June 2005

Homosexuality presents a tremendous challenge to Darwinism. Many animals, including humans, exhibit clear homosexual mating behaviors and associated traits. In some species, up to a few percent of the population is homosexual. While scientific data is weak compared to other studied behaviors, homosexuality is known to be widespread, persistent across species, and is not a cultural artifact. Therefore, homosexuality is most likely a genetically determined characteristic.

According to Darwin, genetically-linked behaviors die out if they do not confer a survival benefit. Which raises the question, how can homosexuality confers survival advantages on the individual, if it has no way to propagate without a way to procreate?

On possibility is that homosexuality might confer a mild genetic advantage if related group members are more likely to survive due to the assistance of a helpful "uncle". In this manner, part of the uncle's genetic code is indirectly preserved across generations. Another possibility is that homosexuality may arise as the unintended consequence of a common mutation. Or, it may emerge as side-effect from a different, environmentally fit characteristic which persists by conventional evolution between generations..

A mystery, indeed.

But what if homosexuality were a result of sperm competition? Sperm, of course, benefit most greatly if they can actually fertilize an egg, and pass their genetic code onto another generation. In many animals, the male (and very frequently, the female) are promiscuous- so sperm from more than one suitor vie simultaneously to fertilize an egg. But, if one sperm loses the battle of procreation, the next best strategy might be for the losing suitor's sperm to kill off the winner's zygote.

Now, sperm could try to directly kill the zygote, but natural selection would just as rapidly evolve a countermeasure for so profound a loss. On the other hand, if the competing sperm simply changed the egg's biochemistry, so the animal became a homosexual, then the gene would be deleted from the pool, but at a much later date. A gay time bomb, so to speak.

Recent investigations show sperm carry and deliver a number of active molecules beyond DNA. And, other investigators believe even small amounts of hormones in the environment can strongly affect embryonic development and sexual maturation. So this hypothesis- that the "losing" sperm retaliate by chemically modifying the development of the winning zygote- is plausible. One place to look for evidence is the rate of homosexuality in babies conceived by in-vitro fertilization- where sperm competition is absent. If the rate were different than expressed in the general public, further studies are warranted.

Over time, we are likely to discover the egg's development is crtically influenced by the local chemical environment, and the local chemical environment may be radically shaped by the actions of millions of second-place sperm.