Introduction

In discussions about the causes of biological diversity, viewpoints have often been grouped into ‘isms’ (neutralism, selectionism, gradualism, punctuationism), and debated as if they were coherent warring parties. This kind of usage seems more political than scientific. Fortunately, the current group of ‘isms’ is disappearing. Experiments prove hypotheses wrong, however passionately their proponents believe in them. The latest approximations to the truth do not often emerge as victories, or even as compromises, but rather as mosaics of old and new.

Vestiges of the ‘isms’ still remain. For example, it has been customary either to argue that evolutionary replacements and polymorphisms are both predominantly driven by selection or that both are predominantly neutral. Yet, as first pointed out by Fisher (1930), most replacements could be driven by selection even if most polymorphisms were neutral (because selected replacements would contribute little to the standing variation). It is worth considering Fisher's argument, which did not allow for many polymorphisms maintained by selection, to see whether it is compatible with current data.

The debates between “neutralists” and “selectionists” were initially about amino acid replacements in evolution. Here it has transpired that the selectionist view was nearer the truth.