Evolutionary psychology assumes that everything about human psychology must have once been a successful adaptation millions of years ago, increasing the likelihood of an individual reproducing their selfish genes. This seems plausible, if scientifically unprovable, in the cases of lust or hunger or even in more complex matters, such as humour and malice. But for a great many other human characteristics it is harder to see.

I have sometimes considered creating a board game called Evolution, in which the task is to explain how being pug ugly or obnoxious or mentally handicapped (or going to see Richard Curtis films post-Four Weddings and a Funeral) increases the chance of reproduction and why they have survived. My favourite is homosexuality.

While having sex with your own gender may lead to all sorts of jolly results, one thing it cannot lead to is babies. Surely, oh surely, even the most fanatical evolutionist must baulk at the idea that being gay or lesbian has been selected as part of our genotype because it increases the likelihood of a gay person's genes being reproduced?

Well, they have risen to the task: it has been argued that because a gay man shares half his genes with siblings, by not having children himself he increases their survival rate by devoting his resources to his nephews and nieces. His genes prevail through them.

This is pure speculation and a politically correct fig leaf for a theory that nearly always ends up supporting right-wing politics (that the poor are poorer, stupider and madder than the rich because of their genes is a corollary of evolutionary reasoning that is rarely mentioned). Where is the evidence that Fred Flintstone's gay brother fought off sabre-toothed tigers to protect Fred's offspring, or that when he died he left his spear to them? Where are the studies testing if people with gay or lesbian aunts and uncles thrive over time and reproduce their genes more than people without gay relatives?

I do not know why so many antireproductive traits have endured in humans. As an explanation of animal evolution, I find Darwin's theory hard to fault, but when it is extended to every aspect of human psychology it begins to creak badly.

Evolutionists have a habit of moving from animals to humans - 'because this or that is true of monkeys, it must be true of us'. But I believe humans are a different logical type because of our possession of self-consciousness, resulting from language. This makes us almost certainly the only entities capable of true volition and the first species to have become masters of the environments to which we are adapting.

It also makes us the first to pose a serious threat to its survival, through ecological or nuclear catastrophe. Never mind. Just before the last evolutionist expires because all life on earth is about to be eradicated and our genes completely destroyed, he will be advancing a theory for how this eventuality reflects events in the primordial swamp that enable us to better reproduce our selfish genes.

· oliver.james@observer.co.uk. Oliver James is unable to enter into any personal correspondence