Homosexual sapiens

25 January 2008

At first glance homosexuality is an evolutionary paradox, placing it firmly on biology’s ‘to explain list’. Until fairly recently most explanations of homosexuality adopted a social constructivist position largely rejecting the input of evolutionary biology. In fact, there is good evidence for homosexuality having a strong genetic component. Homosexuals have more homosexual siblings than heterosexuals and mono-zygotic twins show a greater agreement in sexuality than di-zygotic twins. Male homosexuality is particularly associated with the maternal inheritance and a certain region of the X chromosome has been highlighted as important. Homosexuality is, in part at least, heritable. This places it clearly under the scrutinising gaze of natural selection. Homosexuality earned its title as a paradox because it seemingly affects an individual’s reproductive output. But natural selection is much more subtle than this. Reproductive success depends not only on the individual’s own output, but also that of related individuals who share the same genes. Another subtlety is that genomes are not harmonious collectives, but have many levels of genetic conflict, such as between maternal and parental genes which may favour different behaviours. Yet another relates to the female choice and the role of sexual selection (think of the elaborate displays of peacocks), another to the role of balanced polymorphisms; having an intermediate behaviour may be better than either extreme. Evolution has so many different interweaving components that even the most unintuitive of nature’s repertoire can usually be explained and validated by experiments that come out of the predictions from such explanations. Homosexuality is no exception. There are numerous hypotheses to explain the origins and maintenance of homosexuality which rest on a different aspect of evolution. There are also many that integrate different ideas and place them in the context of an ancestral human population. For example Rahman and Wilson proposed that feminine behavioural traits became adaptive in males as they would mediate aggression, aiding same-sex affiliation and the maintenance of alliances. Such behaviour was then selected for by females as it would reduce infanticide and facilitate paternal care. This perhaps resulted in a period of ‘runaway sexual selection’ allowing the spread of genes promoting homosexual behaviour maintained by balanced polymorphism, in the same way as sickle-cell anaemia alleles are maintained in malaria regions. Similarly selection for masculinising alleles in females may have aided intra-female alliances with “powerful females”. This is just one of many suggested scenarios. Can evolutionary biology incorporate homosexual behaviour? There is a plethora of biological hypotheses to explain the origin of homosexuality which have a sound theoretical basis. Answering my friend’s question appears to be the easy bit, the hard part is deciding which explanation is most likely as most rely on “reverse engineering” and are difficult to test. But what are the moral implications of such explanation? Is there a biological reason to discriminate against homosexuals? Certainly not. Homosexuality has had a constant presence in our evolutionary history, as such it cannot be seen as maladaptive. If it was, natural selection would have weeded it out. It is even plausible, if not likely, that the selective pressures that have produced homosexuality have played a major role in human evolution!