Species Selection

    Suppose that larger-bodied species lineages tend to speciate more rapidly, and that the average rate of extinction (the inverse of the species duration) is also less for larger species. Over time, this will result in a species trend, in which the clade becomes more speciose and each species on average larger. Careful examination of the clade shows that (a) speciation events that produce smaller-bodied species are equal in number to those that produce larger, and (b) that they contribute the same total amount of morphological change, so that the direction of speciation plays no role in the formation of the trend.

Text material © 2005 by Steven M. Carr