Homo Mysterious: Evolutionary Puzzles of Human Nature (18 page)

There is also a case to be made for homosexuality having evolved via strict group selection, independent of any inclusive fitness benefits, as follows: If groups containing large numbers of homosexuals reproduced less rapidly, they would be less likely to overexploit their resource base.
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Other models have also been proposed, focusing on social interaction rather than resource exploitation. For example, if homosexuality correlates with greater sociality and social cooperation, then groups composed of proportionately more homosexuals might function more smoothly, and also occupy better habitats, which in turn would contribute to greater reproductive success for their genes.
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Of course, this presupposes that a society containing significant numbers of homosexuals would be less fraught and conflict ridden than one with fewer, which would seem to necessitate far more tolerance and acceptance of same-sex preference than is true today, at least in the industrialized West. The theory, at least, is intriguing and consistent: Given that male–male competition for access to females is a frequent cause of violence—among animals as well as humans—a case could be made that the larger the number of gay males, the less competition among heterosexual males for access to females, since gay males, once they self-identify, are no longer reproductive threats to their heterosexual colleagues.

On the other hand, there might be a corresponding increase in competition among gay males themselves! This could, however, be expected to be less intense than among straight males, since the latter compete for females, a “resource” that lends itself to being reproductively monopolized—at least for 9 months at a time—whereas
male physiology makes it more possible, at least in biological terms, for gay lovers to “share.”

There is another generalized explanation for the evolution of altruistic behavior, disconnected from group selection and which, unlike kin selection, makes no assumptions about genetic relatedness between altruist and beneficiary. Technically but incorrectly known as reciprocal altruism,
^ix
it is simply a biological version of “you scratch my back, I’ll scratch yours” or “one good turn deserves another.” The basic idea, first elaborated by sociobiologist Robert L. Trivers in 1971, is that individual 1 could be selected to do something that benefits individual 2 if there is sufficient likelihood that some time in the future, the tables will be turned whereupon individual 2 repays the debt.
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As Trivers demonstrated, a system of this sort could evolve so long as the cost to individual 1 for her initial action is relatively low, and the benefit to individual 2 is high. The biggest fly in the reciprocating ointment, however, is the temptation to cheat; that is, for individual 2 to accept aid when in need, but then decline to reciprocate when the situation reverses and it is time for individual 2 to repay the favor.

For all its appeal—emotional as well as intellectual—it has proven difficult to identify clear-cut examples of reciprocity among animals. (The sole uncontested case involves vampire bats, who indulge in a heartwarming practice whereby those individuals who have successfully flown and fed by night regurgitate part of their blood meal to others less fortunate. Grateful recipients are then likely to reciprocate when they are successful and their earlier benefactors would otherwise go hungry.
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) Human beings, on other hand, are the reciprocators par excellence. We share all sorts of things, often exchanging favors when not trading actual objects.

The connection of all this to homosexuality may seem obscure. Indeed, it may well
be
obscure! But here is the idea. Reciprocity doubtless plays a role in cementing mateships, as it does with friendships. The fact that gays and lesbians are every bit as prone to invest in their same-sex partners as are heterosexuals may seem
to be an evolutionary anomaly, insofar as the latter are far more likely to reproduce. Two considerations are relevant here: For one, as already noted, many homosexuals are actually bisexual, or otherwise capable or inclined to reproduce, so that mutual aid within a same-sex partnership can contribute at least somewhat toward the fitness of each. For another, let’s assume that there is a kin-selected payoff to homosexuality, via assistance rendered to one’s genetic relatives: This could lead to a generalized inclination to be helpful and thus for homosexual partners to express such inclinations as well. It also would not preclude taking care of oneself via reciprocity with a partner, especially when “altruistic” assistance to genetic relatives is not immediately necessary or feasible.

In addition, it has been suggested—although not as yet clearly demonstrated—that homosexuals tend to be especially talented at social skills such as caregiving. If so, then they could conceivably be especially prone to mutually fitness-enhancing reciprocity. If not, then we must look elsewhere. But where? Aside from kin selection, reciprocity, and group selection, are there other promising hypotheses for the genetically based evolution of homosexuality? The answer is yes.

A range of possibilities revolve around what geneticists call “heterosis,” resulting in “balanced polymorphisms.” Consider a well-known illness, sickle cell anemia, for which there are two relevant alleles: Call them Sn for normal red blood cells and Ss for sickle cell.
^x
Ss generates a biochemical anomaly as a result of which the sufferer’s red blood cells collapse into an erratic sickle shape, causing them to stick together and clog blood vessels. In double-dose (“homozygous”) form—that is, when individuals are SsSs—the sickling allele is often lethal. One would therefore expect that it would quickly disappear from the human gene pool. Indeed, it is largely absent from most populations, except for certain regions
that have a long history of malaria. It turns out that for complex reasons, heterozygous individuals—who are SnSs and therefore carry a single dose of the sickling gene—are more malaria resistant than are homozygous normals (SnSn). As a consequence, the otherwise deleterious sickling gene has been retained in the human gene pool, even though it is strongly selected against in double-dose form.

For our purposes, the idea would be that perhaps a genetic predisposition for homosexuality, even if a fitness liability when homozygous, could have been retained over evolutionary time if it somehow conveys a compensating benefit when combined with one or more other alleles.
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No precise candidate genes have yet been identified, but for now, the possibility cannot be excluded.
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Moreover, several suggestive qualitative arguments can in fact be made. Here goes.

