同性恋进化之谜

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 choiceto 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.

But since gay and lesbian people have fewer children than straight people, a problem arises.

"This is a paradox from an evolutionary perspective," says Paul Vasey from the University of Lethbridge in Canada. "How can a trait like male homosexuality, which has a genetic component, persist over evolutionary time if the individuals that carry the genes associated with that trait are not reproducing?"

Scientists don't know the answer to this Darwinian puzzle, but there are several theories. It's possible that different mechanisms may be at work in different people. Most of the theories relate to research on male homosexuality. The evolution of lesbianism is relatively understudied - it may work in a similar way or be completely different.

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The genes that code for homosexuality do other things too

The allele - or group of genes - that sometimes codes for homosexual orientation may at other times have a strong reproductive benefit. This would compensate for gay people's lack of reproduction and ensure the continuation of the trait, as non-gay carriers of the gene pass it down.

There are two or more ways this might happen. One possibility is that the allele confers a psychological trait that makes straight men more attractive to women, or straight women more attractive to men. "We know that women tend to like more feminine behavioural features and facial features in their men, and that might be associated with things like good parenting skills or greater empathy," says Qazi Rahman, co-author of Born Gay; The Psychobiology of Sex Orientation. Therefore, the theory goes, a low "dose" of these alleles enhances the carrier's chances of reproductive success. Every now and then a family member receives a larger dose that affects his or her sexual orientation, but the allele still has an overall reproductive advantage.

Another way a "gay allele" might be able to compensate for a reproductive deficit is by having the converse effect in the opposite sex. For example, an allele which makes the bearer attracted to men has an obvious reproductive advantage to women. If it appears in a man's genetic code it will code for same-sex attraction, but so long as this happens rarely the allele still has a net evolutionary benefit.

There is some evidence for this second theory. Andrea Camperio-Ciani, at the University of Padova in Italy, found that maternal female relatives of gay men have more children than maternal female relatives of straight men. The implication is that there is an unknown mechanism in the X chromosome of men's genetic code which helps women in the family have more babies, but can lead to homosexuality in men. These results haven't been replicated in some ethnic groups - but that doesn't mean they are wrong with regards to the Italian population in Camperio-Ciani's study.

Gay people were 'helpers in the nest'

Some researchers believe that to understand the evolution of gay people, we need to look at how they fit into the wider culture.

Paul Vasey's research in Samoa has focused on a theory called kin selection or the "helper in the nest" hypothesis. The idea is that gay people compensate for their lack of children by promoting the reproductive fitness of brothers or sisters, contributing money or performing other uncle-like activities such as babysitting or tutoring. Some of the gay person's genetic code is shared with nieces and nephews and so, the theory goes, the genes which code for sexual orientation still get passed down.

Sceptics have pointed out that since on average people share just 25% of their genetic code with these relatives, they would need to compensate for every child they don't have themselves with two nieces or nephews that wouldn't otherwise have existed. Vasey hasn't yet measured just how much having a homosexual orientation boosts siblings' reproduction rate, but he has established that in Samoa "gay" men spend more time on uncle-like activities than "straight" men.

"No-one was more surprised than me," says Vasey about his findings. His lab had previously shown that gay men in Japan were no more attentive or generous towards their nieces and nephews than straight, childless men and women. The same result has been found in the UK, US and Canada.

Vasey believes that his Samoan result was different because the men he studied there were different. He studied the fa'afafine, who identify as a third gender, dressing as women and having sex with men who regard themselves as "straight". They are a transgender group who do not like to be called "gay" or "homosexual".

Vasey speculates that part of the reason the fa'afafine are more attentive to their nephews and nieces is their acceptance in Samoan culture compared to gay men in the West and Japan ("You can't help your kin if they've rejected you"). But he also believes that there is something about the fa'afafine way of life that means they are more likely to be nurturing towards nieces and nephews, and speculates that he would find similar results in other "third gender" groups around the world.

If this is true, then the helper in the nest theory may partly explain how a genetic trait for same-sex attraction hasn't been selected away. That hypothesis has led Vasey to speculate that the gay men who identify as men and have masculine traits - that is to say, most gay men in the West - are descended from men who had a cross-gendered sexuality.

Gay people do have children

In the US, around 37% of lesbian, gay, bisexual and transsexual people have a child, about 60% of which are biological. According to the Williams Institute, gay couples that have children have an average of two.

These figures may not be high enough to sustain genetic traits specific to this group, but the evolutionary biologist Jeremy Yoder points out in a blog postthat for much of modern history gay people haven't been living openly gay lives. Compelled by society to enter marriages and have children, their reproduction rates may have been higher than they are now.

How many gay people have children also depends on how you define being "gay". Many of the "straight" men who have sex with fa'afafine in Samoa go on to get married and have children.

