Homosexuality: The Evolutionary Paradox

As the astute have noticed, I tend to intersperse a sensible post amongst the insanity. This is such an occasion..........   

Unfortunately, this post is a little long as it is not possible to make something complex, simple, just comprehensible.  

I confess I’m fan of evolutionary theory; tis an elegant theory which explains so much in biology. In fact, I would argue that modern biological thought would be meaningless without a contemplation of the evolutionary process. Dissenters are quick to point out the ‘hard questions’ which need to be addressed. There is nothing wrong with that. Any successful theory should be able to provide answers to the topic it relates to, otherwise it requires modification or should be discarded for a theory best able to explain this illusion we call reality.

A common question I'm asked is: How is homosexuality maintained in the population when evolutionary pressure would be expected to remove the trait from the gene pool? The idea being that in comparison to heterosexuals, homosexuals would leave behind less offspring. Therefore, over the generations homosexuality would be expected to die out. As this has not occurred then surely, evolution through natural selection, is not occurring and, therefore, the theory is in dire jeopardy. Those who oppose evolution, usually on the basis of religious dogma, are quick to point out this so called ‘insurmountable problem’ for evolution. This is a form of specious reasoning called: ‘reasoning from personal incredulity’. Just because you can't conceive of an explanation does not necessarily mean an explanation is unavailable.     

Before launching into the topic in earnest, I would just like to say a few words about what this post is not about. I’ll not be discussing any other aspect of homosexuality. Homosexuality will not be considered within the framework of morality, religious or otherwise, or in any other context not relating to cold, hard, biological science. These other aspects are worthy of discussion and debate, but they hold no interest for me here.

First off, before deciding whether evolution is on the ‘ropes’, it is a good idea to establish if there is a problem to answer. The problem would only exist if it could be demonstrated that homosexuality is in some way determined by genetics. It is conceivable that homosexuality is wholly determined by the environment. When I say environment, in this context, I mean the foetal environment. Therefore, for instance, a spike in a maternal hormone during a critical stage in the development of the embryonic brain may cause changes which result in sexual attraction to the same sex. I mention this as a hypothesis, as far as I am aware there is no evidence that this actually occurs, in humans at least. I propose it as a hypothetical, but plausible scenario. If homosexuality could be shown to be caused by environmental conditioning, without genetic input, then evolution does not have a case to answer.

The best way to see if a complex behavioural trait has a genetic component is to look at identical twins. Identical twins are genetically identical and therefore, ideal for looking at possible genetic influences. To summarise: The data suggest that if one identical twin is homosexual then the other twin has a higher chance of being homosexual than would be expected in comparison to the general population. This is strong evidence for a genetic component in at least ‘certain forms’ of homosexuality. Of course, it is probably naive to expect a single ‘homosexual gene’ to be the cause. It is conceivable that a number of genes are involved and this does not rule out the possibility of an environmental influence. In other words, the real life situation is likely to be biologically complex.   

Several models have been proposed to explain how the trait of homosexuality could be maintained in a population. I’ll consider only one explanation. This is probably the simplest model to contemplate. I mention it as an illustration. Frankly, I do not think homosexuality is determined in this manner, but it will illuminate the general principle I’m trying to convey.

Overdominance

The overdominance model requires just a single gene to determine homosexuality. In this scenario, parents would carry a copy of the homosexual gene but also a copy of a corresponding heterosexual gene. It is to be remembered that genetic traits have two genes- each one inherited from each parent. Because the ‘heterosexual’ gene is dominant it overrides the effect of the ‘homosexual’ gene in the parents, thus both parents are heterosexual. For our purposes, I’ll use the notation: Homosexual gene= HOM, and the Heterosexual gene= HET. Therefore, both parents can be represented as: HET/HOM. In the offspring, there are three genetic possibilities:

HET/HET

HET/HOM

HOM/HOM     

Of these three possibilities, HOM/HOM would be the only case which would confer homosexuality simply because there is no dominant HET gene. The HOM/HOM possibility would be expected to occur, in theory, in 25% of the offspring.  

You may well ask how the HOM gene could be maintained as recipients of HOM/HOM would be expected to have reduced reproductive fitness and hence the HOM gene would eventually be eliminated from the population. The gene could only be maintained if the HET/HOM genetic configuration conferred some form of selective benefit. Remember, individuals with this genetic configuration would be heterosexual but would be carriers of the trait for homosexuality. Let us hypothesise that HET/HOM individuals, on average, tend to have more children than all other combinations. It seems obvious that individuals with HOM/HOM would have fewer children. But why should HET/HOM confer an advantage over HET/HET? Here we can let our imaginations run riot. You may contend that this model seems a little strained however, we do know that an analogous genetic situation occurs. 

Sickle cell anaemia is a disease of Africa, Asia and the Mediterranean lands. It is a genetic abnormality which causes red blood cells to form an abnormal shape. Individuals with the condition have a reduced capacity to carry oxygen and the abnormal red blood cells accumulate in blood vessels causing blockage, pain and organ damage. Individuals with sickle cell anaemia have severe medical problems and have a lowered life expectancy. Their reproductive fitness is reduced in comparison to individuals without the disease. Like the scenario previously outlined, two disease genes are required which are inherited from carrier parents. Both parents carry the trait but are not afflicted with the condition. Interestingly, the bad gene occurs at a relatively high frequency in these populations. As it confers disease when inherited in a double dose it would be expected to be rapidly eliminated from the population. This is particularly important when we consider that effective treatments to alleviate symptoms have only become available in relatively recent times. Therefore, for instance, selection against the trait would have been particularly strong a 100 years ago. This has not happened and suggests that sickle cell anaemia is being maintained in populations by positive selection. It turns out that carriers of the condition are protected from malaria. Malaria is endemic in the regions where sickle cell trait occurs. It is estimated that one million people die each year from malaria. Thus having a degree of protection from malaria is an advantage and increases the reproductive fitness of those individuals who carry the sickle cell trait over those who do not. In this manner, a disease gene is maintained in the population.  

As I mentioned previously, there are other models with regard to homosexuality which attempt to explain the 'evolutionary paradox'. They rely on a multi-gene approach and are subtle and invariably more complex than the simple model outlined above. Suffice it to say, biologists are not stumped for hypothesises and explanations. In fact, this can be extended to other areas of biology which have been seen, usually by none biologists with an agenda, as difficult for evolutionary theory to explain. Again, theories abound and in many cases solid evidence is readily available.

Evolution is not a problem for biology, but has remained for the last 150 years, its ultimate solution.