Evidence from species relationships

On the above view, many long-distance migrants, especially among passerines, have evolved from tropical or subtropical ancestors. One line of evidence cited by Rappole & Jones (2002) in support of this view is based on species relationships. Of the 338 species of migratory landbirds that breed in temperate and boreal North America, 162 (48%) have conspecifics and 78% have congeners that breed in the Neotropics. For the Asian migration system, the numbers are similar: 106 (31%) of 338 species of migrants have conspecific populations that breed in the tropics, while 262 species (78%) have congeners. For the Palaearctic-African system, 301 (16%) of 186 species of migrants have conspecific populations that breed in the tropics, while 139 (75%) have tropical breeding congeners. These numbers, they argued, imply that most of the long-distance migrants that breed in the Nearctic and Palaearctic regions are tropical in origin, although several (e.g. Phylloscopus, Dendroica, Vermivora, Empidonax, Vireo and Myiarchus) have undergone radiations after colonising the northern continents, and subsequent to the evolution of a migratory lifestyle (Rappole 1995). Almost certainly, much of the evolution of these groups pre-dates the last glaciation, and may date back to a much earlier period. There is, of course, no reason why colonisation of the tropics by the descendants of extra-tropical species should not have contributed to this pattern of shared species and genera, so without supplementary information (see later), it is hard to assess the value of this evidence alone. In any case, the evolution of species should not be confused with

1Correcting Rappole & Jones (2002), who gave 42 such species, but this number included some breeding in North Africa, north of the Sahara. The total figures given by Rappole & Jones for North America and Asia are also greater than those given by other researchers, and cited in Chapters 24 and 25.

the different process of the evolution (or modification) of current migration patterns within species. A species may have evolved in a region far removed from that in which it became migratory.

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