of these alleles, "C", is dominant to all the others, and any CC or Cx moths are dark (where "x" is any other allele). Another allele, "c", is recessive to all the others, and cc moths are light. Against the originally pale lichens the darker moths were easier for birds to pick out, but in areas, where pollution has darkened the lichens, the cc moths had become rare. Haldane mentions that in a single day the frequency of cc moths might be halved. Another potential problem is that if "ten other independently inherited characters had been subject to selection of the same intensity as that for colour, only , or one in 1024, of the original genotype would have survived." The species would most likely have become extinct; but it might well survive ten other selective periods of comparable selectivity, if they happened in different centuries. Selection intensity Haldane proceeds to define the intensity of selection regarding "juvenile survival" (that is, survival to reproductive age) as , where is the proportion of those with the optimal genotype (or genotypes) that survive to reproduce, and is the proportion of the entire population that similarly so survive. The proportion for the entire population that die without reproducing is thus , and this would have been if all genotypes had survived as well as the optimal. Hence is the proportion of "genetic" deaths due to selection. As Haldane mentions, if , then . The cost Haldane writes I shall investigate the following case mathematically. A population is in equilibrium under selection and mutation. One or more genes are rare because their appearance by mutation is balanced by natural selection. A sudden change occurs in the environment, for example, pollution by smoke, a change of climate, the introduction of a new food source, predator, or pathogen, and above all migration to a new habitat. It will be shown later that the general conclusions are not affected if the change is slow. The species is less adapted to the new environment, and its reproductive capacity is lowered. It is gradually improved as a result of natural selection. But meanwhile, a number of deaths, or their equivalents in lowered fertility, have occurred. If selection at the selected locus is responsible for of these deaths in any generation the reproductive capacity of the species will be of that of the optimal genotype, or nearly, if every is