Why do species have a skin? Investigating mutational constraint with a fundamental population model

Abstract

Species defy the naive Darwinian expectation that they will continuously adapt and spread. A possible reason that species have this 'skin' is that range-expanding mutations are not advantageous where they occur, and do not occur where they are advantageous. We study the dynamics of such 'pioneering' mutations across the uninterrupted edge of a minimum-contact (meta)population whose range is defined by phenotypically flexible adaptation to a latitudinal environmental gradient. The stochastic spread of these pioneering mutants is a function of the latitude of occurrence and the distance by which they can extend the species range. This produces a 'sieve', which filters out both smaller and larger mutations, but becomes less stringent with increased phenotypic flexibility. Although this will put a brake on evolution, the species can still expand from a hot zone within its own edge, where mid-sized mutations both occur and succeed. The dynamics of the edge therefore cannot completely explain the skin but, combined with the blocking effects of interspecies competition and the exhaustion of appropriate mutation, are capable of explaining range stability, the biodiversity gradient, and punctuated equilibrium. © 2010 The Linnean Society of London.