Matt Manley for The Chronicle Review
Critics claim that evolutionary
biology is, at best, guesswork. The reality is otherwise. Evolutionists have
nailed down how an enormous number of previously unexplained phenomena—in
anatomy, physiology, embryology, behavior—have evolved. There are still
mysteries, however, and one of the most prominent is the origins of
homosexuality.
The mystery is simple enough. Its solution,
however, has thus far eluded our best scientific minds.
First the mystery.
The sine qua non for any trait to have evolved
is for it to correlate positively with reproductive success, or, more
precisely, with success in projecting genes relevant to that trait into the
future. So, if homosexuality is in any sense a product of evolution—and it
clearly is, for reasons to be explained—then genetic factors associated with
same-sex preference must enjoy some sort of reproductive advantage. The problem
should be obvious: If homosexuals reproduce less than heterosexuals—and they
do—then why has natural selection not operated against it?
The paradox of homosexuality is especially
pronounced for individuals whose homosexual preference is exclusive; that is,
who have no inclination toward heterosexuality. But the mystery persists even
for those who are bisexual, since it is mathematically provable that even a
tiny difference in reproductive outcome can drive substantial evolutionary
change.
J.B.S. Haldane, one of the giants of
evolutionary theory, imagined two alternative genes, one initially found in
99.9 percent of a population and the other in just 0.1 percent. He then
calculated that if the rare gene had merely a 1-percent advantage (it produced
101 descendants each generation to the abundant gene's 100), in just 4,000 generations—a
mere instant in evolutionary terms—the situation would be reversed, with the
formerly rare gene occurring in 99.9 percent of the population's genetic pool.
Such is the power of compound interest, acting via natural selection.
For our purposes, the implication is
significant: Anything that diminishes, even slightly, the reproductive
performance of any gene should (in evolutionary terms) be vigorously selected
against. And homosexuality certainly seems like one of those things. Gay men,
for example, have children at about 20 percent of the rate of heterosexual men.
I haven't seen reliable data for lesbians, but it seems likely that a similar
pattern exists. And it seems more than likely that someone who is bisexual
would have a lower reproductive output than someone whose romantic time and
effort were devoted exclusively to the opposite sex.
Across cultures, the proportion of
the population who are homosexual is roughly the same. What maintains the
genetic propensity for the trait?
Nor can we solve the mystery by arguing that
homosexuality is a "learned" behavior. That ship has sailed, and the
consensus among scientists is that same-sex preference is rooted in our
biology. Some of the evidence comes from the widespread distribution of
homosexuality among animals in the wild. Moreover, witness its high and
persistent cross-cultural existence in Homo
sapiens.
In the early 1990s, a geneticist at the
National Institutes of Health led a study that reported the existence of a
specific allele, Xq28, located on the X chromosome, that predicted gay-versus-straight
sexual orientation in men. Subsequent research has been confusing, showing that
the situation is at least considerably more complicated than had been hoped by
some (notably, most gay-rights advocates) and feared by others (who insist that
sexual orientation is entirely a "lifestyle choice").
Some studies have failed to confirm any role
for Xq28 in gay behavior, while others have been supportive of the original
research. It is also increasingly clear that whatever its impact on male
homosexuality, this particular gene does not relate to lesbianism. Moreover,
other research strongly suggests that there are regions on autosomal (nonsex)
chromosomes, too, that influence sexual orientation in people.
So a reasonable summary is that, when it comes
to male homosexuality, there is almost certainly a direct influence, although
probably not strict control, by one or more alleles. Ditto for female
homosexuality, although the genetic mechanism(s), and almost certainly the
relevant genes themselves, differ between the sexes.
Beyond the suggestive but inconclusive search
for DNA specific to sexual orientation, other genetic evidence has emerged. A
welter of data on siblings and twins show that the role of genes in homosexual
orientation is complicated and far from fully understood—but real. Among
noteworthy findings: The concordance of homosexuality for adopted (hence
genetically unrelated) siblings is lower than that for biological siblings,
which in turn is lower than that for fraternal (nonidentical) twins, which is
lower than that for identical twins.
Gay-lesbian differences in those outcomes
further support the idea that the genetic influence upon homosexuality differs
somewhat, somehow, between women and men. Other studies confirm that the
tendency to be lesbian or gay has a substantial chance of being inherited.
Consider, too, that across cultures, the
proportion of the population that is homosexual is roughly the same. We are
left with an undeniable evolutionary puzzle: What maintains the underlying
genetic propensity for homosexuality, whatever its specific manifestations?
Unlike most mystery stories, in which the case is typically solved at the
finish, this one has no ending: We simply do not know.
H
ere are some promising
possibilities.
Kin selection. Scientists speculate that
altruism may be maintained if the genes producing it help a genetic relative
and hence give an advantage to those altruistic genes. The same could be true
of homosexuality. Insofar as homosexuals have been freed from investing time
and energy in their own reproduction, perhaps they are able to help their
relatives rear offspring, to the ultimate evolutionary benefit of any
homosexuality-promoting genes present in those children.
