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HISTORICAL BIOGEOGRAPHY OF THE NEW WORLD SOLITAIRES (MYADESTES SPP.)
Auk, The, Jul 2007 by Miller, Matthew J, Bermingham, Eldredge, Ricklefs, Robert E
SPECIES FORMATION DEPENDS on a fine balance between dispersal, necessary for establishing allopatric populations, and reduced gene flow, which permits genetic divergence. How this balance is achieved and the relationship between dispersal capacities and the diversification of clades are poorly understood (Coyne and Orr 2004). This conflict could be resolved if phases of population expansion and contraction alternated, providing opportunities for the colonization of new regions and periods of reduced gene flow between populations necessary for the accumulation of species differences. Such historical patterns of population expansion and quiescence (Ricklefs and Bermingham 2002) could potentially be visualized by overlaying phylogenetic trees on the geographic distribution of contemporary taxa. Periods of expansion accompanied by long-distance dispersal would appear as nodes separating isolated populations or species in different areas; several such populations formed contemporaneously would suggest a transient phase of expansion occurring over a large area. Alternatively, dispersal might continue at a rate sufficient to establish new populations but not to prevent their genetic divergence (Johnson et al. 2000), in which case one would not expect to see contemporaneous formation of multiple independent lineages, but rather a more haphazard branching pattern within the phylogenetic tree. Testing these alternatives in a phylogenetic framework will ultimately require large samples of clades.
Here, we examine relationships among a single, moderately sized clade of passerine birds, the solitaires of the genus Myadestes (Turdidae), to explore a phylogenetic approach to understanding the balance between dispersal and restricted gene flow. Myadestes includes 13 species of generally sedentary forest songbirds that nonetheless are broadly distributed throughout the Western Hemisphere in montane forests of the Americas from Alaska to Bolivia, throughout much of the West Indies, and, most remarkably, as one of only six passerine lineages to have become established on the Hawaiian Islands, -4,000 km from potential continental source populations (Ridgely and Tudor 1989, American Ornithologists' Union [AOU] 1998). Thus, although Myadestes has achieved remarkable feats of long-distance colonization, continued dispersal has not prevented the formation of highly differentiated populations. However, unlike other genera with multiple sympatric species (e.g., Vireo, Dendroica, Vermivora), species of Myadestes do not occur sympatrically (as "biological" species) except on the island of Kauai in the Hawaiian chain and locally in northern Mesoamerica.
Most species of Myadestes are tropical or Andean. Only one, M. townsendi, occurs north of the Mexico-United States border. It is the only member of the genus that exhibits substantial migratory movements, though some tropical species undertake localized altitudinal migrations (Howell and Webb 1995). Individuals of M. townsendi also are frequent autumn vagrants to eastern North America, as much as 2,000 km from the nearest breeding areas of the species (Bowen 1997). Within the New World tropics, other species of Myadestes are more-or-less continuously distributed, with allopatric replacement, wherever mountain ranges exceed 1,000 m in elevation. Several of these species, including the Andean M. ralloides and West Indian M. genibarbis, have large geographic ranges, allowing comparison of genetic differentiation within and between species and assessment of the genetic cohesion of species by means of gene flow. Assuming that molecular divergence is related to time, we ask whether the colonization of Hawaii and the West Indies, as well as the continental range expansion of Myadestes through Mesoamerica to the Andes, were contemporaneous and might have coincided with marked changes in climate during the late Tertiary.
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