A few months ago, I was talking with a friend about Universal Darwism and the power of genetics, and without hesitation she asked, “Well, what about homosexuality? Surely that would have been removed from the gene pool centuries ago?”. As the evolutionary pragmatist, I completely agreed with her. If being gay was indeed nature over nurture, it would have been removed from the gene pool millenia ago, but the liberal in me felt that homosexuality must be genetic (as opposed to a life choice etc.). This conflict (along with the obvious observation that homosexuality is still pervasive in society) had me utterly stumped.
Let’s tackle the problem of evolving homosexuality out of the gene pool first — why did I believe that homosexuality should have been breed out? Imagine two groups, Group A, which makes up 99.9% of the population and Group B, which makes up the remaining 0.1%. Now imagine that Group B is a mere 1% more efficient at reproducing than Group A. The chart below shows how the two populations would progress (as a percentage of the total population) if Group A had 2 children each and Group B at 2.02 (1% more).
Unbelieveably, despite only making up 0.1% of the initial population, it takes as little as 700 generations for Group B to equal Group A and under 1,400 for their positions to be reversed! Anyone with a financial/economic background will appreciate the power of compound interest here, the same phenomena is exacerbated when applied to evolution, given both the size of the populations and years over which we look at the effects.
So what does this have to do with homosexuality? Simple — homosexual partners cannot reproduce (naturally), therefore cannot pass their “gay gene” down to their children. So, as in the example above, if a 1% decrease in reproductive capability leads to practical extinction within 1,400 generations, how has homosexuality (with a 100% decrease in reproductive capability!) not been erased after 50,000 years?
I should point out now that we don’t actually know the answer to this question. Anything subsequently is conjecture — well thought-out, intelligent conjecture, but conjecture nonetheless!
The first obvious argument is that maybe being gay isn’t genetic — it is a choice after all! Nurture winning over nature. This riled me, as it went against what I believed but seemed a scientifically valid argument. Thankfully, there’s plenty of data to the contrary. In the US, about 3% of men and 2% of women are gay, however if you look at sisters of homosexual siblings, this increases to 6% and 8% for brothers. In short, if your sibling is gay, you are up to 3x more likely to be gay yourself. Now of course, siblings are often brought up in the same environment, so this very well may be nurture still in play. So can we prove a genetic connection further?
One way is to look at concordance, a term in genetics that applies to twins and the probability that they will both have a certain characteristic, given that one of the pair has the characteristic (Wikipedia). Twins are either monozygotic (MZ), identical twins with 100% gene crossover or dizygotic (DZ), fraternal/sororal twins with 50% gene crossover. Without going into the specifics of the various studies (i.e., Homosexual Orientation in Twins: A Report on 61 Pairs and Three Triplet Sets), it has been observed that the concordance for homosexual orientation in MZ twins is above 50% and considerably lower in DZ twins. Looking beyond twins, the concordance is lower in biological siblings than twins and lower still in adoptive siblings. This is exactly what you would expect if genetics exerted a strong influence on sexual orientation. However, the fact that concordance in MZ twins isn’t 100% implies that genetics isn’t the only influence — environment must play a part too.
So while we’ve not ruled out nurture completely, it is highly likely that genetics plays a very strong part in an individual’s sexual orientation. Now the question is, given the adverse effect homosexual orientation has on reproduction, why do “gay genes” exist?
Altruism & kin selection
The first theory is around kin selection. This was first proposed by Darwin in Origin of the Species, to help explain phenomena such as altruism. In essence, kin selection refers to behaviours that favour the reproductive success of one’s relatives (as opposed to offspring), often even to the detriment of one’s own health or even life. Put another way, if I help my sister or uncle, or indeed anyone with some part of my genetic make-up, I am (at least in part) helping my genes to propagate more successfully. For example, in many social insect colonies (bees, ants etc.), the workers are sterile. Why then would a bee work for the greater good of the colony, “knowing” that the genes are not to be passed on to subsequent generations? Kin selection is certainly one evolutionary explanation for this. Kin selection and the evolutionary benefits of altruism are extensively documented and out of scope of this blog entry — but Richard Dawkins explains it beautifully in The Selfish Gene for those looking for a more robust explanation.
