Although geneticists now accept that much allozymic variation is adaptive, ecologists tend to ignore all non-visible variation; in practice they treat inherited variation as if it was neutral. Studies on the House mice of Skokholm Island, Wales have shown that strong natural selection may operate under conditions where the animals are physiologically stressed (particularly by cold). The resulting genetic changes mean that the genetic constitution of the population can change considerably over periods of a few months; this seems likely to be the reason why physiological traits which affect survival in one year are poor predictors of survival in another year. Morphometric and haematological traits are associated with variation at some allozymic loci (especially Hbb); although such traits directly affect phenotypes, their changes with time are complex. Any full description of the ecological response of a population to a variable environment clearly requires a better understanding of adaptation and gene-environment interactions than exists at the moment; deterministic models and those assuming equilibrium seem to be inappropriate.