"The category of same-sex sexuality becomes very diffuse when you take a multicultural perspective," says Joan Roughgarden, an evolutionary biologist at the University of Hawaii. "If you go to India, you'll find that if someone says they are 'gay' or 'homosexual' then that immediately identifies them as Western. But that doesn't mean there's no homosexuality there."

Similarly in the West, there is evidence that many people go through a phase of homosexual activity. In the 1940s, US sex researcher Alfred Kinsey found that while just 4% of white men were exclusively gay after adolescence, 10% had a three-year period of gay activity and 37% had gay sex at some point in their lives.

A national survey of sexual attitudes in the UK last yearcame up with lower figures. Some 16% of women said they had had a sexual experience with another woman (8% had genital contact), and 7% of men said they had had a sexual experience with a man (with 5% having genital contact).

But most scientists researching gay evolution are interested in an ongoing, internal pattern of desire rather than whether people identify as gay or straight or how often people have gay sex. "Sexual identity and sexual behaviours are not good measures of sexual orientation," says Paul Vasey. "Sexual feelings are."

Qazi Rahman says that alleles coding for same sex attraction only explain some of the variety in human sexuality. Other, naturally varying biological factors come into play, with about one in seven gay men, he says, owing their sexuality to the "big brother effect".

This has nothing to do with George Orwell, but describes the observation that boys with older brothers are significantly more likely to become gay - with every older brother the chance of homosexuality increases by about a third. No-one knows why this is, but one theory is that with each male pregnancy, a woman's body forms an immune reaction to proteins that have a role in the development of the male brain. Since this only comes into play after several siblings have been born - most of whom are heterosexual and go on to have children - this pre-natal quirk hasn't been selected away by evolution.

Exposure to unusual levels of hormone before birth can also affect sexuality. For example, female foetuses exposed to higher levels of testosterone before birth show higher rates of lesbianism later on. Studies show that "butch" lesbian women and men have a smaller difference in length between their index and ring fingers - a marker of pre-natal exposure to testosterone. In "femme" lesbians the difference has been found to be less marked.

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.

William Rice, from the University of California Santa Barbara, says that it may be possible to explain this by looking not at our genetic code but at the way it is processed. Rice and his colleagues refer to the emerging field of epigenetics, which studies the "epimarks" that decide which parts of our DNA get switched on or off. Epimarks get passed on to children, but only sometimes. Rice believes that female foetuses employ an epimark that makes them less sensitive to testosterone. Usually it's not inherited, but occasionally it is, leading to same-sex preference in boys.

Dr William Byne, editor-in-chief of the journal LGBT Health, believes sexuality may well be inborn, but thinks it could be more complicated than some scientists believe. He notes that the heritability of homosexuality is similar to that for divorce, but "social science researchers have not… searched for 'divorce genes'. Instead they have focused on heritable personality and temperamental traits that might influence the likelihood of divorce."

For Qazi Rahman, it's the media that oversimplifies genetic theories of sexuality, with their reports of the discovery of "the gay gene". He believes that sexuality involves tens or perhaps hundreds of alleles that will probably take decades to uncover. And even if heterosexual sex is more advantageous in evolutionary terms than gay sex, it's not only gay people whose sexuality is determined by their genes, he says, but straight people too.

在过去的20年里，许多关于同性恋的生物学起源的科学文献相继发表——就在上周又一篇相关文章（同性恋存在遗传基因）得以发表，并成为科学正统。但是这是如何与达尔文的进化论相符的呢？

马克里莫和赖安·路易斯的畅销歌曲《Same Love》（同样的爱）已经成为美国支持同性婚姻运动的非正式颂歌，这首歌表达了许多同性恋者对自己性取向的感受。

歌曲讽刺了那些“认为同性恋只是一种选择，可以通过治疗和宗教被治愈——可以人为地改变这种倾向”的人。少数同志对此不以为然，仍然坚持性取向是一种社会建构，他们有意识地选择自己的同性伴侣，并为此感到骄傲。

但是科学观点是站在马克里莫这边。自上世纪90年代早期，研究者就已指出，同性恋者的兄弟及母系亲属也是同性恋的情况更为普遍，这可能受某种遗传因素影响。尽管无法确切证实，但诸如异性恋者和同性恋者大脑的物理区别，以及动物界令人眼花缭乱的同性恋行为的研究却依旧与此有关。

但是，我们知道，男女同性恋者的后代都比异性恋者要少，这里就出现了一个问题。

“从进化论角度来看，这有些矛盾，”加拿大莱斯布里奇大学教授的保罗·维西（Paul Vasey）说。“如果说携带这种遗传基因的个人不会生育后代，那么男同性恋这种包含遗传组分的性状在进化过程中是如何得以延续呢？”

