Animal Diversification

The animals and plants we see today are the result of past evolutionary processes that shaped their adaptation and diversification. To understand how past evolution has shaped the communities we see today, I have begun an analysis of large-scale patterns of diversification using data extracted from molecular phylogenies and the fossil record.

In one analysis, Dr. Jonathan M. Brown, Grinnell College and I have been collecting molecular phylogenies from the primary literature. From these phylogenies we calculate a number of parameters to quantify the rate of diversification and other statistics for each clade. The data we have collated so far is available here. Our analyses of these data indicate that most groups of animals and plants have diversified at very comparable rates to produce today's biodiversity. This was a hugely surprising result, given the great disparity in the numbers of species in various groups (e.g., compare the numbers of species in the various insect orders in the Tree of Life).

Fig. 4 from McPeek and Brown (2007)I have collected another data set from the literature to compare to these results from molecular phylogenies. This data set contains estimates for the number of extant species in each order of insects, teleost fishes, amphibians, reptiles, birds and mammals. These data show a startling result, namely that animal diversity is a linear function of clade age. Clade age or crown group age is the estimated date of the last common ancestor of the species that are alive today. The fact that this relationship is linear implies that all these groups are diversifying at a constant and common rate. This relationship also explains 40% of the total variation in species richness.

Thus, data from molecular phylogenies and the fossil record are consistent in the story they tell about the tempo of diversification of animals.

Selected Publications

McPeek, M. A. 2007. The macroevolutionary consequences of ecological differences among species. Palaeontology 50:111-129.

McPeek, M. A., and J. M. Brown. 2007. Clade age and not diversification rate explains species richness among animal taxa. American Naturalist 169:E97-E106.

Mittelbach, G, G., D. Schemske, H. V. Cornell, A. P. Allen, J. M. Brown, M. Bush, S. Harrison, A. Hurlbert, N. Knowlton, H. A. Lessios, C. M. McCain, A. R. McCune, L. A. McDade, M. A. McPeek, T. J. Near, T. D. Price, R. E. Ricklefs, K. Roy, D. F. Sax, D. Schluter, J. M. Sobel, and M. Turelli. 2007. Evolution and the latitudinal diversity gradient: speciation, extinction, and biogeography. Ecology Letters 10:315-331.