同性戀進化之謎

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.在過去的20年裏，許多關於同性戀的生物學起源的科學文獻相繼發表——就在上週又一篇相關文章（同性戀存在遺傳基因）得以發表，併成爲科學正統。但是這是如何與達爾文的進化論相符的呢？

馬克裏莫和賴安·路易斯的暢銷歌曲《Same Love》（同樣的愛）已經成爲美國支持同性婚姻運動的非正式頌歌，這首歌表達了許多同性戀者對自己性取向的感受。

歌曲諷刺了那些“認爲同性戀只是一種選擇，可以通過治療和宗教被治癒——可以人爲地改變這種傾向”的人。少數同志對此不以爲然，仍然堅持性取向是一種社會建構，他們有意識地選擇自己的同性伴侶，併爲此感到驕傲。

但是科學觀點是站在馬克裏莫這邊。自上世紀90年代早期，研究者就已指出，同性戀者的兄弟及母系親屬也是同性戀的情況更爲普遍，這可能受某種遺傳因素影響。儘管無法確切證實，但諸如異性戀者和同性戀者大腦的物理區別，以及動物界令人眼花繚亂的同性戀行爲的研究卻依舊與此有關。

但是，我們知道，男女同性戀者的後代都比異性戀者要少，這裏就出現了一個問題。

“從進化論角度來看，這有些矛盾，”加拿大萊斯布里奇大學教授的保羅·維西（Paul Vasey）說。“如果說攜帶這種遺傳基因的個人不會生育後代，那麼男同性戀這種包含遺傳組分的性狀在進化過程中是如何得以延續呢？”

對於這一進化論之謎，科學家尚不能明確解釋，但已建立了一些理論。不同的人羣可能受到不同機制的影響。大部分理論與男同性戀的研究有關。而女同性戀的進化相對來說研究較少——這二者之間起作用的機制可能相似，也可能截然不同。

控制同性戀取向的基因還有其他作用

控制同性戀傾向基因遺傳形態的等位基因——或者基因組——在其他時候可能也會產生較強繁殖效益的優點。這可以彌補同性戀人士較少生育後代的劣勢，攜帶這種基因的非同性戀者也會將其遺傳給後代，這就保證了這種性狀的延續。

這種基因傳遞有兩種或者更多方式。其中一種可能性是，這種等位基因能夠產生一種心理特質，能夠使男異性戀者更吸引女性，或者是女異性戀者更吸引男性。《生而爲同：性取向心理生物學》一書的合著者卡齊·拉赫曼（Qazi Rahman）這樣說道，“我們知道，女性越來越喜歡擁有更多女性行爲特徵和麪部特徵的男性，這樣的人一般更具有良好的家庭教養和更強的同情心。”因此，這種理論認爲，即使只攜帶少量這種等位基因也會增加攜帶者繁殖成功的機會。有時某位家庭成員會遺傳到的大量該種等位基因，因而影響了他/她的性取向，但是總體而言這種等位基因仍具有繁殖優勢。

“同性戀等位基因”彌補繁殖劣勢的另一種方式是在異性中起相反作用。比如說，能夠使攜帶者更吸引男性的等位基因對女性而言就會明顯成爲繁殖優勢。如果這種等位基因存在於男性的基因序列裏，那麼就會產生同性吸引，但是隻要這種情況極少發生，那麼等位基因依舊具有純粹的進化優勢。

有一些證據可以證明第二種理論。意大利帕多瓦大學的安德里亞（Andrea Camperio-Ciani）發現，男同性戀者的母系女性親屬比異性戀男性的母系女性親屬有更多後代。這暗示了男性基因序列中的X染色體上存在某種未知機制，能幫助家族中的女性孕育更多後代，卻也會導致男性的同性戀的傾向。雖然並未對其他種族進行同樣的實驗——但這並不意味着安德里亞對意大利人種進行的研究就是錯誤的。

一些研究者相信，要想理解同性戀人羣的進化，我們需要研究他們是如何融入更廣泛的文化中的。

保羅·維西在薩摩亞羣島上的研究主要集中在一個叫做“親緣選擇”的理論，這一理論又稱“巢中幫手”假說。該理論認爲，雖然同性戀者會缺少子嗣，但是他們會通過提高兄弟姐妹的生存繁殖的能力，貢獻金錢或者是幫助其照看和指導後代的行爲來彌補。一些男同的基因序列可能會與其侄女外甥們相同，因此，這個理論認爲管控性取向的基因依舊會遺傳下去。

懷疑論者指出，平均來說人類僅有25%的基因序列與這些侄女外甥們相同，（這些親屬更高的生存率可以彌補同性戀者自己不生育的損失）每少生育一個自己的後代，同性戀者都將需要撫育兩個本來可能不會出生的甥侄才能彌補。維西還沒有測算出同性取向者促進其兄弟姐妹繁殖的準確比率，但已經確定的是，薩摩亞的“同志”們相比“直男”來說會花費更多的時間照料自己的侄女外甥們。

“沒有人會比我更驚訝了，”提起他的研究結果，維西說道。他之前的研究顯示，日本的男同性戀者與異性戀無子女的男女相比，並沒有對他們的甥侄們表現出更多關注和慷慨。而在英國、美國和加拿大的研究結果也不盡相同。

維西認爲，之所以薩摩亞的研究結果不同是由於研究對象也與衆不同。他研究的fa'afafine被定義爲“第三性者”，他們穿戴成女性，並與自認是“直男”的男性發生性關係。他們是一個跨性別的種羣，不喜歡被人稱爲“基佬”或是“同性戀者”。

