Partial migration, it may be recalled, is the tendency for only some individuals in a population to migrate. Where individuals leave a high-latitude population each year to winter at a lower latitude, they encounter on their return to breeding areas other individuals that have stayed there year-round. The two sectors of the population can then interbreed (though not necessarily as often as expected by chance), so that resident and migratory sectors are not reproductively isolated. In such species, migration is typically stimulated by competition, including dominance interactions in relation to prevailing food supplies, so that in any one year the subordinate members of a population are most likely to migrate (Chapter 12).
In contrast, where individuals leave a low-latitude population to breed at higher latitude, while other conspecifics remain behind to breed at low latitudes, the two sectors of the population become reproductively isolated because they breed in different areas usually at partly different times. This separation could lead to divergence of the two populations, as in the Prairie Warblers mentioned above, first to subspecific then perhaps to specific level. Because individuals from one population can then reproduce only if they migrate to breed at the higher latitudes to which they have become adapted, migration can at the same time become genetically (rather than behaviourally) controlled - obligate rather than facultative. The two processes leading to genetic control of migration and genetic divergence are, therefore, likely to occur hand in hand. This type of partial migration was called population partial migration by Jahn et al. (2004), in order to draw the distinction between 'individual level' and 'population level' processes, the one in which migration is condition dependent (from breeding to wintering areas), involving only a proportion of individuals moving each year from high to low latitudes, and the other in which it is genetically dependent (from wintering to breeding areas), involving all high latitude individuals wintering at low latitude. Possibly both result at different stages in the same process of spread from low to high latitudes. Initially, as birds spread out of an ancestral range, natal phi-lopatry leads them to develop a measure of reproductive isolation in their new breeding area, which accentuates as they spread further, and breed at a progressively different time, and in a different daylength regime.
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