Abrupt changes in migration routes

Are there any known ways in which a sudden change in migration route could evolve? Clearly, if a single step mutation took a bird to a new wintering area where it could survive, this genetic change could be passed on to subsequent generations, leading to the establishment of a new wintering area, as in the central European Blackcaps Sylvia atricapilla discussed in Chapter 20. This is one way in which a big directional change could occur. However, most such mutations are likely to take birds in inappropriate directions or across hostile terrain that they would be ill-equipped to cross, leading to their elimination. Even if they survive, they will be few in number and most likely, for lack of choice, will breed subsequently with birds with normal directional preferences, and the resulting hybrids will move in an inappropriate direction. This type of selection would also hinder the merging of populations with different directional tendencies (see later).

Other suspected mechanisms through which new routes and wintering areas could evolve in one step far from the original include 'mirror-image migration' and 'reversed direction migration', as discussed in Chapter 10, the latter operating mainly on a west-east axis. Again, the directional error would need to be under genetic control, and favoured by selection for it to be passed on to future generations, and the process could be facilitated if the aberrant individuals mated with one another, rather than with birds with 'normal' directional preferences.

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