Number of words: 581

The neo-Darwinian synthesis, as opposed to Darwin’s own version of his theory, is centred on the gene as the unit of natural selection. Genes are discrete entities that can be counted in a population, more or less ignoring the fact that they are in fact sitting in the cells of organisms. Each gene has a frequency in the ‘gene pool’, which is approximated by the number of reproductive individuals possessing it. Successful genes are those whose frequency increases at the expense of unsuccessful alternatives, which decrease in frequency. Genes that cause an animal to be good at caring for its offspring tend to increase because they are carried in the bodies of the offspring cared for. Hamilton realised (s Fischer and Haldane had too, sort of, but they didn’t make much of it) that offspring are not the only category of relatives who share genes, and so therefore might be beneficiaries of evolved caring.

Hamilton derived a simple rule (now call Hamilton’s rule): any gene ‘for’ altruism towards kin will tend to spread through the population, if the cost to the altruist C is less than the benefit B to the recipient devalued by the degree of relatedness r between the two. The degree of relatedness r is a proportion (i.e. a number between 0 and 1) which Hamilton showed how to calculate (its exact meaning is hard, though not impossible, to explain intuitively). Between full siblings, it is 0.5. Between uncles and nephews it is 0.25, between first cousins 0.125. Hamilton had a special interest in social insects, and he made brilliant use of his theory of kin selection in explaining how ants, bees, wasps and (in a rather different way) termites evolved their remarkable habits of social altruism.

A typical underground nest of ants is a factory of propagating genes and spreading them around the countryside. The genes are churned out from the factory packaged up inside the winged bodies of young queens and males. These flying ants (which you might not realize are ants because of their unfamiliar wings) erupt from holes in the ground and fly up to mate on the wing. During her mating flight, each female (young queen) collects a lifetime’s supply of sperm, which she will store inside her body and eke out over the course of a long life. Laden with sperm, the mated female flies off and settles down to dig a hole and found a new nest. In some species, she bites or breaks off her wings, as she will need them no more in her role as subterranean queen.

Most of her offspring will be sterile workers, but the important children from the point of view of gene propagation are the young (winged) queens and males. Workers (all female in the case of ants, bees and wasps; male and female in the case of termites) normally have no prospect of passing on their genes through offspring, and devote their efforts to feeding and caring for their fertile collateral relatives, young queens and males, their siblings or nieces, for instance. A gene that makes a sterile worker care for a sister who is destined to become  a queen can pass into future gene pools, carried there in the body of the young queen. The caring behaviour will be passed on to worker daughters, who will consequently care for young queens and males who can pass it on.

Excerpted from ‘An Appetite For Wonder: The Making of a Scientist’ by Richard Dawkins