After doing a recent podcast episode with Jake Farr-Wharton on The Imaginary Friends Show I thought I’d repost this article from Evolution 101 written by Dr. Zachary Moore on the evolution of homosexuality. I’ve added a few pictures and links to the text. Enjoy!
Why did homosexuality evolve? I realize that, just as with evolution, homosexuality is still somewhat of a controversial issue in pop culture (well, at least in American culture, for my international listeners). But nothing’s more interesting then sex, and what could be better than sex and evolution?
The common argument goes like this: if evolution is true, then only those individuals who are able to reproduce will contribute offspring to the next generation. Thus, individuals who are homosexuals will not be able to reproduce, their genes will not be passed on to the next generation, and so if there is some genetic or biological reason for homosexuality, evolution should have removed it a long time ago.
First of all, is homosexuality a specifically human behavior? If it is a fundamentally biological behavior, there should be some other species which share it. And, in fact, there are close to 500 known species which are known to engage in homosexual behavior, including elephants, dolphins, sheep, bears, deer, rats, cats, dogs, cows, rabbits, kangaroos, squirrels, whales, bats, pigs, mice, goats, as well as just about every other primate. And that’s just the mammals! There are many more birds, fish, reptiles, and even insects which have also engaged in homosexual behavior.
So it really doesn’t seem as if homosexuality is really all that uncommon. But so what? Why should homosexuality be a trait found in so many organisms if it’s so fatal to the evolution of the species.
Well, the answer is, as with most things I discuss here, that sex really isn’t black and white. And homosexuality isn’t fatal to the evolution of species. Remember the definition I gave for evolution way back in the first podcast- “change in allele frequency in a given population over time.” There’s a reason why I specified “population,” and not “individual.” Individual organisms don’t “evolve” any more than a single pixel makes up a picture on your computer screen. What is necessary for evolution to take place is for there to be a group of individuals, a population, within which genes can change and flow.
Now, it certainly is the case that, for most organisms which utilize sex, heterosexual sex is required for propagation. But consider- not all species employ strictly monogamous sexual strategies. For many species, males compete for control of several females, meaning that there are many males who are left out in the cold, so to speak, with nothing but each other and raging libidos. One hypothesis fits this scenario- homosexuality occurs in these organisms to placate the male aggression that is left over after competition for females.
But that doesn’t mean that homosexuality is always a consolation prize. Among the American Bison, male-male intercourse accounts for almost half of all mating, and not just among the losers. Both parties seem to enjoy themselves, with the subordinate male even accommodating the advances of the dominant male. The same phenomenon can be seen in bighorn sheep, where the male being mounted even adopts the arched-back posture called “lordosis,” which is typically associated with the female sexual response. Clearly, these animals seem to be enjoying what they’re doing.
But the males don’t get to have all the fun. Female homosexuality is also common, with female antelope mounting each other in simulation of heterosexual courtship behavior when males are not present. In bonobo chimpanzees, the female-dominated social network is composed of close bonds which are shown by frequent homosexual interactions between female members of the group. In fact, more than half of an adult female bonobo’s sexual interactions will be homosexual in nature. (An in depth paper on homosexuality in primates)
So how, you’re probably wondering, do these populations ever manage to reproduce with so much homosexuality? Well, the reason is because, as I said before, it’s not that black and white. Sure, individuals engage in homosexuality some of the time, or even a lot of the time, depending on the species. But not all of the time- they still find time to mate heterosexually. Sex seems to be a very fluid trait in many animals- pretty much any sexual configuration that can be performed within anatomical limits is done by some kind of animal. Sorry to say, but although humans can be kinky, we’re just not that original.
Now, you remember that I said that evolution takes place in populations, not individuals? Well, consider the social benefits of a population in which all members can share the close bonds of a sexual relationship, not just males and females. Clearly, in the case of bonobo chimpanzees, the bonds formed between females by homosexual relations are socially stabilizing. A stable society is much more likely to promote successful reproduction of young. Thus, homosexuality would be an evolutionarily beneficial behavior.
But what about some molecular evidence? Well, if you’re hoping that a “gay gene” has been found you’re not in luck. One hasn’t been found, although more and more scientists are starting to look at the genetics of homosexuality. Most likely, homosexuality as a behavior is a more complex phenomenon than just blue or brown eyes- a number of factors are considered- including the number of older male siblings a person has. Scientific research out of Toronto has shown that the more older male siblings a man has, the more likely he is to be a homosexual. The hypothesis is that the mothers becomes immunologically sensitized to the successive male fetuses within her, since they contain male proteins that she is not used to. According to this hypothesis, by the time the youngest male child is being carried in utero, she has developed anti-male antibodies which effectively diminish the normal masculinization process, resulting in a tendency towards homosexuality. But there may be some other benefits to the mother- a recent study from Italy showed that the maternal relatives of homosexual men have more children than the maternal relatives of heterosexual men. If this is repeated, it would suggest that there is a reproductive benefit to women whose DNA tends to result in homosexual male children- they have more children overall, meaning that their evolutionary fitness is actually increased because of the fact that they have homosexual sons. This is a fascinating possibility, especially because a better understanding of the genes involved in this phenomenon could have a major influence on our understanding of reproduction in general, and could point towards some better therapeutic targets for women who have problems with fertility.
All right- well, that was a lot to chew on for this week. To review- homosexuality is not a strictly human trait- it is practiced commonly throughout the animal kingdom. It has a clear evolutionary benefit in that it fosters better socialization among members of both genders. In humans, the evidence strongly suggests some kind of genetic component in the development of homosexuality, although the specific genes have not yet been discovered.
Before I sign off, I do want to make it crystal clear that the discussion here is in no way establishing a moral position in favor of, or against homosexuality. To do either would be to commit a clear naturalistic fallacy- to say that because something is natural, it is either right or wrong is clearly illogical. The moral discussion of homosexuality is reserved for other, non-scientific settings. Thanks for listening, and have a great week. I’ll see you next time.
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