Geneticists recognize something called “linkage disequilibrium,” which is simply a fancy way of saying what should be obvious to anyone who has taken a high school biology course: Genes aren’t piled up randomly and independent of each other, like beans in a bottle. Rather, they are arranged, in a line, on chromosomes. And this physical
linkage
is bound to create a degree of
disequilibrium
, if we compare the frequency of genes as they actually exist with what would be expected based simply on the fitness value of each one taken separately. Even if a particular gene has an unhelpful effect on fitness, it could still be maintained in substantial numbers if it is physically linked—on the same chromosome—to some other gene conveying a benefit that more than compensates for the first one’s disadvantage.

Even such linkage should eventually become less effective, however, since chromosomes (the X and Y sex chromosomes excepted) regularly break and then reattach, occasionally crossing over with their matching “homolog.” This enforced separation of genes along a chromosome provides an opportunity—at least in theory—for every gene to be evaluated independent of those to which it had at one time been linked. But this sorting-out process takes time and depends on many other factors. Crossing over does not reliably set the stage for a lower fitness trait to be selected against.

There is also another, parallel process that could result in a lower fitness genetic tendency being retained over evolutionary time.
Known as “pleiotropy” (
pleio
= “many,”
tropy
= “turn” or “influence”), it speaks to the rather common situation in which a single gene has multiple effects, which sometimes seem quite disconnected. Thus, genes “for” eye color in fruit flies often also influence the details of how the affected animals court—which means, of course, that the genes could just as well be identified as “for” courtship, with a pleiotropic side effect upon eye color! Something comparable could be going on with respect to sexual preference, if genes “for” same-sex preference also generated, let’s say, more efficient kidneys or better eyesight. The result would then be the maintenance of homosexuality, not because it is adaptive in itself but in spite of the fact that it might be maladaptive, so long as its genetic underpinning were wedded to some positive aspect of anatomy, physiology, or behavior that more than compensates.

Sadly, for those of us who like our explanations simple, this idea, too, is still awaiting empirical support.

For yet another plausible idea whose only drawback is a lack of evidence, there is always the prospect that homosexuality-promoting genes simply arise unusually often, via mutation. There is mathematically solid theory showing that “mutation-selection equilibrium” could maintain even a trait as dramatic as exclusive homosexuality if the mutation rate were high enough and selection against such a trait were sufficiently low.
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But there is a problem here, too, in that selection should, over time, work against such a mutational tendency, if alternative allele forms that are more stable convey higher fitness. (And an even greater problem is that such mutations have yet to be discovered!
^xi
)

Male homosexuality is more frequent than its female counterpart, at least among human beings. Of an anthropological sample of 21 different societies, 11 (52%) had male-only homosexuality, 9 (43%) had both male and female homosexuality, and only 1 (5%) had female-only homosexuality.
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Moreover, it appears that in every human society, the frequency of male homosexuality (around 3%) is half again higher than female (around 2%). This pattern is suggestive biologically no less than sociologically and could be due to any number of reasons, ranging from greater male vulnerability
to in utero stress to a presumed lower threshold for male sexuality in general.

Developmental processes are often described as being relatively “plastic,” if they are subject to substantial variation depending on environmental conditions, or “canalized,” if they aren’t. Of course, such terminology is simply descriptive; it doesn’t explain why this difference exists. (We might call it the Rumpelstiltskin effect, whereby people are prone to think that a phenomenon has been effectively dealt with and will obligingly disappear—like that annoying, magical dwarf in the famous fairy tale—once it has been named.)

It appears that homosexuality is more plastic in women and more canalized in men, and in this case, at least, such greater plasticity among women is consistent with expectation based on evolution. For one thing, we can anticipate that compared to men, women would be more inclined to switch from heterosexuality to homosexuality with age, since women experience menopause, after which any preexisting heterosexuality ceases to convey a direct reproductive payoff, and which in turn would give greater scope for “sexual fluidity.” This is what happens.
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By the same token, biologists would predict that since men retain the capacity to reproduce into old age, they should be less predisposed to switch from heterosexuality to homosexuality as they get older, and this, too, is the case.
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There is growing evidence that much sexual preference is fluid, among both sexes, but especially women. Considerable debate and uncertainty swirl around how to describe and classify human sexual preferences, as well as how to explain them. Finnish researchers, for example, have data suggesting that nearly 33% of men and 66% of women have the potential for genetically influenced homosexuality.
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It appears that on balance, more women than men are capable of adopting either same- or opposite-sex preference, with the “decision” for women especially likely to be influenced by the particular characteristics of any potential partner. Men, by contrast, are more prone to have a given sexual orientation first—either homosexual or heterosexual—and then secondarily to find suitable partners who match their existing preference.

Prominent among the defining genetic differences between men and women is the fact that males are XY and females XX. A sex-linked gene present on the X chromosome would therefore
be effectively unpaired (since the Y chromosome carries very few genes, and those it possesses lack partner alleles on the X). Accordingly, any such X-carried trait is more likely to be expressed in males. A recessive allele present in a woman’s X chromosome, by contrast, stands a chance of being overridden by an alternative allele on the other X. In this regard, it is interesting that among birds, in which females are the “heterogametic sex,”
^xii
female homosexuality is more frequent than its male counterpart—the opposite of what occurs in mammals.
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Going further, let’s note that a new mutation (or any already-present homosexuality-promoting allele) on a Y chromosome would likely be expressed in all of a man’s sons, whereas a similar gene on a father’s X chromosome may or may not be expressed among his daughters, since unlike a Y-chromosome gene, one carried on a paternal X chromosome would have to contend with the corresponding alleles on the other X chromosome contributed by the mother. It might later, of course, be expressed in his grandsons, but as the generations proceed the analysis becomes harder to make and any relationship is more likely to be missed.