if we eliminate culture and look towards animals we still find plenty homosexual activity.
From ducks to lions, to fruit flies, if a species has sex, members of that species have
homosexual sex.
For example, nine out of ten mountings in Giraffes have been observed between two males,
and in sheep research has found that eight percent of rams prefer a male partner
when given a choice.
It is very clear, that the presence homosexual behavior in a species is the norm
and not the exception!
However, if evolution is to be involved we must first ask ourselves;
does same-sex sexual behavior have a genetic basis?
To answer this we can turn to twin studies; if we compare genetically identical twins
with non-identical twins and pairs of identical twins are more likely to share a trait,
be it height, eye colour, or sexual preference, than non-identical twins are
we have found evidence of a genetic component.
And indeed, in the case of sexuality we do find that identical twins are more likely
to share sexual preferences than non-identical twins.
So the consensus here is that homosexuality does have a genetic component.
But before we look at why, a few caveats to keep in mind.
Human experimentation is … well … complicated … and animal studies are not much easier
when it comes to understanding the motives behind the behavior.
Furthermore, it’s difficult to separate whether traits are caused by genetics
or the effects of such things like the pre-birth environment.
One last caveat before we look a little deeper, most of the research here has been concerned
with male homosexuality, we might assume that female homosexuality works in a similar way,
but that is just an assumption.
But that won’t stop us from trying to understand this conundrum,
and we have a few hypotheses to look at.
To understand this you must know that you have two copies of each gene in your genome,
one from each parent, different copies of these genes have subtly different effects;
such as the difference between blue and brown eyes;
the different possible versions of a gene are called an alleles.
So today I want you to think about genes encoding for sexuality with differ variants,
alleles, encoding for homosexuality and heterosexuality.
For our first hypothesis we can suggest that
the genes involved may encode for other things too.
The reproductive losses due to homosexuality are then offset
by other positive benefits to the individual.
It’s been suggested that alleles encoding for homosexual behavior could be beneficial
in terms of an individual’s ability to form social bonds and build alliances.
For our hunter gather ancestors, the social benefits of bonding with other members of
your hunting party might be crucial to survival
therefore being selected for as a survival strategy rather than a reproductive one.
Second we come to the Inclusive fitness hypothesis.
Under this hypothesis even though homosexual individuals are less likely to reproduce themselves
their siblings and relatives can and, according to this hypothesis,
with the help of their homosexual relatives
are able to raise more children than they would have without that Help
Since these relations share some of the homosexual’s genes
these genes are passed on to the next generation.
Maintaining them in the population.
Our next hypothesis for the maintenance of homosexual alleles is called over dominance.
Under this scenario an individual with two copies of a homosexual allele
suffers a fitness penalty due to their homosexual behaviour.
However, an individual carrying both a homosexual allele and a heterosexual allele
would have a fitness advantage over an individual with two heterosexual alleles.
That advantage that people with two different types of alleles have maintains the homosexual
allele in the population even though those with two copies of the homosexual allele
reproduce less frequently.
The final hypothesis I will introduce today is sexual antagonism.
This suggests that while an allele might confer a fitness cost in one sex,
for examples in males’ resulting in homosexuality - in the opposite sex this same allele confers
a fitness advantage, resulting in those females reproducing more,
thereby maintaining the allele in the overall population.
So which of these hypothesis is the most likely to be true.
Well, research in fruit flies has demonstrated same-sex sexual behavior is a complex trait
with many genes involved as well as significant environmental causes.
And with this complexity in mind, these hypotheses are not mutually exclusive;
it’s quite possible that there is an element of truth in all of these ideas.
but it has been shown that those with both the homosexual and heterosexual alleles
of some genes involved do have higher rates of reproduction,
thus supporting the over dominance hypothesis;
however sexual antagonism is not eliminated either
as females with a genetic makeup associated homosexuality
also have a higher reproductive rate.
The exact evolutionary reason behind the presence of homosexuality might not be particularly clear,
but one thing is agreed upon however.
There are many factors determining sexuality, including some out of the control of the individual
such as the genetic component;
and no about of electroshock therapy, or religious studies, will change these genes
maybe we could play god with CRISPA-CAS, but I’m not sure the religious folk will love that solution.
So, as far as we can tell, that is why homosexuality evolves, but what about how other things evolve?
Well, it just so happens that a few other YouTubers
have just uploaded videos on the how and why things evolve,
from the evolution of the sexes themselves to facial expression
So check out this playlist, I made, of all the videos.