Potential Evolutionary Role for Same-Sex Attraction

  • metta

    Posts: 39149

    Feb 11, 2010 11:24 PM GMT

    Potential Evolutionary Role for Same-Sex Attraction

    http://www.sciencedaily.com/releases/2010/02/100204144551.htm
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    Feb 12, 2010 1:01 AM GMT
    I always thought male homosexuality was meant to give a selective advantage to straight men. The higher the number of gay men, the lesser the number of available men for straight women. But I don't know enough of evolutionary theory to know if a "sex war" makes sense.
  • MSUBioNerd

    Posts: 1813

    Feb 12, 2010 2:27 AM GMT
    bachian saidI always thought male homosexuality was meant to give a selective advantage to straight men. The higher the number of gay men, the lesser the number of available men for straight women. But I don't know enough of evolutionary theory to know if a "sex war" makes sense.


    Then, as an evolutionary biologist, allow me: it doesn't.

    Evolutionary biology is a pretty large topic, so covering it all would be well beyond the scope of a posting here in the forum. For an excellent introduction into how evolutionary genetics work, I suggest The Selfish Gene, by Richard Dawkins. But, I'll take a stab at covering a few points of how think about evolution in a way consistent with biology.

    First, some terminology. Genes are sections of DNA (or, in the case of some viruses, RNA) that produce some function when they're converted into RNA and (usually) from there to DNA -- though there are some that provide their function as RNA. Variants of a gene are called alleles, and they can differ between different individuals in the population. Thus, if there were just a single gene controlling eye color (there isn't -- multiple genes are involved), there would be an allele for blue, an allele for green, an allele for brown, an allele for gray, etc.

    Point number one: Genes care nothing about the fate of the species. To the extent that they care about anything, it is getting copies of themselves into the next generation.

    Darwinian thought is oftentimes expressed as the simplification of "survival of the fittest", which is only partially accurate. Darwin's ideas are essentially covered by 4 major points, the last of which is a necessary result of the previous three.
    1) In any given population, in any given trait, there is some degree of variation.
    2) Some percentage of that variation is typically heritable; that is, offspring resemble their biological parents, on average, more than they do other adults.
    3) For virtually all species, more individuals are born than are capable of surviving and reproducing themselves.
    4) As a consequence, those variations which enhance the ability of the individuals carrying them to survive and reproduce will tend to increase in frequency over time.

    However, all of that is true whether or not the species as a whole would be better off if a particular variant didn't occur. Current ecological thought is that there are too many people around for our planet to sustain. Therefore, it would be better if we had fewer children. Now, let's imagine that there's some genetic variation that causes people who possess it to regulate the number of children that they have by how many people the environment can sustain, while most individuals will tend to have more children than the planet can support. If we accept for the sake of this argument that there are indeed too many people around, the species would be better if everyone ended up with the genetic variant that caused people to regulate the number of kids they had in accordance with assessments of sustainability. However, those with the variation will tend to have fewer kids than those without. Therefore, in each successive generation, a smaller and smaller percentage of people will carry this variation. Because this variant decreases the probability that those who possess it will have children -- and therefore decreases the probability that the variant itself will be represented in the next generation -- it is an evolutionary dead end, even though the species as a whole would benefit from it.

    Therefore, with this in mind, the argument that male homosexuality exists to give a benefit to straight males is fundamentally flawed. The individuals benefiting from the genetic variation do not possess the variation itself, and are not statistically more likely to possess it than a randomly chosen individual. As such, evolution cannot favor this variation for itself -- any favoring evolution could do for that variation would have to be for some other reason, such as that the variation occurred some other genetic variation that did increase the reproductive success of those who carried it.

    This is a common problem people have when trying to think about evolution; they think in terms of what would be good for the species, not in terms of what would be good for that particular genetic variant.

    Point number 2: Fitness, in the evolutionary sense, is completely different from fitness in the exercise sense.

    Evolutionary fitness is a measurement of the number of copies of your genetic variants have made it into the next generation. Most of us evolutionary biologists will even tell you that it's the relative fitness that matters more than the absolute fitness: that is, how many copies of your variants make it into the next generation compared to average, not the raw number. If you have 2 kids when the average is 1, your genes are spreading. If you have 3 kids when the average is 4, your genes are contracting. However, in the larger sense, evolution cares more about the number of grandkids you have than kids. The woman who has 3 children who all make it to adulthood well enough to find mates and provide her a total of 6 grandkids is doing better than the man who has 10 kids of which only 3 survive to adulthood, 2 of those never have kids, and the final 1 has 4 children.

