Evolutionary paleontologists often think of their work as involving a kind of two-step procedure: they begin by identifying and documenting patterns in the fossil record, then they try to draw inferences about the underlying evolutionary processes that give rise to those patterns. The terms “pattern” and “trend” are not exactly interchangeable, but they are close. A trend is one kind of pattern. Scientists standardly define a trend as any persisting directional change in some interesting variable over time. The debates concerning PE and species selection are really debates about patterns and trends. For example, PE involves a claim about how to see certain patterns in the fossil record; while species selection involves a claim about the processes that give rise to such patterns. At this point in the book, I want to throw the door open to some deeper philosophical questions about patterns and trends.

In this chapter, I explore two preliminary questions about trends. First, what does it mean to say that a trend is or isn't real? And second, how is the study of trends related to questions about evolutionary progress?