EVOLUTIONARY biologists have always had a problem with homosexuality. If it
has any genetic basis, and if homosexuals tend to have fewer children than
heterosexuals, then a full-blooded Darwinian view would suggest that the genes
involved should have disappeared long ago.
That there is at least some genetic basis to homosexuality is suggested by
the work of Dean Hamer, who made headlines in 1993 with his discovery of the
鈥済ay gene鈥�. Hamer, of course, never claimed that genes 鈥渃aused鈥� homosexuality,
just that some genes made homosexuality more likely to occur during the
interaction between genes and environment that creates any behaviour. But that
still left evolutionary biologists with the problem of explaining why those
genes had not vanished.
Now the problem has grown more difficult again with the publication of Bruce
Bagemihl鈥檚 book Biological Exuberance: Animal Homosexuality and Natural
Diversity. The book leaves little doubt that homosexuality is found
everywhere you look in the higher animal kingdom (see p32, 鈥淨ueer Creatures鈥�). Indeed, the more you
look, the greater the diversity of sexual relationships that appear among
mammals and birds. To their shame, animal researchers have been rather slow to
report observations of the true range of sexual behaviour. But it all adds to
the challenge of explaining the evolution of homosexuality.
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One common view, put forward by Hamer and many others, is that 鈥済ay genes鈥�
survive because they confer a significant evolutionary advantage that outweighs
any reduction in the number of children born to homosexuals. It might be, for
example, that a gene that is linked to homosexuality in men has a different
effect when it is inherited by women and actually boosts the birth rate. Or it
might be that many men who carry the 鈥済ay gene鈥� are not homosexual but are more
successful as heterosexuals, fathering more children. Depending on which
theorist you fancy, that could be because heterosexual men carrying the 鈥済ay
gene鈥� have a greater interest in sex, or because they are more sensitive and
creative and so more attractive to women.
Alternatively, you can take a step further and argue for a kin or
group-selection view of the diversity of sexual behaviour. If, for example,
genes associated with homosexual behaviour are also linked to altruism, then
animals that don鈥檛 themselves reproduce may increase the reproductive success of
relations who share some of their genetic makeup. So 鈥済ay genes鈥� could confer an
advantage to the group. The existence of 鈥渉elper birds鈥� that assist in rearing
kin but do not reproduce has long been explained by just such theories.
At this point, there is a serious risk that we are just plain making up 鈥淛ust
So鈥� stories. It is both a weakness and a strength of evolutionary theories that
they can explain almost anything: testing the theories is a lot more difficult.
For animal populations, years of observations must be made and the real
relationships between kin worked out by DNA tests. The small band of field
researchers patiently collecting such hard evidence deserves applause.
Information on the behaviour of humans is perhaps even harder to come by.
Indeed, with such little real information available, it is possible that
homosexuality in humans is not the 鈥渞eal鈥� biological phenomenon it appears. A
general assumption is that homosexuals have fewer children than heterosexuals.
Hamer鈥檚 own survey showed that this currently appears to be the case, at least
in the US. But other surveys show that this was much less true twenty years ago.
Travel further back in time and who knows whether homosexuality necessarily
meant fewer children.
To put it another way, the notion of homosexuality is itself a rapidly
changing social construct. The idea of who is homosexual and how homosexuals
normally behave may have changed over the past hundreds and thousands of years.
杏吧原创s studying homosexuality or seeking 鈥済ay genes鈥� thus risk studying a
shifting social idea rather than a fixed biological phenomenon.
Bagemihl鈥檚 book might perhaps reinforce this view by charting the enormous
diversity of sexual behaviour in the animal world. In the human world, the
desire to categorise can both define and create social behaviour.
Evolutionary biologists must tread a careful path鈥攁nd keep an eye out
for quicksand.