Wednesday, February 11, 2009

Chapter VII

Permit a humble supplicant to pose a question to the biologists of this outfit. In regards to warning calls, it was said the caller announcing his location to the predator by making noise isn't too much of a disadvantage. I would agree. Even if it is a disadvantage, it probably isn't so much of a disadvantage that the trait would be deselected. Callers rarely are killed as a result of their behaviour. Therefore, even if the behaviour isn't advantageous to the individual (while it is undoubtedly advantageous to the group) would not the trait still be passed down because it is not so disadvantageous as to warrant deselection - especially if it confers a general advantage to the whole?

Cannot this logic be applied to those species that produce sterile offspring? If giving birth to some sterile offspring does not destroy utterly the parent's prospects of reproductive success, since they may produce other offspring that are fertile, why should a variation with the tendency to produce some sterile offspring be wiped out of existence if producing sterile offspring does not hinder their fitness as an individual and enhances the fitness of the family or group?

I throw this question out into the void.

1 comment:

Dr. Fox said...

Hi David,

The question you're asking is essentially a quantitative, empirical one. It all comes down to just how disadvantageous the altruistic act is to the one who performs it, and how indirectly beneficial it is, via its beneficial effect on relatives (or members of the same group, if one is thinking in terms of group selection rather than kin selection).

A famous theoretical result in evolutionary biology, due to Bill Hamilton, is called Hamilton's Rule: altruism is favored by selection if and only if r > b/c, where r is the coefficient of relatedness of the altruist to the beneficiary, b is the fitness benefit of the altruistic act to the beneficiary, and c is the cost of the altruistic act to the individual performing it. Hamilton's Rule is the core of the idea of kin selection. Obviously, I'm glossing over a lot of technical details here, but the basic idea should be clear enough. Altruism is more likely to be favored by (kin) selection if relatedness of altruist and beneficiary is high (i.e. they're close kin), if the act is highly beneficial to the beneficiaries, and if the act is of low cost to the performer. In the limit of maximally-high relatedness (i.e. r=1; this is your relatedness to your identical twin), selection will favor any act for which the fitness benefits exceed the costs (which should make intuitive sense).

The other issue is that not all traits favored by selection necessarily increase in frequency if there are other factors at work. In particular, genetic drift (roughly speaking, random variation in the realized fitnesses of identical organisms) imparts an element of chance to the direction of evolution, especially in small populations. Put simply, if intrinsically-fit individuals are simply unlucky, their traits will not be passed on in future generations. Genetic drift is most important in small populations; in large populations bad luck and good luck tend to cancel out (to a precise extent that can be quantified mathematically).

I raise the issue of drift because it gets to your suggestion that some altruistic acts might have such low costs as to be effectively cost-free, so that it's not at all surprising that they would evolve. How low the cost must be before an act is "effectively" cost-free is a function of population size. In a small population, the effect of genetic drift would be to swamp the weak selection disfavoring altruists who perform a low-cost act and favoring "selfish" individuals who don't. However, in a large population even tiny differences in fitness are "visible" to natural selection and will affect which traits increase in frequency and which decline.

If you go back to earlier chapters, you'll see that Darwin himself was aware that large populations create the optimal conditions for natural selection to act (in part for other reasons besides those noted above).

As an aside, I don't actually recall off the top of my head any specific studies on the risks taken by alarm callers. I'm merely sure such studies must exist, and that at least some of them likely reveal that giving an alarm call isn't necessarily as risky as it might appear (which doesn't mean it's risk free).