[zkhalique]

Perhaps those who wanted more than that out of life had less genetic fitness than those who wanted exactly that out of life, so most people want that out of life.

On the other hand, I'm not sure how to explain the presence of homosexuality in a population when the theory of evolution predicts that any such trait (which leads to much lower genetic fitness) would have disappeared a long time ago. How does it stay at this level from generation to generation?

[sevenless]

The basic flaw in this is that there's no straightforward heritability for homosexuality, so regardless of the evolutionary pressures, it's not something that could be obviously 'selected against'.

Homosexual men throughout history did reproduce, though. In many ancient societies, norms were such that you had kids (if you survived long enough), and what you did with other men on the side was more or less your business. The Romans had a lot of gay sex, but didn't have a concept of a gay man, rather an effeminate man (who liked to bottom). So homosexuality is not really a puzzle that begs for a solution. The reality is, that the reasons we have sex do not have much to do with reproduction. Western society is still rather prudish about acknowledging this.

It's worth pointing out also, men generally pass their genes on a whole lot less than women do. Genetic evidence shows women were twice as likely as men to reproduce. So it's not like being heterosexual meant you'd have kids, being homosexual meant you wouldn't. I guess monogamy would reduce the disparity between men and women here.

http://www.eurekalert.org/pub_releases/2004-09/uoa-ges091604...

[bhaak]

I think I recall some articles about studies that showed that homosexual people (that don't reproduce themselves) are useful as caretakers for close relatives as they want to ensure the survival of similar genes.

Your question is similar to the question why old humans don't just die off when they can't reproduce anymore. This is most obvious with women when they enter menopause but also the quality of sperm declines with men with age.

They are either useful to some extent to reproduction or improving the survival rate of the offspring, or at least the evolutionary pressure is too small to eradicate these traits.

But as we have descended from group animals, I suspect the former.

[vidarh]

A gene survives and spreads if the gene helps increase the average fitness of the part of the population having the gene.

There is nothing about evolution that requires a gene to benefit any given individual. There are plenty of species where individuals are metaphorically "thrown under the bus" for the survival of genetically similar enough individuals to be worth it.

And for any given individual, a gene that confers high fitness on the population it exists in as a whole can still be disastrous in certain configurations.

An example of that would be the sickle-cell trait, which causes lots of deaths amongst those unlucky enough to get it from both parents, as it leaves them prone to sickle cell anaemia and related problems.

It survives in many regions such as West Africa because while receiving it from both parents cuts an average 20-40 years of your life expectancy in developed countries, "just" having the trait from one parent is a net positive because it makes it a lot less likely you will get malaria, which is a far bigger killer in countries with poor healthcare.

This is the convoluted way of saying that you can't look at the effect on individual survival - there can be convoluted mechanisms that makes a gene that makes your copies less likely to be propagated more fit on average.

[ta0967]

explain the presence of homosexuality in a population when the theory of evolution predicts that any such trait (which leads to much lower genetic fitness) would have disappeared a long time ago

one's homosexuality may improve fitness of one's family: if we're brothers and i'm homosexual, we won't be splitting resources between your and my children, and i'll be an extra "parent" looking after your kids. genetically speaking, that is still a (qualified) success for me.

also, genes don't give a damn about individual's reproduction, and local optima are rampant.

edit: what siblings have said.

[joeyo]

Re: homosexually, there are several possibilites. A plausible explanation is that it's linked with genes that confer fitness to siblings. As with most traits, however, it's likely to have both heritable as well as environmental antecedents.