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Narrator:

Listen to part of a conversation in a class on animal behavior. Professor:

All right, so we've... we left off with a discussion of altruism. Who can remember what altruism is?

Student:

Um, it's.. .uh, it's when you have an animal that acts to benefit another animal or to the detriment of itself.

Professor:

Yes. Perfect. Now, why would an animal ever do something like that? Well, I'd like to quote from Haldane. In the early 1930s, J.B.S. Haldane the late, great geneticists said that he would lay down his own life to save two of his brothers, or eight of his cousins. That might sound crazy to some of you, but, evolutionarily speaking, Haldane's statement makes perfect sense. You see, for each allele, tsk, Haldane received from his parents, his brother each... his brothers each had a 50% chance of receiving the same allele. Consequently, it's, uh, statistically expected that two of his brothers would

pass on as many of Haldane's particular combination of alleles to the next generation as Haldane himself would, if not more. By the same token, tnk, Haldane and a first cousin (Someone's first cousin is the same as their cousin. Compare second cousin.) would share an eighth of their alleles. You see, their parents, which are siblings, would each share half their

allele

Any one of two or more alternative forms of a gene that may occur alternatively at a given site on a chromosome.

Alleles may occur in pairs, or there may be multiple alleles affecting the expression of a particular trait. If paired alleles are the same, the organism is said to be homozygous for that trait; if they are different, the organism is heterozygous. A dominant allele will override the traits of a recessive allele in a heterozygous pairing (see dominance and recessiveness). In some traits, alleles may be codominant (i.e., neither acts as dominant or recessive). An individual cannot possess more than two alleles for a given trait. All genetic traits are the result of the interactions of alleles.

alleles, and each of their children would receive half of these, of which half on the average would be in common: one-half times, one-half times, one-half equals... anyone?

Student:

One-eighth.

Professor:

So, eight first cousins would therefore pass on as many of those alleles to the next generation as Haldane would himself.

That takes us to William Hamilton. If you don't know who William Hamilton is, well... he'wrote the most influential explanation for the origin of altruism. Hamilton saw Haldane's point clearly: natural selection will favor any strategy that increases the net flow of an individual's alleles to the next generation.

So, Hamilton showed that by directing aid toward kin, or genetically close relatives, an altruist could increase the reproductive success of its relatives enough to compensate for the reduction in its own fitness. Because the altruist's behavior increases the number of alleles in relatives, altruistic behavior will be favored by natural selection. Now, selection that favors altruism directed toward relatives is called kin selection — That's a key term. Although the behaviors being favored are cooperative, the genes are actually behaving selfishly, because they encourage the organism to support copies of themselves in other individuals. I hope that makes sense. The animal is being altruistic, but the underlying genes are still selfish.

Hamilton's kin selection model predicts that altruism is likely to be directed toward close relatives. The more closely related two individuals are, the greater the potential genetic payoff. This relationship is descried.. .described by Hamilton's rule. You know, the one made by William Hamilton. Hamilton's rule states that altruistic acts are favored when the benefits of an altruistic act divided by its costs are greater than the inverse倒數 of its relatedness血緣關聯性. I know that sounds complicated, the formula

You won't be responsible for this formula, but you should know the rationale behind it.

Student:

That seems pretty hard to measure. I mean, R, relatedness, how, how is that measured?

Professor: ,

They call that the relatedness coefficient相關性係數. It is roughly the proportion of genes the two animals share. Umm, siblings would be half related, meaning they would have a coefficient of one half. Cousins, first cousins, like in the example, would have a coefficient of one eighth. But I'm really not a mathematician.

Many examples of kin selection are known from the animal world. For example, Belding's ground squirrels地面灰鼠 give alarm calls when they spot a predator, such as a coyote or a badger. That might actually encourage the predator to attack the calling squirrel, so giving a signal places the caller at risk. The social unit of a ground squirrel colony consists of, um, a female and her daughters, sisters, aunts, and nieces, the whole deal. Males in the colony are not genetically related to these females. By marking all squirrels in a colony with an individual dye pattern on their fur and by recording which individuals gave calls and the social circumstances of their calling, researchers found that females who have relatives living nearby are more likely to give alarm calls than females with no kin nearby. Males tend to call much less frequently.... as would be expected as they are, uh, not related to most colony members.