Why Worry about Species?

Goldschmidt (1940) defined macroevolution as evolution above the species level and envisaged speciation as the crossing of a threshold of major genomic reorganization. Although his specific ideas of change are now outmoded, the mechanisms and effects of speciation are still hotly debated, and many still see speciation as a vault through the looking glass, leaping past new evolutionary thresholds. The punctuated equilibrium hypothesis, for example, is in search of a mechanism that focuses most morphological change at the time of speciation. The crux of the matter is how to relate genetic and phenotypic variation within a population to divergence between species. Are speciation and the subsequent genetic divergence merely an extrapolation of within-population variation, or is there a consistent jump in genetic and phenotypic difference? If speciation is a special time of reorganization, then the elaboration of large phenotypic differences in evolution would be enhanced both by the rate of speciation and by extinction that is selective relative to a suite of morphologies. Macroevolutionary questions place a magnifying glass on our understanding of speciation and its effects.

Our discussion can best be framed as a series of questions:

1. What are species?

2. Are the genetic differences between species of the same sort as intraspecific differences?

3. Does speciation accelerate differences important in major evolutionary change? Indeed, is that what speciation is about?

4. If the differences are significant, then does this make a difference to theories concerning the process of macroevolution?

5. Are species accidents or adaptations?