On the Engine of Speciation

Abstract

A major biological problem is how phenotypic clusters, known as species, separated one from the other by prominent phenotypic gaps, are produced. Palaeontological evidence suggests that species remain remarkably stable over long periods of time., When phenotypic change occurs it tends to be abrupt, again producing phenotypic gaps, but now between successive species. A surprisingly simple mechanism might explain both phenomena. Because, by definition, fit traits replace less fit traits, fit traits tend to become common, while maladaptive traits tend to develop low to very low allelomorphic frequencies. Sexual creatures would therefore be expected to prefer mates sporting predominantly common features. This is termed koinophilia. When two polygenic traits initially formed independent, continuously variable, phenotypic clines on a continuous resource gradient, a stochastic computer model of koinophilia invariably caused thede novoevolution of distinct morphospecies separated by prominent phenotypic gaps, involving both traits, under a wide range of selection criteria. Koinophilia reproductively isolated the morphospecies from one another, suggesting that this might be the crucial first step in the development of other barriers to hybridization. The rapidity with which koinophilia canalized the initial continuum of interbreeding phenotypes into reproductively isolated species, and its subsequent defence of those phenotypes against invasion by unusual or unfamiliar phenotypes, might be a paradigm of punctuated equilibrium.