Paleontology is not simply about documenting patterns and trends in the fossil record. Paleontologists seek to go beyond the surface patterns in order to draw conclusions about the underlying evolutionary processes. If the patterns told us nothing about the underlying processes, then it would be impossible to sustain the claim that studying the fossil record can teach us anything about how evolution works. In this chapter I will continue to use Cope's rule of size increase as a case study. I will try to highlight some of paleontology's recent successes in teasing out the causes of evolutionary size increase, while at the same time raising some skeptical doubts and concerns. While it is sometimes possible to draw interesting conclusions about the causes, or underlying dynamics, of evolutionary trends, epistemological modesty is called for. There is only so much that the fossils can reveal.

Varieties of evolutionary trends

The first step is to draw some distinctions among different kinds of historical trends. Paleontologists usually distinguish within-lineage trends from amonglineage trends (McShea 1998; Alroy 2000). A within-lineage trend is one that results primarily from evolutionary forces – especially natural selection – that do their work within evolving populations. An among-lineage trend is one that results primarily from species sorting, or the differential extinction and speciation of lineages. Here it's important to recall that species sorting does not imply species selection.