Introduction

Allopatry, meaning 'in another place', describes a population or species that is physically isolated from other similar groups by an extrinsic barrier to dispersal. From a biogeographic perspective, allopatric species or populations are those that do not have overlapping geographic ranges (Figure 1a). In contrast, 'sympatric' (Figure 1b) and 'parapatric' (Figure 1c) are used to describe species or populations that have overlapping and abutting ranges, respectively.

In the fields of ecology and evolutionary biology, allopatry is typically used to describe populations that are isolated to such a degree that dispersal and gene flow is reduced to negligible levels, such that populations are evolutionarily independent. For that reason, allopatry is not usually defined by the characteristics of barriers per se, but by the effects of barriers on gene flow and genetic differentiation. For example, a wide river may restrict the dispersal of many small terrestrial mammals, but the same barrier will not hinder the movements of most birds. 'Allopatry' and 'allopatric' are therefore encountered in discussions of speciation, the process by which one species splits into two. In fact, allopatry is often used synonymously with 'allopatric speciation', the process in which intrinsic (i.e., genetic) reproductive isolation evolves between geographically separated populations. Although new species may also arise from sympatric and parapatric populations, all evolutionary biologists agree that allopatry is a common, if not the most common, mechanism by which new species arise.

Some ecologically similar species have allopatric distributions despite the absence of obvious barriers between them. Distributional studies of species on island archipelagos often reveal that particular pairs or combinations of species never co-occur, suggesting that interspecific interactions, rather than physical barriers to

Figure 1 Diagrammatic representation of (a) allopatric, (b) sympatric, and (c) parapatric distributions. Allopatric taxa are often separated by a physical barrier to dispersal, such as a river or mountain range. Sympatric taxa may have partially or completely overlapping geographic distributions, whereas parapatric taxa have abutting or continguous distributions.

dispersal, maintain some allopatric distributions. In the absence of experiments directly testing the competitive exclusion of one species by another, these negative or 'checkerboard' distributions provide evidence for interspecific competition. A corollary of this pattern is the prediction that in sympatry, coexisting species should be more dissimilar in ecologically important traits than would be expected by chance.

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