对于这一进化论之谜，科学家尚不能明确解释，但已建立了一些理论。不同的人群可能受到不同机制的影响。大部分理论与男同性恋的研究有关。而女同性恋的进化相对来说研究较少——这二者之间起作用的机制可能相似，也可能截然不同。

控制同性恋取向的基因还有其他作用

控制同性恋倾向基因遗传形态的等位基因——或者基因组——在其他时候可能也会产生较强繁殖效益的优点。这可以弥补同性恋人士较少生育后代的劣势，携带这种基因的非同性恋者也会将其遗传给后代，这就保证了这种性状的延续。

这种基因传递有两种或者更多方式。其中一种可能性是，这种等位基因能够产生一种心理特质，能够使男异性恋者更吸引女性，或者是女异性恋者更吸引男性。《生而为同：性取向心理生物学》一书的合著者卡齐·拉赫曼（Qazi Rahman）这样说道，“我们知道，女性越来越喜欢拥有更多女性行为特征和面部特征的男性，这样的人一般更具有良好的家庭教养和更强的同情心。”因此，这种理论认为，即使只携带少量这种等位基因也会增加携带者繁殖成功的机会。有时某位家庭成员会遗传到的大量该种等位基因，因而影响了他/她的性取向，但是总体而言这种等位基因仍具有繁殖优势。

“同性恋等位基因”弥补繁殖劣势的另一种方式是在异性中起相反作用。比如说，能够使携带者更吸引男性的等位基因对女性而言就会明显成为繁殖优势。如果这种等位基因存在于男性的基因序列里，那么就会产生同性吸引，但是只要这种情况极少发生，那么等位基因依旧具有纯粹的进化优势。

有一些证据可以证明第二种理论。意大利帕多瓦大学的安德里亚（Andrea Camperio-Ciani）发现，男同性恋者的母系女性亲属比异性恋男性的母系女性亲属有更多后代。这暗示了男性基因序列中的X染色体上存在某种未知机制，能帮助家族中的女性孕育更多后代，却也会导致男性的同性恋的倾向。虽然并未对其他种族进行同样的实验——但这并不意味着安德里亚对意大利人种进行的研究就是错误的。

一些研究者相信，要想理解同性恋人群的进化，我们需要研究他们是如何融入更广泛的文化中的。

保罗·维西在萨摩亚群岛上的研究主要集中在一个叫做“亲缘选择”的理论，这一理论又称“巢中帮手”假说。该理论认为，虽然同性恋者会缺少子嗣，但是他们会通过提高兄弟姐妹的生存繁殖的能力，贡献金钱或者是帮助其照看和指导后代的行为来弥补。一些男同的基因序列可能会与其侄女外甥们相同，因此，这个理论认为管控性取向的基因依旧会遗传下去。

怀疑论者指出，平均来说人类仅有25%的基因序列与这些侄女外甥们相同，（这些亲属更高的生存率可以弥补同性恋者自己不生育的损失）每少生育一个自己的后代，同性恋者都将需要抚育两个本来可能不会出生的甥侄才能弥补。维西还没有测算出同性取向者促进其兄弟姐妹繁殖的准确比率，但已经确定的是，萨摩亚的“同志”们相比“直男”来说会花费更多的时间照料自己的侄女外甥们。

“没有人会比我更惊讶了，”提起他的研究结果，维西说道。他之前的研究显示，日本的男同性恋者与异性恋无子女的男女相比，并没有对他们的甥侄们表现出更多关注和慷慨。而在英国、美国和加拿大的研究结果也不尽相同。

维西认为，之所以萨摩亚的研究结果不同是由于研究对象也与众不同。他研究的fa'afafine被定义为“第三性者”，他们穿戴成女性，并与自认是“直男”的男性发生性关系。他们是一个跨性别的种群，不喜欢被人称为“基佬”或是“同性恋者”。

维西推测，fa'afafine对他们的侄女外甥们更为关注的部分原因在于他们在萨摩亚文化中的接纳程度远高于西方和日本的同性恋者（“你不可能在亲属排斥你的情况下帮助他们”）。但是，他也认为fa'afafine生活方式里的某些因素使他们可能更倾向抚育自己的侄女外甥们。维西推测，世界上其他的“第三性”人群应该也是相似。

如果这是正确的，那么“巢中帮手”理论可能在一定程度上解释了同性相吸这一遗传性征为什么没有被自然淘汰。这一假说促使维西产生另一个推测：被视为男性且具有男性特征的男同性恋者——大部分的西方男同性恋者即是如此——是从跨性别性征者进化而来。

