Why Did Homosexuality Evolve?

All right, now to start things off with some listener email. I’m very pleased to have received my first critical email. You might think that podcasting on a topic that is so controversial in the popular culture would get me lots of criticism, but I’ve only received positive comments so far. That is, unless you check out the reviews on the iTunes page- there have been a number of 1-star reviews. But this email, although it’s critical, is so polite I just have to share it with you right off the bat.

Dear Dr. Zach,

I hope you are well and your weekend is running smoothly.

The Infidel Guy recently turned me onto your podcast, apparently he holds you in high esteem.

I was excited to hear the gospel of evolution from an educated person, as most of the times I hear it spouted are from high-schoolers and fellow workers, who are innocent in the fact that they believe in evolution simply because it has been crammed down their throats.

I expected your podcast to at least acknowledge the number of evolutionary frauds that have been presented(all of them) and the number of evolutionary evidences that stand before science(none of them).

I was dissapointed to hear your podcast sounds like someone reading a public school text-book. I was hoping to at least hear about microbial evolution or evidences that may be true that I haven't heard of, instead I hear things like, "We Suppose" "The evidence suggests" "The Fossil Record" and I wonder exactly which evidences have solidified your understanding of evolution, because it is all clearly a religion, and the most boring fairy tale ever told, to those of us who understand the theory.

The main evidence that it seems you are relying on for the foundation of your religion is the occurence of variation within a species. The correct term is variation, evolutionists such as yourself have misnomered it to be "Micro-Evolution" so you can piggy back upon something that has actually happened. Micro-evolution(I'll use your word) will NEVER lead to a new improvement of a species or the formation of a new species.

I hope you have a good Sunday, please consider spending it in Church.

Yours,
C.S.

Well, C.S., as it happens I do spend my Sundays in church. Although, I can’t see what that has to do with evolutionary theory. I’m glad to hear that you connected to this podcast through the Infidel Guy- as my long-time listeners know, he was the one who first conceived of Evolution 101. I certainly haven’t had evolution “crammed down my throat” as you say, I’ve been learning as much as I can on my own, since it is so scientifically compelling. Hopefully, I can communicate enough of what I know to creationists who have had nothing but pseudoscience crammed down their throats, instead of real science. I’m not sure which evolution “frauds” that you’re talking about- although I can hazard a guess. Nebraska Man? Piltdown Man? The thing is, these just aren’t scientifically relevant anymore- who do you think recognized them as frauds? Scientists! The only people who are interested in them anymore are creationists, because they think that bringing them up can poke holes in evolutionary theory now. That’s just not the case- if you’ll check out my podcast on human evolution, you’ll see that they’re not even part of the equation. Also, I’m not quite sure why you think I haven’t presented any evidence for evolution. I did a whole series on the molecular evidence for evolution which I’m quite proud of. Evolution is decidedly not a religion- it makes no claims about a deity, its advocates don’t get together once a week to sing songs about how wonderful evolution is, and I don’t pray to Charles Darwin. Regarding “micro-evolution,” actually it is creationists who use this term much more often than those who accept science. I’ve already discussed the reason why there isn’t a difference mechanistically between micro-and macro-evolution, any more than the difference between micro- and macro-economics.

All right, well, thanks from C.S., and now for a more inquisitive question:

The environmental pressures that lead to the reproductive selection of certain changes or mutations seem to be at a very gross level, such as at the level of reproductive success or survival.

How, then, does evolution effect something like eye color, skin color, size, cholesterol production in the liver, the presence or absence of a pinky toe, intellect, etc. that don't seem to directly effect reproductive success or survival? How does/did our body/species select for subtle changes that are effectively invisible to natural selection?

From H.G.

The reproductive success of a population isn't just one trait- instead, fitness is a comprehensive quality that takes into account lots and lots of traits. Any individual trait that gives any reproductive advantage, no matter how small, will be increased in a population over many generations.

For any given trait, we can arrive at conclusions based on what we presently know about the relative fitness associated with the presence or absence of that trait. For example, eye color (essentially the presence or absence of pigment in the iris) has historically been segregated geographically. The lack of pigment (blue eyes) occurs with the greatest frequency in Northern European populations. We know that this region tends to experience less direct sunlight, especially at the higher latitudes. We also know that other organisms that are exposed to very little light also lose pigment over time. This is either because the selective pressure to keep pigment drops (it does cost energy to make pigment molecules, after all) or because there is a selective advantage to have little pigment. Skin color is likely due to the latter- Northern Europeans also lack much pigment in their skin. There may be an advantage to their ability to synthesize Vitamin D in these environments, but I'm not sure if this has been established empirically.

There are some traits that are obviously of no bearing to reproductive success whatsoever. For example, the ability to roll your tongue. I can do it, others can't, and unless it's linked to some other important trait, it seems to be just a weird genetic coincidence. It might be left over from some earlier period in human evolution, perhaps from our smaller, fruit-eating primate ancestors. For now, though, it's a trait that just randomly shuffles through the generations, until some change in our environment makes it imperative for reproductive success.

All right- well that’s enough questions for this week, on to the main topic: 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.

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.