    We gay men are probably more fit than the average straight man in the US -- we go to the gym more, we eat a healthier diet, etc. But we also have fewer kids, and a higher percentage of the kids we do have are adopted, so from an evolutionary standpoint, we're less fit on average.

    Point number 3: Virtually no "Gene for X" reported in the media is actually a gene for X, and X is virtually never controlled by a single gene.

    There are a number of traits that are caused by single genetic variants. There is an allele that causes sickle cell anemia, for instance, when both copies of the gene are of that particular allele. It's found in relatively high frequency in people whose ancestors lived in certain tropical areas, though, because when the person has just one allele that would cause sickle cell, and one that doesn't, they not only don't have sickle cell disease, they're also less likely to die from malaria than someone who has 0 copies of the sickle cell allele.

    However, the vast majority of traits that interest us (height, weight, intelligence, longevity, appearance, etc.) and which affect the so-called diseases of affluence (cancer, heart disease, stroke, diabetes, etc) are controlled by dozens to thousands of genes. Even eye color, which many of us were taught was a single gene with 3 alleles back in high school is actually controlled by substantially more than 1 gene; the two best understood are one which is involved in pigment production, and another involved in the thickness of one type of fiber, which influences color by the degree light scatters when interacting with it.

    Further, genes are often named for what happens when the gene is broken in some way. p53 is a well-studied gene because of a variant that is associated with a higher risk of cancer. However, the gene's function is not to cause cancer. Instead, the gene functions as a regulator of the cycle of cellular growth and reproduction. When it malfunctions in a certain way, it causes that cell to become cancerous. People who carry certain specific alleles of p53 are at a higher risk to develop cancer because their copy of the gene is more likely to break down in these specific ways.
  • Anto

    Posts: 2035

    Feb 12, 2010 2:36 AM GMT
    MSUBioNerd,

    The problem with the idea of a selfish gene is that it doesn't make sense. The genes don't 'care' about anything. And I don't understand why it gets reduced like it does and not take into account human involvement, like people deciding and manipulating things or even how the environment shapes evolution. It's not just about what genes do alone.
  • MSUBioNerd

    Posts: 1813

    Feb 12, 2010 2:36 AM GMT
    (cut off from above)

    Beyond those general points of evolutionary biology, the article itself has a number of flaws in it. First off is that the heritability of homosexuality, in most studies I've seen, is essentially 0, although the articles just states that it appears it is heritable because gay people are less likely to have children than straight people are. That's not how you determine heritability. You use twin and adoption studies. In adoption studies, heritability estimates are essentially 0 -- the probability that a child will turn out to be gay is the same whether or not the genetic parent is gay, and whether or not the adoptive parent is gay. In twin studies, the heritability is generally greater than 0, but less than 1(which would mean that every identical twin of a gay man is himself gay). The twin studies have come under further criticism for selection bias -- twins where both are gay are more likely to respond to questionnaires of their orientation than are twins where one is gay and the other is straight.

    More reliable, for a scientific standpoint, is the birth order effect, which has shown up in many studies at statistically significant rates. Basically, the more previous male pregnancies a woman has had, the greater the probability that the male fetus gestating in her will turn out to be gay, regardless of whether those previous pregnancies resulted in older brothers in the house or not. No such effect seems to exist for lesbians. There is also some marginal support for the idea of a genetic role of one particular region of the X chromosome -- region Xq28. In some studies, gay males are more likely to have a particular variant here, and women with such a variant have more children than women without that variant. However, other studies fail to find this, and the whole thing is at the border of statistical significance/insignificance.

    Then there's the fact that even if it's true that gay men in Samoa are more likely to provide for their nieces and nephews, there's no guarantee that that's genetic. It could well be cultural, as cultural is quite powerful among humans. Culture causes a number of gay men to live in the closet and marry women, for example.

    Then there's the math behind kin selection, touched on only briefly in the article. Many of us gay guys are/will be great uncles. But are we such great uncles that our siblings have 4 more children than they would if we weren't around to help out? It takes, on average, 2 children, or 2 siblings, or 4 nieces/nephews, or 8 first cousins, etc to carry one copy of each of our genetic variants. Less than that, and evolution will work against that variant.
  • DCEric

    Posts: 3713

    Feb 12, 2010 2:44 AM GMT
    MSU, my brain is full. May I be excused?