同性恋者也有孩子

在美国，大约37%的女同、男同、双性恋和变性者有孩子，其中约60%是亲生子女。根据威廉姆斯学院的研究，每对有孩子的同志伴侣平均会有两个孩子。

这些数据可能不高，还不足以维持这一群体的基因特质，但是进化生物学家杰里米·约德（Jeremy Yoder）在一篇博文中指出，近现代许多同性恋者并没有公开地过着同性恋生活。迫于社会结婚生子的压力，他们的生育率可能比现在的同恋者要高。

有多少同性恋者会有孩子呢，这取决于你怎样定义“同性恋”。在萨摩亚群岛，许多和fa'afafine发生过关系的“直男”仍会结婚生子。

“如果从多元文化观的视角来看，同性恋的范畴就会变得十分宽泛”，夏威夷大学进化生物学家Joan Roughgarden说道，“如果你到印度去就会发现，如果有人自称是‘同性恋’，那你就可以直接认定他们是西方人。但这并不意味着印度没有同性恋者。”

西方也是一样，有证据表明，很多人都有过一个阶段的同性恋行为。上世纪40年代，美国性学教授阿尔弗雷德·金赛（Alfred Kinsey）发现，尽管仅有4%的白人男性青春期后就确定自己是纯同，其他有10%的男性有过三年的同志行为阶段，37%的男性曾与同性发生过性关系。

去年在英国进行的一项全国性观念调查得到的数据相对较小。大约16%的女性表示她们曾与其他女性有过性体验（其中8%有过生殖器接触），7%的男性表示他们曾与其他男性有过性体验（其中5%有过生殖器接触）。

但是大多数研究同性恋进化的科学家感兴趣的是一种持续的内在性欲模式，而不是人们自认为是同性恋还是异性恋，或者发生同性性行为的频率。“性别身份和性行为并不能准确区分性取向，”保罗·维西表示，“性欲感觉才是。”

卡齐·拉赫曼表示，控制同性吸引基因遗传形态的等位基因仅能解释一部分性取向行为。其他自然变化的生物学因素也在其中发挥作用，卡齐认为，大约有七分之一的男同的性取向受“兄长效应”的影响。

这与乔治·奥威尔《1984》书中的“老大哥”没什么关系。据观察，有兄长的男孩更有可能成为男同性恋——并且每多一个兄长，其成为同性恋的机会就会增加约三分之一。没有人知道其中的原因，有一种理论是，每次怀上男孩，母体就会产生某种蛋白免疫反应，而这种蛋白质恰好影响男性大脑发育。由于这一免疫反应只会在生产多个子女后才会发生——这些兄弟姐妹大部分会是异性恋，并会继续生育后代——这种孕期特异性才没有被演变淘汰。

出生前受到异常水平的荷尔蒙刺激也会影响人的性取向。例如，在出生前受到较高水平睾丸素刺激的女性胎儿以后成为女同性恋的比率更高。研究显示，充当“丈夫”角色的女同性恋者和男性一样，食指与无名指的长度有略微差异——这表明了胎儿出生前受到睾丸素的刺激。作为“妻子”角色的女同性恋者的这种差异却不太显著。

另一种兄弟——同卵双胞胎——也带来了一个棘手的问题。研究发现，如果同卵双胞胎中的一个是同性恋，那么大约有20%的概率另一个人也拥有同样的性取向。虽然这比随机性的可能性要高，但对于两个拥有相同基因序列的人来说，还是比预期要低。

来自加州大学圣芭芭拉分校的威廉·赖斯（William Rice）表示，通过研究基因序列的处理过程而不是基因序列本身，我们有可能对此作出解释。赖斯和他的同事参考了新兴的表观遗传学，该学说研究“附因子”，附因子用于调控基因的具体实现。附因子只是偶尔能够遗传给后代。赖斯认为，女性胎儿受某种附因子的影响从而减少了对睾丸素的敏感度。这种附因子通常不会遗传给下一代，但是如果偶然出现遗传，就会导致男孩的同性性倾向。

《同志健康》杂志主编威廉·拜恩（William Byne）博士称性取向很可能是先天的，但他也认为这可能比许多科学家以为得要复杂得多。他注意到，同性恋的遗传性与离婚者类似，但是“社会科学研究者们却没有……对‘离婚基因’进行过任何研究。相反他们专注于研究遗传的性格和喜怒无常的特质这些可能会导致离婚的方面。”

对卡齐·拉赫曼来说，媒体总是报导发现了“同性恋基因”，这把性取向的遗传理论过于简单化了。他认为，性取向涉及几十甚至上百对等位基因，这些基因可能需要数十年的时间才能解密。此外，尽管异性恋在进化过程中比同性恋更具优势，但是卡齐说，并不是只有同性恋者的性取向才是由基因决定的，异性恋者也同样如此。