Darwinian Explenation for Transexuality

[Zor]

No Homophobia here, just honest curiosity.

I was wondering about this. How did homosexuality evolve? It seems to be rather unproductive for individuals to have one gender be attracted to another and yet it happens, not only in H.Sapiens but in other creatures as well, so how did it arise?

Zor

[Zor]

Sorry about the title, could a mod fix that? Even so, even though i know Transexuality is a different matter than Homosexuality, but it would make sense to cover both of them.

Zor

[Darth Raptor]

You're making a few unwarranted assumptions.

First of all, you're assuming that just because it's innate that it's necessarily hereditary. This isn't the case. Environmental factors in prenatal development may account for the development of such traits.

Secondly, even if it was hereditary selective forces may not be able to eliminate it because it's not just linked to one gene, but several. Even then, genes can be recessive and not express themselves in all offspring. There are a lot of other maladaptive traits that haven't been selected out too. That's right; maladaptive. Not every trait has some kind of clear evolutionary advantage. Some are neutral, while others are a genuine handicap.

Finally, history is replete with homosexuals who up and breed anyway.

[Lancer]

Alternatively, the same genes which influence sexuality may confer other advantages, even if the primary trait is recessive. I'm sure you've heard about how even being a carrier for sickle-cell anemia confers a much greater resistance to malaria. A similar mechanism could be at work.

[Alyrium Denryle]

There are several ways it can arise genetically but in the end it provides a reproductive advantage. The key is that it does not need to provide a reproductive advantage directly to the gay person, all it need do is provide a reproductive advantage for relatives OR be a byproduct of something that does.

The one that makes the most sense is the idea that homosexuality (at least in males) is a byproduct of a Quantitative Gene Locus that controls masculine and feminine development in the brain. A QTL is a complex of non-allelic genes that controls for continuous traits, such as human height. In this case, having high heterozygosity yields what we would think of as the typical male, while high homozygosity in one direction yields a male with more "feminine" traits that make him more likely to be able to raise children successfully, thus increasing his reproductive success, the trade off being that the more homozygous he is in that direction, the more likely the guy is of ending up gay, bringing his fitness near zero. (with some bisexuality in the mix) High homozygosity in the other direction leads to the "hypermasculine" individual. You know the type, angry aggressive ladies men that mate a lot but dont invest anything in their offspring. They have high mating success but compared to the guy that settles down and raises a family has low long term inclusive fitness.

Selection will stabilize the system, forming a normalized distribution within the population. This is supported by the fact that Heritability (being variation in gender-behavior and sexual orientation due to genetics) is very high. Twin studies for example show that concordance rates of monozygotic twins is between 50 and 70% even when they are raised apart, and the occurance of homosexuality in other cultures at similar rates (different environments serve as an environmental control). As well the fact that homosexuality is highly correlated with "feminine" gender behavior.

Another model I have seen is one in which a series of genes on the X chromosome that increases female fecundity also increases the chance of homosexuality in her male offspring.

[FA Xerrik]

Environmental factors in prenatal development may account for the development of such traits.

I believe it has been established that a large part of gender-identity for both males and females is linked to a pre-birth release of testosterone in vivo. Both males and females release testosterone while still in vivo, but for males there is a large spike at I believe about the halfway point in development. As I understand it, and my knowledge is limited to a lecture from my Psych class which I do not have the notes from at hand, variations in these testosterone surges have been shown (tentatively) by research to affect both gender and sexual orientation in the child.

Also, in some primate species female homosexuality is used to cement group bonds and establish alliances amongst groups within the larger tribe. The book The Moral Animal contains some fascinating insights into this aspect of homosexuality.

[Alyrium Denryle]

Environmental factors in prenatal development may account for the development of such traits.

I believe it has been established that a large part of gender-identity for both males and females is linked to a pre-birth release of testosterone in vivo. Both males and females release testosterone while still in vivo, but for males there is a large spike at I believe about the halfway point in development. As I understand it, and my knowledge is limited to a lecture from my Psych class which I do not have the notes from at hand, variations in these testosterone surges have been shown (tentatively) by research to affect both gender and sexual orientation in the child.

Also, in some primate species female homosexuality is used to cement group bonds and establish alliances amongst groups within the larger tribe. The book The Moral Animal contains some fascinating insights into this aspect of homosexuality.

Most certainly. There is no reason that those should be mutually exclusive with other models either... after all, Heritability is not 100%...

[FA Xerrik]

Another model I have seen is one in which a series of genes on the X chromosome that increases female fecundity also increases the chance of homosexuality in her male offspring.

That makes a lot of sense. Since testosterone doesn't occur in large amounts naturally in the female body, each male child contributes towards an immune response against testosterone in the womb. Each successive male causes a reduction of testosterone levels in future males, and so the biological changes instigated by these surges are interfered with over time.

