Self-Organization of Population Structure in Biological Systems

Abstract

Conventional wisdom in the field of population genetics suggests that discrete boundaries between distinctive, geographically adjacent biological populations must reflect the influence of external factors, such as differential selection or a barrier to dispersal [Endler 1977]. Therefore, empirical observations of such boundaries are usually taken as evidence of a previous period of geographical isolation, unless there is an obvious change in environment that coincides with the boundary. A justification for this practice was provided in a seminal paper [Avise 1987], which coined the term “phylogeography” and laid the groundwork for a great deal of recent research (mostly empirical) on the spatial distribution of genetic variation within species. However, the argument provided by [Avise 1987] was, at best, a tenuous one that permitted research in this area to proceed without the burden of a seriously complicating factor. They could see no reason to expect such boundaries to emerge intrinsically and the few empirical examples available at that time seemed to be easily explained as cases of secondary contact after a period of complete geographical isolation [Avise 1987]. Since publication of [Avise 1987], spatial boundaries between gene pools currently exchanging migrants on a regular basis have been identified in many widespread species. Reliance on the argument of [Avise 1987] has led many to infer the earlier existence of a now defunct, complete barrier to gene flow between the gene pools.