Unfortunately, available evidence does not
show that homosexuals spend an especially large amount of time helping their
relatives, or even interacting with them. Not so fast, however: Those results
are based on surveys; they reveal opinions and attitudes rather than actual
behavior. Moreover, they involve modern industrialized societies, which
presumably are not especially representative of humanity's ancestral
situations.
Some recent research has focused on male
homosexuals among a more traditional population on Samoa. Known as fa'afafine, these men do
not reproduce, are fully accepted into Samoan society in general and into their
kin-based families in particular, and lavish attention upon their nieces and
nephews—with whom they share, on average, 25 percent of their genes.
Social prestige. Since there is some
anthropological evidence that in preindustrial societies homosexual men are
more than randomly likely to become priests or shamans, perhaps the additional
social prestige conveyed to their heterosexual relatives might give a
reproductive boost to those relatives, and thereby to any shared genes carrying
a predisposition toward homosexuality. An appealing idea, but once again, sadly
lacking in empirical support.
Group selection. Although the great majority
of biologists maintain that natural selection occurs at the level of
individuals and their genes rather than groups, it is at least possible that
human beings are an exception; that groups containing homosexuals might have
done better than groups composed entirely of straights. It has recently been
argued, most cogently by the anthropologist Sarah B. Hrdy, that for much of
human evolutionary history, child-rearing was not the province of parents
(especially mothers) alone. Rather, our ancestors engaged in a great deal of
"allomothering," whereby nonparents—other genetic relatives in
particular—pitched in. It makes sense that such a system would have been
derived by Homo sapiens, of
all primate species the one whose infants are born the most helpless and
require the largest investment of effort. If sufficient numbers of those
assistants had been gay, their groups may have benefited disproportionately.
Alternatively, if some human ancestors with a
same-sex preference reproduced less (or even not at all), that, in itself,
could have freed up resources for their straight relatives, without necessarily
requiring that the former were especially collaborative. Other group-level
models have also been proposed, focusing on social interaction rather than
resource exploitation: Homosexuality might correlate with greater sociality and
social cooperation; similarly, it might deter violent competition for females.
Balanced polymorphisms. Perhaps a genetic
predisposition for homosexuality, even if a fitness liability, somehow conveys
a compensating benefit when combined with one or more other genes, as with the
famous case of sickle-cell disease, in which the gene causing the disease also
helped prevent malaria in regions where it was epidemic. Although no precise
candidate genes have been identified for homosexuality, the possibility cannot
be excluded.
Sexually antagonistic selection. What if one or more genes
that predispose toward homosexuality (and with it, reduced reproductive output)
in one sex actually work in the opposite manner in the other sex? I prefer the
phrase "sexuallycomplementary selection":
A fitness detriment when genes exist in one sex—say, gay males—could be more
than compensated for by a fitness enhancement when they exist in another sex.
One study has found that female relatives of
gay men have more children than do those of straight men. This suggests that
genes for homosexuality, although disadvantageous for gay men and their male
relatives, could have a reproductive benefit among straight women.
To my knowledge, however, there is as yet no
evidence for a reciprocal influence, whereby the male relatives of female homosexuals
have a higher reproductive fitness than do male relatives of heterosexual
women. And perhaps there never will be, given the accumulating evidence that
female homosexuality and male homosexuality may be genetically underwritten in
different ways.
A nonadaptive byproduct. Homosexual behavior might be
neither adaptive nor maladaptive, but simply nonadaptive. That is, it might not
have been selected for but persists instead as a byproduct of traits that
presumably have been directly favored, such as yearning to form a pair bond,
seeking emotional or physical gratification, etc. As to why such an inclination
would exist at all—why human connections are perceived as pleasurable—the
answer may well be that historically (and prehistorically), it has often been
in the context of a continuing pair-bond that individuals were most likely to
reproduce successfully.
There are lots of other hypotheses for the
evolution of homosexuality, although they are not the "infinite
cornucopia" that Leszek Kolakowski postulated could be argued for any
given position. At this point, we know enough to know that we have a real
mystery: Homosexuality does have biological roots, and the question is how the
biological mechanism developed over evolutionary time.
Another question (also yet unanswered) is why
should we bother to find out.
There is a chilling moment at the end of Ray
Bradbury's The Martian
Chronicles,when a human family, having escaped to Mars to avoid
impending nuclear war, looks eagerly into the "canals" of their new
planetary home, expecting to see Martians. They do: their own reflections.
It wasn't terribly long ago that reputable
astronomers entertained the notion that there really were canals on Mars. From
our current vantage, that is clearly fantasy. And yet, in important ways, we
are still strangers to ourselves, often surprised when we glimpse our own
images. Like Bradbury's fictional family, we, too, could come to see humanity,
reflected in all its wonderful diversity, and know ourselves at last for
precisely what we are, if we simply looked hard enough.
Unlike the United States military, with its
defunct "don't ask, don't tell" policy, many reputable investigators
are therefore asking ... not who is
homosexual, but why are
there homosexuals. We can be confident that eventually, nature
will tell.
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