[Alyrium Denryle]

Another model I have seen is one in which a series of genes on the X chromosome that increases female fecundity also increases the chance of homosexuality in her male offspring.

That makes a lot of sense. Since testosterone doesn't occur in large amounts naturally in the female body, each male child contributes towards an immune response against testosterone in the womb. Each successive male causes a reduction of testosterone levels in future males, and so the biological changes instigated by these surges are interfered with over time.

Fraternal birth order effect. Increases the chance of homosexuality by a third for each successive male offspring.

But that is not what I was referring to... I was referring to a hypothesis that homosexuality is contributed to by genetic factors that make the mom more fertile, but increase the chance of homosexuality. Such as genetically caused decreased sensitivity to tesosterone.

[The Duchess of Zeon]

The biological expression of male-to-female transsexualism is in the BSTc region of the brain, which is twice the size in men as in women, and is known to control sex-expression in rats. So if we set the male size as 1, the female size as 50% of that, we find out that the size of the average male to female transexual is 47% of 1 (6% smaller than the female). The two studies that have been done--both by autopsy, as this region can't be effectively measured with existing imaging equipment--agreed on this, and controlled for hormone use (several transexuals who had been off of hormones for years, even more than a decade for various health reasons, were available in the sample), and on the male side, exposure to feminizing hormones (men who had tumors which caused estrogen use, estrogen use as a treatment for cancer) and lack of testosterone (sundry gonadal issues provided that one).

In all cases the males had a correctly sized BSTc region despite feminizing hormone exposure and in all cases the transexuals had a female-sized one despite their absence for some times a decade or more before the study. I think in one case there was even one who had never taken hormones and transitioned without them, though I can't definitely recall.

Now, what is obvious is that, first of all, transsexualism, like homosexualism, is an irrevocable fact of brain structure. Then we get down to the trickier issue of cause.

We know that the BSTc region is not influenced by feminizing hormones or lack of male hormones as an adult. There's two possibilities:

1. It's influenced by such hormone exposure in a natal stage when the brain is developing.

2. It isn't hormone exposure as such.

I tend to follow the second one. My theory is that it's a straightforward issue of error rate. Let's consider the fact that nothing works perfectly. One third of pregnancies are spontaneously aborted at a stage where the woman never knows she was pregnant; there are an immense number of disorders which outright destroy the fetus before birth, and plenty of gonadal intersex conditions beyond that.

So it occurred to me that the possibility is simply that the chemical reactions which occur during development failed. Look at this way; the baseline for all human beings is female. So everything is going to develop female, regardless of your genes, until your Y chromosome when you're a guy actively starts telling your body to develop masculine characteristics, right? This is all done by bio-chemical processes, and none of those chemical processes are perfect. In short, for the BSTc region to develop wrong, there may simply be a single chemical process in the entire series of brain development which doesn't work. Without that trigger, the region stays at default (female) and never develops male characteristics. It's very simple, very straightforward, and incredibly bad luck for the transexual individual.

An interesting consequence of that is that even with all forseeable advanced technology we couldn't completely eliminate transsexualism; you can't make any process absolutely 100% perfect. In the long term probably the only effective treatment would be to detect the disorder through scanning of the fetus in the womb, and then if it is present, hitting the fetus with androgen blocking drugs, mimicing the natural condition of Androgen Insensitivity Syndrome, and therefore, the result (i.e. a perfectly female body but still an XY chromosome pattern).

In all cases--and regardless of cause--these people are supremely unlucky. They're literally born with a brain structure incompatible with their bodies.

[Illuminatus Primus]

Not everything is evolved; sometimes environmental factors change the genetically typical outcome. Sometimes a weird mix of alleles that did not adapt together produces an odd result which does not pertain to adaptability at all (esp. possibly true in our unnatural modern environment).

[Alyrium Denryle]

Not everything is evolved; sometimes environmental factors change the genetically typical outcome. Sometimes a weird mix of alleles that did not adapt together produces an odd result which does not pertain to adaptability at all (esp. possibly true in our unnatural modern environment).

That last sentence was scientifically incoherent

[Illuminatus Primus]

There are reasons that proto-gay traits or whatever could evolve in a population even if actually catagorically not wanting to have sexual intercourse with the opposite sex is not a preferred trait. They could then become expressed just by probability in some individuals in ways that would not be useful the way that the original trait came about. Some men have unusually large penises which are probably more of a hindrance to successful sexual relations with the opposite sex than an aid. That doesn't mean a tendency toward large penises did not serve some need at some point. However, nowadays, everyone survives and many reproduce regardless of their fitness. We don't have strong selection pressures.

[Turin]

Alyrium, could there also be an advantage for the gene even if it inhibits reproduction if it provides some factor in kin selection?