    /On a side note, you sort of touched on birth order being slightly significant, I've noticed that in families where there are two children of the same sex, the younger one tends to be taller. Do you know of any studies on this? Curious minds want to know.
  • Posted by a hidden member.
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    Feb 12, 2010 2:48 AM GMT
    This theory has been around for decades and makes sense to me.
  • MSUBioNerd

    Posts: 1813

    Feb 12, 2010 2:52 AM GMT
    Anto saidMSUBioNerd,

    The problem with the idea of a selfish gene is that it doesn't make sense. The genes don't 'care' about anything. And I don't understand why it gets reduced like it does and not take into account human involvement, like people deciding and manipulating things or even how the environment shapes evolution. It's not just about what genes do alone.


    The fact that genes don't actually care about anything -- not having brains -- is why I phrased it as "to the extent that they care about anything".

    The selfish gene principle actually makes a tremendous amount of sense. To paraphrase Dawkins, chickens are just the most efficient means of reproducing that chicken genes have yet found. The fact that a genetic element will operate in such a way that it will increase its own representation in then gene pool, regardless of whether that increased representation harms the individual carrying that gene, explains a great number of things that make little to no sense otherwise in biology.

    The environment certainly shapes how genes are expressed and effect the individuals carrying them. The genetic variation that causes phenlyketonuria, for example, causes substantial mental retardation -- if the person consumes phenylalanine while developing. With a careful diet throughout childhood and adolescence, mental function is essentially normal. But that doesn't change anything about the argument about how evolutionary genetics will favor genetic elements that increase their own representation in the gene pool even at the expense of those who carry them.

    Standard example from population genetics: A given species of bird averages successfully rearing one chick per couple per year, starting at age 3. In this species, individuals who die from old age tend to die at the age of 10. A new mutant arises which begins breeding at age 2 but dies of old age at age 9. Otherwise, the mutant and the wild-type birds are identical; they have the same number of chicks per year, the same survival probability of avoiding predators, etc.

    Question: Does natural selection favor this mutant, favor the wild type, or neither?

    Answer: Natural selection favors this mutant.

    The percentage of individuals who survive to age X is always greater than the percentage of individuals who survive to age X+1. In this species, the mutant and the wild-type will have the same reproductive success per year from ages 3-8. The wild-type also gets a round of reproduction at age 9, the mutant gets a round of reproduction at age 2. Because more birds are alive at age 2 than are alive at age 9, the round of reproduction at age 2 averages more chicks than the round of reproduction at age 9. Therefore, the mutant has more chicks, on average, than the wild-type does, and has a higher evolutionary fitness even though is succumbs to old age earlier. This mutation is expected to spread through the population, and will eventually become the new wild-type.
  • Posted by a hidden member.
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    Feb 12, 2010 5:21 AM GMT
    MSUBioNerd said
    Then, as an evolutionary biologist, allow me: it doesn't.

    Evolutionary biology is a pretty large topic, so ...

    ...Therefore, the mutant has more chicks, on average, than the wild-type does, and has a higher evolutionary fitness even though is succumbs to old age earlier. This mutation is expected to spread through the population, and will eventually become the new wild-type.


    * swooon *
  • metta

    Posts: 39149

    Feb 12, 2010 9:08 AM GMT
    fascinating stuff. Thanks everyone!
  • metta

    Posts: 39149

    Feb 12, 2010 9:15 AM GMT
    How Gay Uncles Pass Down Genes


    http://alturl.com/ftct

    http://news.yahoo.com/s/livescience/20100211/sc_livescience/howgayunclespassdowngenes;_ylt=AmBJbf8RXzLZEuNry.Ge2tkPLBIF;_ylu=X3oDMTJ2OG1nbTM1BGFzc2V0A2xpdmVzY2llbmNlLzIwMTAwMjExL2hvd2dheXVuY2xlc3Bhc3Nkb3duZ2VuZXMEcG9zAzIzBHNlYwN5bl9zdWJjYXRfbGlzdARzbGsDaG93Z2F5dW5jbGVz
  • Posted by a hidden member.
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    Feb 12, 2010 9:22 AM GMT
    This is what I know and feel.

    1. I am attracted to men sexually and not women
    2. I was born gay. Tits NEVER did a thing for me!
    3. I was not sexually molested
    4. There is a gay gene... My Great Aunt was a lesbian...I just found out!
    5. My only younger brother was also BORN gay. 4 years younger!
    6. I tried to be straight and it did not work...How many straights try to be GAY?
    7. Having same sex attraction is not bad....having same sex is not bad EITHER..
    8. Society and Religion have complicated the issue only because of the media. It's compounded now that gay people want equal rights.
    9. COCK is meaningful and Pussy is not.
    10. I still like fancy Vs. Simple. Mr. Rogers say's men are fancy!
    11. Being gay is OK!
    12. If God say's love one another.....I see only hate from most religions....
    13. I think when we wake up from this nightmare, the great and powerful universe will say, "So did you enjoy your one and only human existence"?

