Selfish Genes

From SkepticWiki

1 Introduction
2 Example: all successful genes are selfish
3 Example: Genes versus survival
4 Example: Genes versus reproduction
5 Example: Genes versus the good of the species
6 Example: Genes that cause extinction
7 Related Articles

[edit] Introduction

Your genes: they're utterly amoral and they live inside you. Be afraid.

The phrase "selfish gene" was coined by Richard Dawkins to remind people that natural selection favors, not genes that are good for their carriers, nor genes that are good for the species in which they occur, but genes that are good for themselves: that is, genes that are good at promoting their own propagation through the gene pool.

We should emphasize that Dawkins did not mean that we should anthropomorphize genes and consider them as having motives or desires.

[edit] Example: all successful genes are selfish

For most genes, benefiting the organism carrying them is the route to success. To take an example at random, the gene that allows us to metabolize phenylalanine is successful because this is useful to us: it prevents us from dying of phenylketonuria. But this is not to say that the gene is being "altruistic"; the gene needs us to survive in order to propagate itself. Using Dawkin's anthropomorphic language, it is pursuing a policy of "enlightened self-interest"; it so happens that the interests of the gene coincides with our own interests, and the benefit is mutual. But this is not always the case, as we shall see in subsequent examples.

[edit] Example: Genes versus survival

In other cases, the benefit is more ambiguous. The female praying mantis, for example eats the male during copulation, which doesn't seem to benefit him very much (for further examples of this kind, see our article explaining why the phrase "Survival of the Fittest" is misleading). A gene for avoiding females mantises might benefit a male mantis, if we count not being eaten alive as a benefit, but such a gene would never be passed on.

[edit] Example: Genes versus reproduction

Perhaps, you might say, in the light of the previous example, we should redefine "benefit" to be more in line with the law of natural selection: should we not define it in terms of reproductive advantage? But even if we do so, there are genes that put most of their possessors at a reproductive disadvantage. Take, for example, the sterile worker caste found in bees. The genes that make them sterile are the ultimate reproductive disadvantage. The key to this strange situation is that the queen bee carries, but does not express, the genes for being a sterile worker. The existence of the sterile workers ensures that the queen and her grubs are tended to: and tending to the queen and grubs ensures the reproduction of the genes for sterility.

[edit] Example: Genes versus the good of the species

It is easy to see how the selfish action of genes can result in a below-optimum result for the species as a whole. genes cannot get together and agree to practice restraint for the greater good of all. For example, a viral disease might be better able to spread if it didn't incapacitate its victim so much. However, the suffering caused to the victim is a by-product of the reproduction of the virus; in order to reduce it, the virus would have to reproduce at a lower rate. But this would require a gene for not reproducing so much to arise by mutation in one particular virus --- which it might --- and then to spread through the gene pool, which would be in flat defiance of the law of natural selection.

This graph shows how the hourly payoffs of foraging in trees (the green line) and foraging on the ground (the brown line) and the payoff for each mixture of behaviors (the gray line) vary according to how the species collectively allocates its foraging time. The selfishness of genes forces the behavior of the species towards the evolutionarily stable strategy (ESS) rather than to the optimum strategy for the species.

In our article on Evolutionarily Stable Strategies, we discuss the example of birds with a choice between two foraging strategies, each subject to diminishing returns (i.e., the more a strategy is used, the less the payoff) and we showed how the inability of genes to practice collective restraint leads to a below-optimal foraging strategy (see diagram, right). In this example, the good of the species requires genes for less foraging in trees and more foraging on the ground, but the first bird to practice such restraint, when all the other birds were not, would be at a disadvantage, and the gene for such behavior would never manage to get far into the gene pool without being eliminated by natural selection. For the reasons why this should be so, see the main article on Evolutionarily Stable Strategies.

[edit] Example: Genes that cause extinction

The most spectacularly selfish gene of all is the killer X mutation. Not only does it place its carriers at a reproductive disadvantage; it also causes the extinction of the entire species in which it occurs.

Consider a species with sex determined like our own (the "XY" system) so that an individual with two X chromosomes is female; an individual with an X and a Y chromosome is male. On reproduction, each female egg, carrying one X chromosome from the mother, is fertilized by a sperm from the father containing, with equal probability, an X or a Y chromosome, so that the result in the next generation is, again, 50% female, with two X chromosomes, and 50% male, with one X and one Y chromosome.

Now, suppose a new variant of the X chromosome arises with the peculiar effect that, in males, it causes the production of a toxin that kills off all sperm carrying the Y chromosome.

The result of this is that any male with this "killer X" version of the chromosome will exclusively have children carrying at least one copy of the killer X chromosome (one from the male, because no-one can inherit his Y chromosome, and possibly one from the mother, if she also has one or two copies of the killer X chromosome). This means that the killer X chromosome has a selective advantage over the Y chromosome; and it also has a selective advantage over the original X chromosome, by virtue of having an advantage over the Y chromosome that the original X chromosome lacks.

Key: the red curve shows killer X chromosomes as a proportion of all X chromosomes, starting with the killer X as a one-in-a-thousand rarity in generation 0. The blue line shows males as a proportion of the population.

The result of this is shown in the diagram to the right. The killer X chromosome explodes through the gene pool, headlong towards fixation. But at the same time, because males with the killer X chromosome can only pass on X chromosomes, as the proportion of killer X chromosomes in the population rises, so too must the proportion of females.

In the short term, this means that carriers of the killer X chromosome are at a disadvantage, because when the proportion of males in a population falls below 50%, there is a selective advantage to having male descendants (to see why this should be so, consult the section on sex ratios in our article on Evolutionarily Stable Strategies). And it is bad for the species, because in the long run the end result must be that eventually the last male will go extinct, shortly to be followed by his species, in the form of a last generation of virgin spinsters, all or almost all of whom possess two copies of the killer X chromosome.

Again, the killer X genes would do better if only they could collectively organize some sort of system of restraint, for the greater good of all; if, for example, they could agree to let one Y chromosome in ten live, but any such variant would be at a selective disadvantage compared to the more lethal variety. Being "nice" did not help the original X chromosome.

There is still some hope for a species so afflicted. If a mutation arises on the Y chromosome that makes it immune to the effects of the killer X, then this mutation will have a strong selective advantage and spread rapidly through the gene pool, restoring the gender balance and preventing extinction.