I may be misusing that term slightly, so this is what I mean: if not all individuals in a kin group of a social species reproduce because certain members are disinclined to do so ("homosexual homozygous"), then they can assist in the care of their immediate kin's offspring, who have a high chance of being "homosexual heterozygous" themselves. This would improve the chances of the genes involved of being passed on. Right?

[Alyrium Denryle]

Its possible, and that hypothesis is easily testable. Unfortunately it turned out, at least in modern cultures, to be incorrect.

[Turin]

Its possible, and that hypothesis is easily testable. Unfortunately it turned out, at least in modern cultures, to be incorrect.

I meant to include non-human examples of homosexuality in social animals in that statement as well. Obviously in modern human cultures, there's something else going on that destroys that hypothesis. But is that factor (homophobia) also hereditary/genetic or is it solely cultural? I've never seen any evidence supporting the former.

[Alyrium Denryle]

Not sure on that. It in and of itself I would wager is a cultural manifestation of genetically based xenophobia and aggression

[Fire Fly]

Alyrium, could there also be an advantage for the gene even if it inhibits reproduction if it provides some factor in kin selection?

I may be misusing that term slightly, so this is what I mean: if not all individuals in a kin group of a social species reproduce because certain members are disinclined to do so ("homosexual homozygous"), then they can assist in the care of their immediate kin's offspring, who have a high chance of being "homosexual heterozygous" themselves. This would improve the chances of the genes involved of being passed on. Right?

One idea that was tossed around why homosexuality continues to survive despite it not contributing to reproduction is that bisexuals are the ones who are propagating the "gay gene." An alternative idea that was thrown around was that as the mother continues to have more children, she's somehow becoming more "resistant" to the Y chromosome, making subsequent children more likely to be gay. And then there's the classic 1993 twins study which found that twins shared the same genetic markers on the X chromosome that was inherited from the mother.

Kin selection is also a possible explanation for why homosexuality persists although I find it to be a weak argument since altruism hasn't really been a strong human quality until recently. In other animals, altruism is even weaker.

[Adrian Laguna]

The biological expression of male-to-female transsexualism is in the BSTc region of the brain, which is twice the size in men as in women, and is known to control sex-expression in rats. So if we set the male size as 1, the female size as 50% of that, we find out that the size of the average male to female transexual is 47% of 1 (6% smaller than the female).

What about the opposite? I would presume that female-to-male transsexuals would have an approximately "size 1" BSTc region. Does anyone know how that could happen or no idea? Some biochemical process must tell the brain to got male, but I doubt it's testosterone as there is no Y-chromosone around.

[The Duchess of Zeon]

The biological expression of male-to-female transsexualism is in the BSTc region of the brain, which is twice the size in men as in women, and is known to control sex-expression in rats. So if we set the male size as 1, the female size as 50% of that, we find out that the size of the average male to female transexual is 47% of 1 (6% smaller than the female).

What about the opposite? I would presume that female-to-male transsexuals would have an approximately "size 1" BSTc region. Does anyone know how that could happen or no idea? Some biochemical process must tell the brain to got male, but I doubt it's testosterone as there is no Y-chromosone around.

There hasn't been a similar study done on transmen, so no data exists for me to give you an answer to that question.

[Alyrium Denryle]

Alyrium, could there also be an advantage for the gene even if it inhibits reproduction if it provides some factor in kin selection?

I may be misusing that term slightly, so this is what I mean: if not all individuals in a kin group of a social species reproduce because certain members are disinclined to do so ("homosexual homozygous"), then they can assist in the care of their immediate kin's offspring, who have a high chance of being "homosexual heterozygous" themselves. This would improve the chances of the genes involved of being passed on. Right?

One idea that was tossed around why homosexuality continues to survive despite it not contributing to reproduction is that bisexuals are the ones who are propagating the "gay gene." An alternative idea that was thrown around was that as the mother continues to have more children, she's somehow becoming more "resistant" to the Y chromosome, making subsequent children more likely to be gay. And then there's the classic 1993 twins study which found that twins shared the same genetic markers on the X chromosome that was inherited from the mother.

Kin selection is also a possible explanation for why homosexuality persists although I find it to be a weak argument since altruism hasn't really been a strong human quality until recently. In other animals, altruism is even weaker.

Um... no

Maybe you are thinking about non-reciprocal altruism, but that is not kin selection. This is going to take some explanation from the budding behavioral ecologist...

One can split an individual's fitness into two general categories. Inclusive and Exclusive.

Exclusive fitness is strictly an individuals' reproductive fitness.

Inclusive fitness is the sum total of exclusive fitness, but also the fitness of the individual's relatives added together and taking account of the Relatedness Coefficient of each. So if I don't mate but my sister has 8 kids, one can say that my fitness equals 2. This is because my sister shares half my genome and each of her kids shares 1/4th of my genome.