So could homosexual behaviour be kin selection at work? Do gay men and women provide additional care to their kin, therefore increasing the likelihood of them surviving? The answer is unclear — in Western society, there is no meaningful difference between homosexual and heterosexual behaviour here, but in some cultures this behaviour is certainly seen. For example, in Samoa, they treat effeminate men as a third gender — fa’afafine — who are known for their dedication to family.
Kin selection as a theory has a few issues, specifically around the lack of modern evidence to support it. However, it has long been argued (and indeed still prevalent in societies such as certain isolated Amazonian tribes) that child-rearing in ancient societies wasn’t the sole domain of the mother. The advent of industrialisation, a lower child mortality rate and a considerably more mobile populace may have shifted the effectiveness of kin selection.
However, social cooperation and cohesiveness remains an ideal fitness factor. For example, if a population contained a greater degree of homosexuality, this would reduce the male/male competition for females, allowing greater focus on resource utilisation and in turn allowing for a better habitat, a controlled population and a greater reproductive success for everyone’s genes. Some people may find this sort of reasoning hard to swallow, but think back to the compound interest example we started this article with. The tiniest potential increase in fitness will be exploited by evolution on the genetic (as opposed to individual) level.
Heterozygote Advantages and Pleiotropy
Both kin selection and any associated social cohesion theory assumes a positive affect of homosexuality on reproduction. What if homosexuality is an accidental emergence or a by-product of another genetic adaptation? Heterozygote advantage and pleiotropy are two fancy words for describing just this phenomena within genetics.
Sickle-cell anaemia (SCA) is a genetic blood disorder that reduces a red blood cell’s ability to functional optimally, causing greatly reduced life expectancy and, in some severe cases, death. As a genetic defect, evolution should have long selected this gene out of the worldwide population. However, there is an interesting side effect. If a person receives the defective gene from both the mother and their father (homozygotic), they will develop SCA. However if they receive one defective and one healthy allele (heterozygotic), they become a carrier — able to pass on the disease but not exhibit full-blown symptoms. As such, a carrier will produce some sickled cells but enough to remain healthy. Furthermore, these sickled cells provide a key benefit to the carrier — a greatly increased resistance to malaria! This is the heterozygote advantage.
As a result, SCA indeed has been removed from virtually all of the global population, except where malaria is still endemic. So what does this have to do with homosexuality? There have been some suggestions that a gene that increases a tendency toward homosexuality in homozygotic form would still yield a fitness increase in heterozygotic form. In essence, homosexuality is to the sickle cell disease as the heterosexual’s fitness increase is to the malarial resistance. So what is that fitness increase in heterosexuals? We have no idea.
Heterozygotic advantages aside, another emergent behaviour exhibited in genetics is known as pleiotropy — where a single gene has multiple effects, many time with very disconnected effects. For example, the gene that vestigal gene that controls wing development also controls the number of eggs in a fly’s ovaries and decreases life expectancy. The pigmentation gene in cats is pleotropic, with 40% of white fur/blue eyed being deaf. Talking of eye colour and, most interestingly in our case, the gene that controls the eye colour in fruit flies also changes their mating behaviour!
So homosexuality could be a pleotropic effect for additional fitness-increasing benefits we have yet to ascertain.
We have looked at four potential explanations for the existence of homosexuality — kin selection, social cohesion, heterzygotic advantage and pleiotropy. There are plenty of other theories and a lot of research into other behaviours in the animal kingdom (one species of albatross is about 30% lesbian!) which I will leave for the curious reader to research themselves. This blog has been heavily based upon the fantastic Homo Mysterious by David Barash that looks that human behaviours and traits not totally explained by evolution.
While there are no hard and fast answers, I’m confident enough in the evidence that homosexuality has enough genetic benefits (or side effects) to warrant its continued presence within the gene pool. Of course, the ironic conclusion behind all of this is that if religious leaders truly wanted to get rid of homosexuality (or at least greatly reduce it), the easiest path to take would be that of total acceptance! As the tolerance towards homosexuality decreases, it increases the likelihood that homosexuals will repress their true feelings (as millions globally do today) and pass on their genetic material (and therefore homosexual tendencies) to their children.
Admitting this however would be to accept genetics and evolution — and if we were all made in God’s image, then God must be a little bit gay too…