    :-)

    Love you all,
    I do really
  • Posted by a hidden member.
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    Feb 12, 2010 10:19 AM GMT
    MSUBioNerd,

    Thanks for trying to explain the genetic stuff in sort of a "Evolutionary Biology for Dummies" sort of way!

    I have often questioned those who oppose homosexuality as being unnatural, if they've ever considered it as being nature's way of population control? It seems as if most have never done so.

    I don't know if there's ever been any research into that, but it just seems to make sense to me. Whether its 10%, 20% or just 2%, there's always a percentage of homosexuals in the population, in pretty much every culture that we know of.

    Now of course, we have more people who are stepping forth and claiming their orientation, so to the opposition, it appears as if the number of LGBT people is growing; which could be possible since the earth's population has grown so out of control.

    I remember reading an article several years ago saying that it took (x) amount of years to reach 1 billion people, half that time to reach 2 billion and so until we got to our present 6 plus billion people. I'm not sure of the specific time lines/numbers, but I'm sure you know what I'm talking about.

    Would it not then follow that there would be more of us, the more people there are? In an effort to keep the population in check? It's not in any way me field of expertise, but makes sense to me.

    That's my story, and I'm sticking to it!icon_lol.gif



  • GQjock

    Posts: 11649

    Feb 12, 2010 11:35 AM GMT
    While homosexuality is not a traceable "gene" it is a trait
    that is multi-factorial in its phenotype
    Much like musicality or the ability to hunt
    while you cannot trace these abilities to any set of genes they are traceable in a macro-genetic form through the generations

    It is hard to get around the idea where a seemingly nonadvantageous trait like homosexuality (If you use the ability to pass on your own genes as the prize)
    you lose the forest thru the trees
    insects .... bees and ants show this to us all the time
    whole segments of their population forgo the ability to pass on their own genes for the good of the population
    Whether this model is working in human society remains to be proved
    Human society is based in primate interaction
    and homosexuality is known in all its populations

    What is important to know from this is .... if you see behaviors in the natural world
    there is always a reason for them
    evolutionarily it is so difficult to remain alive for generation to generation that all your behaviors need to have a reason for your survival
  • rockleetpt

    Posts: 76

    Feb 12, 2010 11:49 AM GMT
    Parnell_Marcano saidMSUBioNerd,

    Thanks for trying to explain the genetic stuff in sort of a "Evolutionary Biology for Dummies" sort of way!

    I have often questioned those who oppose homosexuality as being unnatural, if they've ever considered it as being nature's way of population control? It seems as if most have never done so.

    I don't know if there's ever been any research into that, but it just seems to make sense to me. Whether its 10%, 20% or just 2%, there's always a percentage of homosexuals in the population, in pretty much every culture that we know of.

    Now of course, we have more people who are stepping forth and claiming their orientation, so to the opposition, it appears as if the number of LGBT people is growing; which could be possible since the earth's population has grown so out of control.

    I remember reading an article several years ago saying that it took (x) amount of years to reach 1 billion people, half that time to reach 2 billion and so until we got to our present 6 plus billion people. I'm not sure of the specific time lines/numbers, but I'm sure you know what I'm talking about.

    Would it not then follow that there would be more of us, the more people there are? In an effort to keep the population in check? It's not in any way me field of expertise, but makes sense to me.

    That's my story, and I'm sticking to it!icon_lol.gif





    That's an assumption in which you give yourself credibility because it makes sense. But in a lot's of complicated (and some simple) things in life, just because it makes sense doesn't mean it's true. Some things are counter-intuitive.

    Fact is there is not yet definitive proof regarding homossexuality being genetic or environmental (starting from the womb). And it's indeed a very complicated issue dealing with population, representation, definition of homossexuality (or rather, non-heterossexuality). There's no simple reason regarding it. I might have my own guesses since I have a straight identical twin brother but I don't KNOW if it's true, it just FEELS like it's true, but, as I said, making sense doesn't necessarly mean that it's true so I obviously I try to keep myself as an open-but-yet-critical-minded regarding stuff like this.