Kin Selection is basically the selective pressure on an organism to maximize their fitness, and it requires trade-offs between making investments in their own offspring vs the offspring of their relatives.

Of course, Kin Selection is really just a subtype of Reciprocal Altruism. I think your primary error is forgetting about the reciprocal part. In every act of so-called altruism found in animals (humans included) there is a benefit gained by the altruistic individual that outshines the cost, or there is one in their evolutionary past. There are several mechanisms for this

1. Kin Selection
As discussed above. In order to maximize overall fitness an individual must make trade offs. This can take place at the level of behavioral decision making. Such as whether an individual invests resources in providing for its own offspring or assisting a sibling. This happens a lot in birds, and is also the reason why in humans the extended family often assists in raising children. It is also why meercats and ground hogs set up colonies with sentries. They are all related and it is in their best interests in terms of fitness to rotate keeping watch for predators

and at the level of family groups, in which individuals might get the shaft in order to increase the reproductive fitness of the closely related family group, such as in ant colonies.

2. Cooperation
There are a LOT of forms of this. Often times you will find the same behaviors with groups of relatives as well. But In here I will put in non-relatives and interspecific interactions.

Male-male reproductive cooperation:
Two unrelated males cooperate to defend a territory. Usually one gets the majority of the matings, but the other increases his matings over what he would normally get. For example, a subordinate male cichlid fish will assist a larger male in defending his territory. The big sexy male will attract females and mate with them, but the smaller male mixes his sperm in and thus gets a percentage of the fertilization and in some cases, sneaks full matings with some of the females in line. This increases his fitness beyond what it would normally be, and the bigger male gets more matings because A) he is not spending all his time defending his territory and B) he does not have to compete for mating with a lot of little males, just one. They both win.

communal rearing of offspring:

Offspring of unrelated offspring are reared communally, often in rotating shifts by adults. You see this in some penguins. Once the hatchlings are old enough to not be completely helpless, the adults will go get food, leaving the young in groups watched over by a few adults. Everyone wins.

Examples and subsets of reciprocal altruism like this abound, show me a species and chances are I can show you an example of reciprocal altruism. In fact, give me a human behavior and I can give you how evolution probably plays into the development of that behavior.

[Fire Fly]

-snip-

For whatever reason, the definition I had of kin selection was completely off mark. I'm not sure what the hell I was thinking at the time...

[Androsphinx]

My wife's parents were (are!) both gay. She claims to take after her father, but I'm not sure if that's a genuine claim based on genetics

[Alyrium Denryle]

My wife's parents were (are!) both gay. She claims to take after her father, but I'm not sure if that's a genuine claim based on genetics

A) The dynamics for females dont have to be the same because of different developmental dynamics.

B) Depending on how the heredity works out, she may end up with combinations of alleles that do not predispose homosexuality

[Turin]

Its possible, and that hypothesis is easily testable. Unfortunately it turned out, at least in modern cultures, to be incorrect.

I meant to include non-human examples of homosexuality in social animals in that statement as well. Obviously in modern human cultures, there's something else going on that destroys that hypothesis. But is that factor (homophobia) also hereditary/genetic or is it solely cultural? I've never seen any evidence supporting the former.

Not sure on that. It in and of itself I would wager is a cultural manifestation of genetically based xenophobia and aggression

You know what? I'm a dumbass for not checking my facts before asking about this idea. Apparently there's been a "Kin Selection Hypothesis" for male homosexuality since the 70s, but there's been other work that's demonstrated a lot of holes in that hypothesis. For example (abstract only):

It is clear from above that the kin selection theory has not been subject to rigorous empirical testing. In fact, the theory has been heavily criticized by several authors (see Buss, 1994; Dickemann, 1995; Kirkpatrick, 2000; McKnight, 1997; Muscarella, 2000; Rahman & Wilson, 2003). There are two major conceptual problems with the theory. Firstly, the advantage of having homosexual family members would need to be very large to offset the lack of direct reproduction; the genes for homosexuality would have to show high penetrance, which is unlikely (Hamer & Copeland, 1994; Rahman & Wilson, 2003). Secondly, homosexuality appears as a poor solution for collateral nepotism (e.g., that seen in asexual worker castes in some insect species) because homosexuals naturally expend energy on pursuing non-reproductive sex rather than assisting kin (Bobrow & Bailey, 2001). Ruse (1982) and Weinrich's (1976) assertions are not compelling either as the available anthropological evidence is inconclusive and begs the question of why high status roles would be provided to non-reproducing homosexuals rather than to reproductively successful "alpha-type" heterosexual males (Kirkpatrick, 2000).

Apologies.