Feb 11, 2010 11:24 PM GMT
Potential Evolutionary Role for Same-Sex Attraction
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.
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.
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.
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!
MSUBioNerd saidbachian 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.
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.
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.
Sedative saidDanielH 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)