Meeting Abstract
19.2 Saturday, Jan. 4 10:30 A comparison of two radiations: Jaw morphology of terrestrial squirrels SWIDERSKI, D.L.*; LI, J.; TRAN, L.A.P.; ZELDITCH, M.L.; Univ. of Michigan, Ann Arbor dlswider@umich.edu
The dynamics of adaptive radiations have captured much attention recently, but equally interesting are groups famed for their morphological conservatism, particularly those that are speciose and geographically widespread. One such group, the chipmunks (Tamias), contains 25 species, many notoriously difficult to discriminate without geographic data. Its sister taxon, the Marmotina (ground squirrels) includes 60 species, many that have evolved to exploit grasslands. In this analysis, we compare the evolutionary dynamics of chipmunks and marmotines, examining jaw size and shape disparity for all chipmunks and 49 marmotines, in a phylogenetic framework based on molecular data for 85 of 91 extant species. As expected, chipmunks are less disparate in both size and shape and its subclade disparity is consistently high relative to the total disparity, sometimes slightly exceeding total disparity. In marmotines, within-subclade size disparity is well below the total disparity because several groups have non-overlapping size ranges. Subclade shape disparities also are much smaller than the total but there is more evidence of later convergence. The most strongly supported model for marmotine size evolution is an early burst; for shape, the initial stage of the radiation is a burst of divergence, then subclade morphology is static for only part of the subsequent history. Size and shape show different evolutionary patterns in chipmunks; size evolves by Brownian motion but shape is constrained; the most supported model is a single stationary peak (Ornstein-Uhlenbeck). While chipmunks initially have low rates of jaw shape evolution, they repeatedly converge to densely fill a smaller morphospace. Marmotines partitioned morphospace into distinct regions early in their radiation into grasslands, and less frequently converge.
