Most of the time I’m focusing on population genetic time scales when I think of evolutionary change. That is, allele frequency shifts within a species level lineage, or narrower. Since this is amenable to experimental analysis obviously there are advantages. But sometimes I really wonder if I’m doing a disservice to myself not paying more attention to examinations of evolutionary change on the scale of tens of millions of years and across whole clades which might have thousands of species. A new paper in PLoS BIOLOGY, Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage
is very interesting. Here’s the author summary:
Animals display huge morphological and ecological diversity. One possible explanation of how this diversity evolved is the “niche filling” model of adaptive radiation—under which evolutionary rates are highest early in the evolution of a group, as lineages diversify to fill disparate ecological niches. We studied patterns of body size evolution in dinosaurs and birds to test this model, and to explore the links between modern day diversity and major extinct radiations. We found rapid evolutionary rates in early dinosaur evolution, beginning more than 200 million years ago, as dinosaur body sizes diversified rapidly to fill new ecological niches, including herbivory. High rates were maintained only on the evolutionary line leading to birds, which continued to produce new ecological diversity not seen in other dinosaurs. Small body size might have been key to maintaining evolutionary potential (evolvability) in birds, which broke the lower body size limit of about 1 kg seen in other dinosaurs. Our results suggest that the maintenance of evolvability in only some lineages explains the unbalanced distribution of morphological and ecological diversity seen among groups of animals, both extinct and extant. Important living groups such as birds might therefore result from sustained, rapid evolutionary rates over timescales of hundreds of millions of years.
As this paper is predicated on nifty statistical analysis one has to be careful at taking the results at face value. Subsequent reanalysis might yield a different conclusion. But it is certainly an intriguing possibility that clade-level selection of some sort might be operating. I’m still very skeptical of what to even think about this, or how to conceptualize the dynamic. But that’s often a good thing.
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