維西推測，fa'afafine對他們的侄女外甥們更爲關注的部分原因在於他們在薩摩亞文化中的接納程度遠高於西方和日本的同性戀者（“你不可能在親屬排斥你的情況下幫助他們”）。但是，他也認爲fa'afafine生活方式裏的某些因素使他們可能更傾向撫育自己的侄女外甥們。維西推測，世界上其他的“第三性”人羣應該也是相似。

如果這是正確的，那麼“巢中幫手”理論可能在一定程度上解釋了同性相吸這一遺傳性徵爲什麼沒有被自然淘汰。這一假說促使維西產生另一個推測：被視爲男性且具有男性特徵的男同性戀者——大部分的西方男同性戀者即是如此——是從跨性別性徵者進化而來。

同性戀者也有孩子

在美國，大約37%的女同、男同、雙性戀和變性者有孩子，其中約60%是親生子女。根據威廉姆斯學院的研究，每對有孩子的同志伴侶平均會有兩個孩子。

這些數據可能不高，還不足以維持這一羣體的基因特質，但是進化生物學家傑里米·約德（Jeremy Yoder）在一篇博文中指出，近現代許多同性戀者並沒有公開地過着同性戀生活。迫於社會結婚生子的壓力，他們的生育率可能比現在的同戀者要高。

有多少同性戀者會有孩子呢，這取決於你怎樣定義“同性戀”。在薩摩亞羣島，許多和fa'afafine發生過關係的“直男”仍會結婚生子。

“如果從多元文化觀的視角來看，同性戀的範疇就會變得十分寬泛”，夏威夷大學進化生物學家Joan Roughgarden說道，“如果你到印度去就會發現，如果有人自稱是‘同性戀’，那你就可以直接認定他們是西方人。但這並不意味着印度沒有同性戀者。”

西方也是一樣，有證據表明，很多人都有過一個階段的同性戀行爲。上世紀40年代，美國性學教授阿爾弗雷德·金賽（Alfred Kinsey）發現，儘管僅有4%的白人男性青春期後就確定自己是純同，其他有10%的男性有過三年的同志行爲階段，37%的男性曾與同性發生過性關係。

去年在英國進行的一項全國性觀念調查得到的數據相對較小。大約16%的女性表示她們曾與其他女性有過性體驗（其中8%有過生殖器接觸），7%的男性表示他們曾與其他男性有過性體驗（其中5%有過生殖器接觸）。

但是大多數研究同性戀進化的科學家感興趣的是一種持續的內在性慾模式，而不是人們自認爲是同性戀還是異性戀，或者發生同性性行爲的頻率。“性別身份和性行爲並不能準確區分性取向，”保羅·維西表示，“性慾感覺纔是。”

卡齊·拉赫曼表示，控制同性吸引基因遺傳形態的等位基因僅能解釋一部分性取向行爲。其他自然變化的生物學因素也在其中發揮作用，卡齊認爲，大約有七分之一的男同的性取向受“兄長效應”的影響。

這與喬治·奧威爾《1984》書中的“老大哥”沒什麼關係。據觀察，有兄長的男孩更有可能成爲男同性戀——並且每多一個兄長，其成爲同性戀的機會就會增加約三分之一。沒有人知道其中的原因，有一種理論是，每次懷上男孩，母體就會產生某種蛋白免疫反應，而這種蛋白質恰好影響男性大腦發育。由於這一免疫反應只會在生產多個子女後纔會發生——這些兄弟姐妹大部分會是異性戀，並會繼續生育後代——這種孕期特異性纔沒有被演變淘汰。

出生前受到異常水平的荷爾蒙刺激也會影響人的性取向。例如，在出生前受到較高水平睾丸素刺激的女性胎兒以後成爲女同性戀的比率更高。研究顯示，充當“丈夫”角色的女同性戀者和男性一樣，食指與無名指的長度有略微差異——這表明了胎兒出生前受到睾丸素的刺激。作爲“妻子”角色的女同性戀者的這種差異卻不太顯著。

另一種兄弟——同卵雙胞胎——也帶來了一個棘手的問題。研究發現，如果同卵雙胞胎中的一個是同性戀，那麼大約有20%的概率另一個人也擁有同樣的性取向。雖然這比隨機性的可能性要高，但對於兩個擁有相同基因序列的人來說，還是比預期要低。

來自加州大學聖芭芭拉分校的威廉·賴斯（William Rice）表示，通過研究基因序列的處理過程而不是基因序列本身，我們有可能對此作出解釋。賴斯和他的同事參考了新興的表觀遺傳學，該學說研究“附因子”，附因子用於調控基因的具體實現。附因子只是偶爾能夠遺傳給後代。賴斯認爲，女性胎兒受某種附因子的影響從而減少了對睾丸素的敏感度。這種附因子通常不會遺傳給下一代，但是如果偶然出現遺傳，就會導致男孩的同性性傾向。

《同志健康》雜誌主編威廉·拜恩（William Byne）博士稱性取向很可能是先天的，但他也認爲這可能比許多科學家以爲得要複雜得多。他注意到，同性戀的遺傳性與離婚者類似，但是“社會科學研究者們卻沒有……對‘離婚基因’進行過任何研究。相反他們專注於研究遺傳的性格和喜怒無常的特質這些可能會導致離婚的方面。”

對卡齊·拉赫曼來說，媒體總是報導發現了“同性戀基因”，這把性取向的遺傳理論過於簡單化了。他認爲，性取向涉及幾十甚至上百對等位基因，這些基因可能需要數十年的時間才能解密。此外，儘管異性戀在進化過程中比同性戀更具優勢，但是卡齊說，並不是只有同性戀者的性取向纔是由基因決定的，異性戀者也同樣如此。