J. G. Cox
Posts: 38
Joined: Dec. 2005
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All traits have both genetic and environmental components; this includes behavioral traits. Sometimes, one or the other seems much more important than the other. For instance, how many heads you have is almost entirely controlled by genes, though certain environmental factors (e.g., not too little food meaning you have none and are dead, or environmental toxins which interfere with development giving you 2) could still exert some influence. Almost all traits are also influenced by more than one gene locus. That is why it is usually incorrent to speak of 'a gene' for something. What is usually meant is that a particular allele of a given gene affects the probability that a given organism will have one value of a trait versus another. Usually, there are multiple genes for which different alleles may affect the same trait.
So, would the existence of certain alleles that increase the likelihood that their carriers are homosexual disprove evolution? First, there is the point that pretty much no one datum or result could disprove evolutionary theory at this point, but that is a topic for another thread. The basic objection would seem to be that such alleles should reduce fitness, and therefore be selected against. This is an interesting question, in fact, as that is part of why some researchers are investigating it. (Notice that they are doing actual research into an interesting evolutionary conundrum, not simply making assertions about its potential implications). Researchers search for alleles influencing homosexuality because it is extremely unlikely that this trait is entirely a consequence of environment.
Let's assume that alleles for homosexuality do in fact reduce fitness. In an evolutionary framework, that would make such alleles a sort of genetic disease. Notice that classifying such alleles as deleterious and 'disease-causing' requires accepting an evolutionary framework. However, our definitions of disease do not correspond to what natural selection might select against. For example, brain tissue is extremely metabolically expensive; in food-limited situations, having too much brain matter could easily lead to reduced fitness, or even early death. In that case, would we declare intelligence to be a disease? Another example; I don't want to have children, which greatly reduces my fitness; am I diseased? Thus, sociological attacks against homosexuality based on it being a 'disease' i.) require acceptance of evolution, and ii.) are guilty of the same logical fallacy as social Darwinism
Let's still assume that alleles which promote homosexuality impose a fitness cost. Why might such alleles persist? Evolutionary theory gives some indication of where to look. One mechanism which might maintain such alleles is persistent mutation of wild-type alleles into mutant type, in this case one promoting homosexuality. In instances where the mutational pathway from wild-type to a particular mutant allele is short and/or probable (e.g., a single point mutation), then selection and mutation can maintain the deleterious mutants at non-trivial frequencies in a population. Another reason why such alleles might persist is simply that natural selection hasn't eliminated them yet. Remember, natural selection is simply an adjustment of the probability distribution governing the fates of different alleles; it is not wholly deterministic. Natural selection is also a continuing process, and just because a given alleles hasn't yet achieved fixation or has not been wholly eliminated does not mean that natural selection is not acting on it.
However, we cannot simply assume that alleles which promote homosexuality do in fact reduce fitness. This is where some of the research that others have pointed to in this thread comes in. Again, evolutionary theory tells us where we should be looking for answers. One possibility is that such alleles might increase inclusive fitness; this is also known as kin selection. Thus, having alleles which promote homosexuality might decrease one's own reproductive output, but might increase that of one's relatives. For instance, a homosexual male might contribute more to raising the offpsring of his sisters and mother than a heterosexual male, or might be less likely to be run off or killed by the dominant male in his social group (thus thus better able to help his relative reproduce). Unfortunately, this is very hard to test because i.) we cannot directly observe the ecological and social conditions under which our ancestors evolved and ii.) doing the appropriate experiments with humans would be considered highly unethical.
Another important point is to remember that natural selection acts on the average fitness derived from having a certain trait or allele. If having alleles which promote homosexuality on average improves fitness, then we would expect to see them in a population, at least at some frequency. This ties in to research which showed the increased reproduction of female relatives of gay men. If the increase in fitness for the females is equal to or greater than the decrease in fitness for the males carrying the alleles, then such alleles would spread in the population.
In summary, the presence of alleles which promote homosexuality would not constitue a 'problem for evolution' for two reasons. First, modern evolutionary theory predicts the presence of some deleterious alleles within a population. Second, we have no idea if such alleles are actually deleterious.
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