    In MY view, the theory "homossexuality is a genetic device to keep population in check" doesn't make sense to me. It raises more questions than it answers. What about bissexuality? Assexuality makes more sense in "keeping the population in check" because having no sexual desire, there is more potential focus towards the comunity in contrast with the homossexual who, having sexual desire and attraction, would devote their time and effort towards the significant other (and this is assuming it's one significant other) , rather than their family/comunity.

    It's just my two cents. Sorry if this is borderline off-topic since I'm more focusing on the causes of homossexuality rather than potential evolution role for such.
  • Posted by a hidden member.
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    Feb 12, 2010 12:36 PM GMT
    MSUBioNerd has really nailed this one, so there's no real point in wading in here, but my two cents:

    The kin-selection hypothesis for homosexuality ain't new (I think it was E.O. Wilson in the 70s who first suggested it, might be wrong), but really founders on a complete lack of evidence.

    As BioNerd sez, this quite clearly could be cultural. Here's a group of people within a society - fa'afafine - with quite a clear role and place in that culture. So much so that they even have a name and an identity (pointing out the obvious - that's not true of a lot of human societies, and, as far as I'm aware, the guys on realjock ain't fa'afafine). The kin-selection argument supports a specific sociological/social anthropological functionalist explanation as well it does a genetic one (better, in fact, I think).

    One more thing: the assumption that homosexual men have lower reproductive fitness than heterosexual men is a modern idea which corresponds to our current culture. I'm not sure whether that was true historically, especially where marriage provided an economic function instead of being due to romantic attachment.

  • Posted by a hidden member.
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    Feb 12, 2010 1:47 PM GMT
    I'll put my two cents in, as a biologist.
    MSU, you said something about ... let me see if I can find it:
    "Therefore, with this in mind, the argument that male homosexuality exists to give a benefit to straight males is fundamentally flawed."

    The way you've presented it is true. If there exists an allele (or set) somewhere in the genome that makes people gay so that they'll have fewer kids and the rest of society will be better off (population reduction) and that's the end of the story, then yes, an allele set could not exists with just those traits because they would get selected out of the gene pool very quickly. Those carrying the genes would have fewer kids and bla bla bla, you get the point, the genes would eventually not be carried by anyone at all.

    But if -- like the classic case of sickle cell which you point to -- being heterozygous for the allele gives you or your offspring an evolutionary advantage, then the homozygous form of the allele would show up, albeit at slightly lower frequencies.
  • Posted by a hidden member.
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    Feb 12, 2010 1:52 PM GMT
    Biologists unite! hahaha...

    Homosexuality has been shown to provide social bonding in many animal populations, thus imparting a survival advantage to the entire group.

    Also, the idea that homosexuality is a population control byproduct of evolution is baseless: homosexuals are perfectly capable of reproduction, and many have exemplified this point by having children.

    From an evolutionary standpoint, the only survival advantage it imparts is group survival in animals where this is needed. Otherwise, if it is a genetic trait, it's one that is neither advantageous or disadvantageous, but simply just "is." Not every trait manifest by evolution has a purpose.
  • Posted by a hidden member.
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    Feb 12, 2010 1:57 PM GMT
    Dude I learned something today




    MSUBioNerd said
    bachian saidI always thought male homosexuality was meant to give a selective advantage to straight men. The higher the number of gay men, the lesser the number of available men for straight women. But I don't know enough of evolutionary theory to know if a "sex war" makes sense.


    Then, as an evolutionary biologist, allow me: it doesn't.

    Evolutionary biology is a pretty large topic, so covering it all would be well beyond the scope of a posting here in the forum. For an excellent introduction into how evolutionary genetics work, I suggest The Selfish Gene, by Richard Dawkins. But, I'll take a stab at covering a few points of how think about evolution in a way consistent with biology.

    First, some terminology. Genes are sections of DNA (or, in the case of some viruses, RNA) that produce some function when they're converted into RNA and (usually) from there to DNA -- though there are some that provide their function as RNA. Variants of a gene are called alleles, and they can differ between different individuals in the population. Thus, if there were just a single gene controlling eye color (there isn't -- multiple genes are involved), there would be an allele for blue, an allele for green, an allele for brown, an allele for gray, etc.

    Point number one: Genes care nothing about the fate of the species. To the extent that they care about anything, it is getting copies of themselves into the next generation.

