Abstract

The life histories of many marine organisms are characterised by high fecundity, planktonic larvae and a remarkable capacity to colonise remote habitats. These species are commonly viewed as consisting of large, undifferentiated populations that are seldom limited by recruitment. This view leads to the expectation that substantial changes in marine populations generally must be brought about by forces acting over large geographical and temporal scales. However, recent surveys of molecular genetic variation suggest a very different perspective. Sharp geographical discontinuities have been found in the absence of any recognisable barriers to dispersal. These may be interpreted in terms of adaptation to local or regional environments. In many species, low levels of genetic variation or high levels of temporal variance in allele frequencies have indicated surprisingly small genetically effective population sizes. An important question raised by these findings is whether the stability of such populations may be critically dependent on the reproductive success of relatively few individuals. If so, then the potential consequences of habitat or range reduction may be more severe than previously anticipated.

Introduction

The concept of biodiversity unites the variety of life-forms that are the products of evolution with the diversity of habitats and ecosystems in which they are found. These two aspects of biodiversity have traditionally been the subjects of separate disciplines: biosystematics and ecology. However their union is now crucial.