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Migration is now generally recognized to be an adaptation that allows an organism to exploit resources that vary in both space and time (Southwood, 1962, 1977). A contemporary perspective of migration in insects (Drake et al., 1995) identifies four primary components of the broad migration phenomenon. These are: (i) the changing habitat arena in which migration occurs; (ii) the pattern of population movements through the arena that successive generations follow ( the pathway); (iii) the syndrome of physiological and behavioural traits that allow the insects to make these migrations and that tend to steer them towards resources as these become available; and (iv) the genetic complex that underlies this syndrome. In this holistic view, the many interactions of these components include the process of contemporary natural selection, which acts especially through changes in the arena - weather and climate effects, variations in habitat quality, the incidence of natural enemies, etc. Selection acts continually as populations move along the pathway, adjusting the frequencies of alleles and (along with the normal processes of inheritance and sexual reproduction) maintaining sufficient variation within the population for it to survive and exploit the changing environment. It seems likely that the form of these processes, and of the genetic complex resulting from them, will depend significantly on the extent to which changes in the arena environment are predictable (Southwood, 1977; Wilson, 1995 ). |
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