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Adaptive evolution toward larger size in mammals

Ben Creisler

A new online paper:

Joanna Baker, Andrew Meade, Mark Pagel, and Chris Venditti (2015)
Adaptive evolution toward larger size in mammals.
Proceedings of the National Academy of Sciences (advance online publication)


There is a long-held notion that bigger body sizes are intrinsically
advantageous. We demonstrate an overwhelming tendency for rapid
morphological change to lead to larger body size in 10 of the 11
largest mammal orders, suggesting that mammals have consistently
evolved toward larger size, most likely as a response to selection
pressure. These results are the first evidence, to our knowledge, from
extant taxa that are compatible with the pattern of increasing body
size through time observed in the mammalian fossil record. By
accommodating variation in the rate of evolution into studies of size
change, we demonstrate that it is possible to detect and reconstruct
accurate historical evolutionary processes by taking advantage of the
wealth of data available from extant species.


The notion that large body size confers some intrinsic advantage to
biological species has been debated for centuries. Using a
phylogenetic statistical approach that allows the rate of body size
evolution to vary across a phylogeny, we find a long-term directional
bias toward increasing size in the mammals. This pattern holds
separately in 10 of 11 orders for which sufficient data are available
and arises from a tendency for accelerated rates of evolution to
produce increases, but not decreases, in size. On a branch-by-branch
basis, increases in body size have been more than twice as likely as
decreases, yielding what amounts to millions and millions of years of
rapid and repeated increases in size away from the small ancestral
mammal. These results are the first evidence, to our knowledge, from
extant species that are compatible with Cope’s rule: the pattern of
body size increase through time observed in the mammalian fossil
record. We show that this pattern is unlikely to be explained by
several nonadaptive mechanisms for increasing size and most likely
represents repeated responses to new selective circumstances. By
demonstrating that it is possible to uncover ancient evolutionary
trends from a combination of a phylogeny and appropriate statistical
models, we illustrate how data from extant species can complement
paleontological accounts of evolutionary history, opening up new
avenues of investigation for both.