Flyway evolution is too fast to be explained by the modern synthesis: proposals for an 'extended' evolutionary research agenda.

In this paper, I argue that to fully grasp the generation and maintenance of variation in the migratory phenotypes of (shore-)birds we need to expand our scientific search image and include developmental processes and non-genetic pathways of inheritance in the explanatory frameworks. Traditionally, studies of micro-evolution of migratory phenotypes were restricted to comparative studies on migratory versus non-migratory taxa, and artificial selection and heritability experiments on quantitative behavioural traits related to migration. Such studies had a focus on the genetic axis of inheritance and were restricted to songbirds. In avian groups such as the shorebird families Scolopacidae and Charadriidae, all but a few island species are migrants, which precludes comparative studies at the species level. Like other taxa, shorebirds have geographically separate breeding populations (either or not recognized as subspecies on the basis of morphological differences) which differentiate with respect to the length, general direction and timing of migration, including the use of fuelling at staging sites and the timing of moult. However, their breeding systems preclude artificial selection and heritability experiments on quantitative traits. This would seem to limit the prospects of evolutionary analysis until one realizes that the speed of evolutionary innovation in shorebird migratory life-histories may be so fast as to necessitate other avenues of explanation and investigation. According to our best current estimates based on mitochondrial gene sequence variation, in Red Knots Calidris canutus considerable phenotypic variation has evolved since the Last Glacial Maximum ca. 20,000 years ago, to the extent that six subspecies are currently recognized. This would be too short a time for the origin of the qualitatively and quantitatively distinct and non-overlapping traits to be explained by random point mutations followed by natural selection, although we cannot dismiss the possibility of previously unexpressed (standing) genetic variation followed by selection. I argue that, to understand the flyway evolution of such shorebirds in the 'extended' evolutionary framework, we need to give due attention to developmental versatility and broad-sense epigenetic evolutionary mechanisms. This means that experimental studies at the phenotypic level are now necessary. This could involve a combination of observational studies in our rapidly changing world, common garden experiments, and even experiments involving global-scale displacements of particular migratory phenotypes at different phases of development. I provide suggestions on how such experiments could be carried out. [ABSTRACT FROM AUTHOR]