The evolutionary puzzle of homosexuality

[xchrom]

http://www.bbc.co.uk/news/magazine-26089486



In the last two decades, dozens of scientific papers have been published on the biological origins of homosexuality - another announcement was made last week. It's becoming scientific orthodoxy. But how does it fit with Darwin's theory of evolution?

Macklemore and Ryan Lewis's hit song Same Love, which has become an unofficial anthem of the pro-gay marriage campaign in the US, reflects how many gay people feel about their sexuality.

It mocks those who "think it's a decision, and you can be cured with some treatment and religion - man-made rewiring of a predisposition". A minority of gay people disagree, maintaining that sexuality is a social construct, and they have made a conscious, proud choice to take same-sex partners.

But scientific opinion is with Macklemore. Since the early 1990s, researchers have shown that homosexuality is more common in brothers and relatives on the same maternal line, and a genetic factor is taken to be the cause. Also relevant - although in no way proof - is research identifying physical differences in the brains of adult straight and gay people, and a dizzying array of homosexual behaviour in animals.

[jpak]

1. Evolution is not perfect and it is not goal oriented

Darwin is safe on this...

[pnwmom]

2. This surprised me:

"Brothers of a different kind - identical twins - also pose a tricky question. Research has found that if an identical twin is gay, there is about a 20% chance that the sibling will have the same sexual orientation. While that's a greater likelihood than random, it's lower than you might expect for two people with the same genetic code."

I had thought the likelihood of an identical twin being gay was more like 50%.

[DetlefK]

3. I read a while ago, that male homosexuality is most likely a genetic side-effect.

The source is a gene of the mother that increases fertility. That gene also has the side-effect that male offspring may be modified in the womb to turn gay.

It makes sense from an evolutionary point of view: If 1 in 10 sons is gay and won't reproduce, but you have twice as many offspring, then you still end up on the plus-side.

(Female homosexuality also stems from a modification of the foetus' brain in the womb when it's subjected to too much of a particular hormone, but the trigger for the modification is unknown so far.)

[Adrahil]

4. It's probably an unusual expression of several genes....

...the link to siblings in larger families is a clue. We know that epigenetics (when genes are turned on and off) is especially active during gestation. We know that these epigenetic effects can shift in women with multiple pregnancies. I think it's most likely that sexual orientation is expressed through a combination of genes, and it's why we see a spectrum of sexuality instead of a simple duality. For example, people's hair and skin colors are not simply discrete values, but a spectrum. In fact, most things about humans are that way. I think sexuality is the same. Some people are MAINLY heterosexual. Some people are MAINLY homosexual. And a LOT of people are somewhere long that spectrum.

[LuvNewcastle]

5. That all sounds pretty complicated, looking at all these alleles and

levels of testosterone and estrogen. It's all kind of comical to me for some reason. I don't think they're ever going to find a 'gay gene' or anything like that, like the gene for eye color.

I think social factors play a role in being gay, too, factors that affect personality traits. I don't think sexuality is something you can completely separate between genetic and social influences. There's no line there when it comes to sexuality, in my opinion. If I'm right, then there are so many different factors governing sexuality that you might as well say it's random. I'm gay because that's the number I drew out of the hat. Holding someone's sexuality against them is like being mad at someone because of the number on their lottery ticket. It's just a fact of life to accept and move on.