    Darwinian thought is oftentimes expressed as the simplification of "survival of the fittest", which is only partially accurate. Darwin's ideas are essentially covered by 4 major points, the last of which is a necessary result of the previous three.
    1) In any given population, in any given trait, there is some degree of variation.
    2) Some percentage of that variation is typically heritable; that is, offspring resemble their biological parents, on average, more than they do other adults.
    3) For virtually all species, more individuals are born than are capable of surviving and reproducing themselves.
    4) As a consequence, those variations which enhance the ability of the individuals carrying them to survive and reproduce will tend to increase in frequency over time.

    However, all of that is true whether or not the species as a whole would be better off if a particular variant didn't occur. Current ecological thought is that there are too many people around for our planet to sustain. Therefore, it would be better if we had fewer children. Now, let's imagine that there's some genetic variation that causes people who possess it to regulate the number of children that they have by how many people the environment can sustain, while most individuals will tend to have more children than the planet can support. If we accept for the sake of this argument that there are indeed too many people around, the species would be better if everyone ended up with the genetic variant that caused people to regulate the number of kids they had in accordance with assessments of sustainability. However, those with the variation will tend to have fewer kids than those without. Therefore, in each successive generation, a smaller and smaller percentage of people will carry this variation. Because this variant decreases the probability that those who possess it will have children -- and therefore decreases the probability that the variant itself will be represented in the next generation -- it is an evolutionary dead end, even though the species as a whole would benefit from it.

    Therefore, with this in mind, the argument that male homosexuality exists to give a benefit to straight males is fundamentally flawed. The individuals benefiting from the genetic variation do not possess the variation itself, and are not statistically more likely to possess it than a randomly chosen individual. As such, evolution cannot favor this variation for itself -- any favoring evolution could do for that variation would have to be for some other reason, such as that the variation occurred some other genetic variation that did increase the reproductive success of those who carried it.

    This is a common problem people have when trying to think about evolution; they think in terms of what would be good for the species, not in terms of what would be good for that particular genetic variant.

    Point number 2: Fitness, in the evolutionary sense, is completely different from fitness in the exercise sense.

    Evolutionary fitness is a measurement of the number of copies of your genetic variants have made it into the next generation. Most of us evolutionary biologists will even tell you that it's the relative fitness that matters more than the absolute fitness: that is, how many copies of your variants make it into the next generation compared to average, not the raw number. If you have 2 kids when the average is 1, your genes are spreading. If you have 3 kids when the average is 4, your genes are contracting. However, in the larger sense, evolution cares more about the number of grandkids you have than kids. The woman who has 3 children who all make it to adulthood well enough to find mates and provide her a total of 6 grandkids is doing better than the man who has 10 kids of which only 3 survive to adulthood, 2 of those never have kids, and the final 1 has 4 children.

    We gay men are probably more fit than the average straight man in the US -- we go to the gym more, we eat a healthier diet, etc. But we also have fewer kids, and a higher percentage of the kids we do have are adopted, so from an evolutionary standpoint, we're less fit on average.

    Point number 3: Virtually no "Gene for X" reported in the media is actually a gene for X, and X is virtually never controlled by a single gene.

    There are a number of traits that are caused by single genetic variants. There is an allele that causes sickle cell anemia, for instance, when both copies of the gene are of that particular allele. It's found in relatively high frequency in people whose ancestors lived in certain tropical areas, though, because when the person has just one allele that would cause sickle cell, and one that doesn't, they not only don't have sickle cell disease, they're also less likely to die from malaria than someone who has 0 copies of the sickle cell allele.

    However, the vast majority of traits that interest us (height, weight, intelligence, longevity, appearance, etc.) and which affect the so-called diseases of affluence (cancer, heart disease, stroke, diabetes, etc) are controlled by dozens to thousands of genes. Even eye color, which many of us were taught was a single gene with 3 alleles back in high school is actually controlled by substantially more than 1 gene; the two best understood are one which is involved in pigment production, and another involved in the thickness of one type of fiber, which influences color by the degree light scatters when interacting with it.

    Further, genes are often named for what happens when the gene is broken in some way. p53 is a well-studied gene because of a variant that is associated with a higher risk of cancer. However, the gene's function is not to cause cancer. Instead, the gene functions as a regulator of the cycle of cellular growth and reproduction. When it malfunctions in a certain way, it causes that cell to become cancerous. People who carry certain specific alleles of p53 are at a higher risk to develop cancer because their copy of the gene is more likely to break down in these specific ways.
  • DiverScience

    Posts: 1426

    Feb 12, 2010 3:00 PM GMT
    Something else to add to the thoughts on evolutionary advantage. Remember that evolution is about populations of a gene, not an individual's genes. That is, a gene that can be disadvantageous in one individual may be advantageous in another.

