ABSTRACT
In 1961, renowned Harvard biologist Ernst Mayr asked himself why the warbler he had been observing on the grounds of his New Hampshire summer home began its southward migration on the night of the 25th of August. The answer, he came to realize, was not a simple one. There were ecological causes: being an insect-eater, the warbler would starve in the New Hampshire winter. There were physiological causes: the warbler had an intrinsic capacity to sense the dwindling number of daylight hours. And there were genetic causes: the warbler’s special sensitivity to environmental stimuli indicating the approach of colder climate was programmed into its very DNA. But on Mayr’s view, these myriad reasons for the warbler’s migration were really just of two kinds: the proximate causes dealing with the physiology of the warbler as it related to the photoperiodicity and air conditions in its environment, and the ultimate causes dealing with the bird’s evolutionary history, the way its genetic constitution had been molded by natural selection over many thousands of generations (Mayr 1961).
