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.
Thanks for sharing my vid /u/Brown_Polar_Bear . I know this is a simplified explanation of a rather complex topic so and comments, criticisms questions are welcome.
Awesome video!! Great explanations and nice graphics.
The narrator is a little hard to understand, maybe the mic needs some tuning? Or maybe he needs to switch up his cadence a bit.
What's with the evolution dump? Like ten videos with that theme.
I'm not complaining, but wow.
Why does a person with both homo and heteo Alleles have more biological fitness? The video says that but doesn't explain why.
Great video. And solid explanation's. I can see that you have putting a lot of work into this video, we'll done
Could you do one on bisexuals as that seems another area that could explained as i didn't even know about these homosexual hypotheses.
Dude needs to hire a narrator. Made it 15 seconds.