    Let me clarify. First, as MSU said, almost no traits are single gene.

    One of the simplest examples you can wrap your head around is sickle-cell anemia. If you have one copy of this gene, you're immune to malaria. If you have two, you've got sickle cell. Then you're fucked, you probably die, and you don't pass on your genes. Seems bad, right?

    Except... what that means is that SICKLE CELL will stay in the population, even though evolutionarily it looks like a horrible disadvantage, because there's such an incredible advantage to the people who only have one copy (malaria resistance). So on a population level, this gene is not a disadvantage but an advantage, even though to a particular individual it may be lethal.

    Now, look at gayness, which we know isn't a mendelian (simple on/off) trait. Imagine it has... 10 genes. Now add in that it's a behavioral trait and thus more highly affected by environment and other external factors. Now say that each of the 10 genes, has a 0 copy, 1 copy and a 2 copy trait associated with it. Ok, we've now got 30 genetic traits, plus the ability to modulate or even mute them via environmental factors, this would explain why we can't pick out traits that "all gay men have" because there's 3^30 or 2810^15, or 205,891,132,094,649 or over 205 TRILLION combinations.

    If one, or two, traits that arise from each of those genes is advantageous, but it requires 5 or more of them to be "gay" and environmental factors that don't mute them, those genes will all stay in the population, and thus, so will "gay" even though on an *individual organism basis* they are disadvantageous.
  • MSUBioNerd

    Posts: 1813

    Feb 12, 2010 3:15 PM GMT
    The responses to Parnell are sound; thoughts about homosexuality existing as a population check are unworkable in an evolutionary context. See point one: you are thinking in terms of what is good for the species, not what is good for the individual carrying the genetic trait. Any time that a genetic trait costs the individual carrying it, and the benefits are not directed toward others who carry that genetic variant (either the same individual, a number of close kin, or a large number of others who will perform the same service in the future), it is unworkable in the long run of evolution. Individuals who are genetically disposed to not reproduce in order to limit the size of the population will lose out to others who do not share that genetic predisposition, and the trait will go extinct.

    Javaman raises a potentially valid point: that is, a genetic predisposition toward homosexuality could persist if it were part of a pleiotropic allele that increased reproduction in another way: higher fitness for heterozygotes, for example, or increased fertility in women with the trait while simultaneously decreasing male reproduction. The rate at which it would persist can be directly calculated from the amount of evolutionary benefit on the one hand, versus the cost on the other. Or, if we know the rate and the cost, we can calculate the benefit necessary.

    To pull a number from the air, let's say that gay men have, on average, 1/3 as many genetic children as straight men do. (To all of you adoptive fathers out there: you are wonderful people. As a society, we need to do more to support adoptive parents, and we need to remove the ridiculous prohibitions in some jurisdictions against gay men adopting children. That being said, we're talking evolutionary genetics here, so it's the genetic children that we're counting. The kids you adopted are presumably the genetic children of straight men, unless you're a second parent adoption from your partner having inseminated a surrogate, or your partner having had children with a woman prior to your presence.) If we assume it's a simple case of one gene, two alleles, two recessive alleles lead to gayness while one recessive and one dominant lead to some sort of evolutionary benefit, if gay men make up 5% of the male population, those heterozygotes would have to have roughly 20% more children than those with two dominant alleles, or else the frequency of homosexuality would decrease.
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    Feb 13, 2010 8:35 AM GMT
    MSUBioNerd said
    Javaman raises a potentially valid point: that is, a genetic predisposition toward homosexuality could persist if it were part of a pleiotropic allele that increased reproduction in another way: higher fitness for heterozygotes, for example, or increased fertility in women with the trait while simultaneously decreasing male reproduction. The rate at which it would persist can be directly calculated from the amount of evolutionary benefit on the one hand, versus the cost on the other. Or, if we know the rate and the cost, we can calculate the benefit necessary.


    Sickle cell anemia's distribution itself is limited to a very specific population group targeting a very specific advantage (greater resistance against malaria). You can even trace the spread of the genotype though population movements.

    The problem is that there is every sort of indication that homosexuality is omnipresent in almost all humans, though of course the actual numbers probably will never be known in our lifetimes due to the cultural stigma attached to it.

