In the case of the New Bedford killifish the situation is similar, but more complicated.

ECONOMIST: Pollution and evolution

Both Hudson tomcod and New Bedford killifish, however, have unusual AHR molecules.

ECONOMIST: Pollution and evolution

Dr Hahn has, however, found nine places along the amino-acid chain of killifish AHR where the link in the chain varies between individuals.

ECONOMIST: Pollution and evolution

It is possible that the tomcod and the killifish will then evolve again, just as the peppered moth did, if their PCB-resistant proteins are not absolutely as good as the original versions which they might not be, given that evolution did not find them before.

ECONOMIST: Pollution and evolution