Tuesday, December 6, 2011

Evolution: Part VI: Cooperation III: Kin Selection

In a number of species, humans included, individuals will often aid others, at their own detriment, when there is no possibility of reciprocation. In extreme examples, this can even extend to total self-sacrifice. How can evolution possibly explain this?
The following material is flagged Green Level. It is intended to reflect material that the author believes to be a matter of consensus among experts in the field. This belief may be incorrect, however; and as the author is not an expert and does not have an expert fact-checking the article, errors may creep in.
It is time we moved beyond the idea of evolution as purely selfish. It is time we looked at it in terms of genes, and how they move through populations. What matters is not the survival of the self, but rather the survival of as much of the self's genome as possible. This is why reproduction happens: in order to preserve the genome.

So, let's look at a population. We will think of this population as a flock of birds. When a predator approaches, a bird has two options: it can sneak away, making its own survival more likely; or it can call out, letting the rest of the flock know a predator approaches but dooming itself. Why would a bird ever cry out?

From a perspective saying that evolution is purely selfish, this would never happen. But it happens all the same, so that must not be strictly true.

But remember: one of the core ideas that absolutely must be true for evolution to work is that descendants must share traits with ancestors, and vice versa. And likeliness to alert the flock at the expense of oneself is a trait. If this trait is inherited, than any given bird's offspring are more or less likely to have it, and whether this is "more" or "less" depends on the parent itself.

So let's look at the probabilities. The "cry out" gene is known to be possessed by this one bird. That bird's parents, offspring, and full siblings each have a one-half chance of having it. Grandparents, grandchildren, aunts, uncles, nephews, nieces, first cousins, and half-siblings have a one-fourth chance. Great-grandparents, great-grandchildren, great-aunts, great-uncles, great-nephews, great-nieces, half-aunts, half-uncles, half-nephews, half-nieces, first-half-cousins, second cousins, and first-cousins-once-removed (whew!) have a one-eighth chance. And so on through the entire flock.

Parenthetical Note:
Anyone familiar with the work of Gregor Mendel will realize that I am absolutely butchering it. For instance, these exact probabilities only work with genomes that have exactly one copy of each trait. If the trait is a dominant and this is a diploid species, as most birds are, the odds of any parent having at least one copy are slightly better than one in two. If it's a recessive, the odds of any parent having at least one rises to one in one. So these exact numbers work for some strange hypothetical haploid species where each individual has two parents, but not for birds or peas, and especially not for bacteria or potatoes. And for that matter, the statistics are leaving out mutations, because the odds of an individual's germ-line mutations affecting a specific gene are pretty small. As in 1 in 100,000, or thereabouts. But it's still pretty close to correct. It's a heck of a lot more accurate than the "electrons are particles whizzing very fast around a nucleus, like planets around a star" Lie To Children in chemistry, or the "centrifugal and Coriolis forces are illusions" one in physics, and those can still make useful predictions. So yeah.

Also, anyone familiar with Bayes' Theorem will notice that it's getting the same treatment. The listed probabilities don't take the background incidence of the trait into consideration. If something's universal, the odds of it being in a given member of the species go way up.
We now return you to your regularly scheduled Topic.
 So, the odds are pretty good that, if you take half of one bird's surviving parents, and half of its offspring, and half of its siblings, and a quarter of its slightly more distant relatives, and an eighth of its extended family, and so on, they add up to more than one bird. So, as far as the gene is concerned, there is more of it outside this bird than inside it, and sacrificing less than half of all existing copies leaves it better off than sacrificing more than half.

This leads to an interesting idea that has shown up in evolutionary thought that, oddly, itself keeps evolving, called group selection. Essentially, this is the idea that, just as individuals compete, so do groups of individuals. To put it in a human perspective: individuals compete, but so do their clans, tribes, nations, and treaty organizations, and cooperation between the individuals of a group leads to more effective functioning as a group. The idea was discredited in its earliest stages (since the death of all individuals leads to the extinction of the group), but the modern and widely-accepted gene-centric view reaches conclusions that are shockingly similar.

Nothing in here is to say, of course, that evolution, competition, and outright conflict never happen between relatives. But it does point to a "no one hits my brother but me" type of attitude between them: that is, they will still compete, but will stand together against an outside threat.

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