    If it did impart some sort of advantage in its heterozygous state (if it was indeed as simple as, it may not, after all, involve something as simple as Mendellian dominance as DiverScience pointed out), that advantage must have manifested very early in human evolution. Let's say... the MRCA must have been at least one of the first thousand humans or so. Such that the subsequent migrations of human populations didn't affect it in the least. It may not even be human at all. But a genetic predisposition that began with any of our animal ancestors and only found actual phenotypic advantage in social animals like humans and apes.

    Because come to think of it. IF it is indeed a form of homozygous variation (I hesitate to call it abnormality) with a particularly advantageous heterozygous form, there would be populations of humans where homosexuality is ENTIRELY absent.

    Unless of course, this advantage that the heterozygous form of the 'gay gene' supposedly imparts is so successful that the carriers eventually drove the non-carriers extinct, in which case the implications are even more... tinfoilhatty. icon_biggrin.gif

    Taking the sickle cell anemia comparison further in which the homozygous phenotype itself is a magnification of the effects of the heterozygous form. We could even conjecture that perhaps the 'gay genes' in their heterozygous state could have imparted something really profound. Something like male social cooperation, maternal instincts, or even the whole predisposition to social structures of the primates (conversely its absence could lead to aggression, territoriality, solitary behavior, etc.). Something as important and indispensable as that. I know, I'm reaching. ^-^ But it is possible, isn't it? Otherwise the sheer success of the gay gene (in terms of its ubiquitousness) is a bit puzzling, especially in light of all it's apparent evolutionary disadvantage.

    It's even more intriguing when you consider the fact that it is animals which also show social structures and a suspected degree of sentience which engage in homosexual activities for pleasure (dolphins, apes).

    But then again things to consider:

    1) As tigerbear said: humans have one major difference from other animals - sentience. It could thus be indeed cultural (with or without genetic predisposition), as evidenced perhaps by the different forms and social roles of homosexuals in human societies.

    EDIT: I'm curious as to how this will turn out in the future. Genes have always been selfish, it has one purpose only: to multiply (one of the most extreme examples of which is Death itself. Though there is indication that aging may have been mostly brought on by mitochondrial genes rather than the 'true' cell DNA). With self-awareness we are now theoretically capable of actually influencing our genetics, either through actual physical changes to the genetic code or through deliberate selection of partners for a particular trait (eugenics). Something that has been absent in animals who rely on genetically encoded attraction factor. In a sense, the mind is a threat to the faithful replication of the DNA. Heck, with this you can say that choosing to be with an individual of the same sex is more uniquely human than the mostly instinct-driven heterosexual unions. Simply because it goes against the usual wishes of the genes. But yeah... one problem: it's not a choice.

    2) We don't have the hard numbers. The repression of homosexuality has been almost total that we probably never will get an accurate snapshot of its occurrence in earlier times and even today. Since homosexuals CAN function outwardly as a heterosexual, there will always be numbers that won't be accounted for, especially in societies where homosexuality carries a stigma (which is pretty much every damn large civilization).

    As such, unlike Sickle cell anemia, we can not ascertain its prevalence. Its spread nor its origin,

    3) And finally, like jimbobthedevil said: not every evolutionary trait has a purpose. A good deal of our genetic code has no real use or are 'turned off' sections inherited from earlier animals*. Consider the fact that the human and chimpanzee DNA differ by only around 1.6%, and more, human and Fruit Fly DNA difference is only 50% despite the sheer difference between them phenotypically. You can't help but wonder what the last 50% does or if they even do anything (they might be nonsense code, I don't know).

    It could just have been a truly harmless mutation that got embedded into the genetic code (humans or human ancestors) early on.

    Notes:
    *Like gills or tails or extra nipples or very long canines, which still manifests every now and then in human children born with genetic mutations. Most don't come from nowhere, they are actually atavisms. Phenotypes that are from genotypic 'memories', long forgotten DNA parts that once were manifested in our animal ancestors.
  • Little_Spoon

    Posts: 1562

    Feb 13, 2010 8:41 AM GMT
    I was thinking it was because there are so many more women than men that it was a way to keep some sort of balance.
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    Feb 13, 2010 8:48 AM GMT
    DanielH saidI was thinking it was because there are so many more women than men that it was a way to keep some sort of balance.


    Not true. The human sex ratio is actually around 1:1 with local variations (e.g. China)
  • Little_Spoon

    Posts: 1562

    Feb 13, 2010 8:49 AM GMT
    Sedative said
    DanielH saidI was thinking it was because there are so many more women than men that it was a way to keep some sort of balance.


    Not true. The human sex ratio is actually around 1:1 with local variations (e.g. China)


    and now I know.