MOLECULAR ADVANCES IN THE STUDY OF GEOGRAPHIC VARIATION AND SPECIATION IN BIRDS

Abstract

Problems in deciphering the patterns and causes of geographic variation and speciation in birds occupied Ned Johnson (e.g., Johnson 1980, Cicero and Johnson 1998, Johnson and Cicero 2002) and many other ornithologists for much of their lives, but the recent onslaught of molecular studies and associated analytical methods are providing breakthroughs in understanding these evolutionary phenomena. In particular, coalescent theory and Markov chain Monte Carlo (MCMC) applications have shown that bird species are sometimes strongly structured into well-differentiated populations by historical subdivision, high philopatry, and small effective population sizes, whereas other species that have recently recolonized parts of their range are effectively panmictic. These are the sorts of results that were impossible to obtain from studies of geographic variation in phenotypic characters alone. Recovery of well-supported species trees from gene trees is much more likely when multiple genes are sequenced, and provides the means for inferring divergence times and patterns and processes of evolution in birds. As in other vertebrates, patterns of cladogenesis in large clades of birds correlate with major paleoenvironmental changes and associated adaptive radiations, reminding us that much of current biodiversity on the planet had its genesis in the distant past. Received 24 July 2006, accepted 5 February 2007. Resumen.Ornitologos como Ned Johnson, entre muchos otros, han dedicado gran parte de sus carreras a analizar las causas y patrones en la variacion geografka y la especiacion en aves (e.g. Johnson 1980, Cicero and Johnson 1998, Johnson and Cicero 2002). Recientemente, los estudios moleculares y los metodos analiticos asociados han provisto de las herramientas necesarias para entender estos fenomenos evolutivos. En particular, el uso de la teoria de coalescencia y los algoritmos asociados a las cadenas de Markov Monte Carlo han demostrado que algunas especies de aves presentan una estructura poblacional muy marcada, producto de subdiviciones historicas, filopatria y tamanos poblacionales efectivos reducidos, mientras que otras especies que recientemente han recolonizado una parte de su area de distribution son panmiticas. Estos resultados son dificiles de obtener estudiando unicamente la variacion geografica de caracteres fenotipicos. Al igual que en otros vertebrados, los patrones de cladogenesis en grandes clados de aves se correlacionan con eventos paleoambientales y radiaciones adaptativas asociadas, recordandonos que gran parte de la biodiversidad del planeta tuvo su genesis en un pasado lejano. Ned Johnson had a consuming interest in the study of geographic variation and its connection to speciation in birds and was a strong advocate for the biological species concept (e.g., Johnson et al. 1999). He and his students and postdocs embraced new methods of detecting and quantifying variation within and among species as the way to make groundbreaking progress in this ever-evolving field and, thus, his contributions were many and varied. A landmark paper from Johnson's laboratory on the nature of genie variation in birds established the importance of neutrality of molecular markers in analyzing genetic structure in avian species (Barrowclough et al. 1985). Subsequent use of putatively neutral markers elucidated some of the major forces acting on avian populations, including founder effects and bottlenecks, genetic drift, gene flow, and geographic Present address: Department of Natural History, Royal Ontario Museum, Toronto, Ontario M5S 2C6, Canada. E-mail: allanb@rom.on.ca