Abstract
Population genetics and phenotypic structures are often predicted to vary along the geographic range of a species. This phenomenon would be accentuated for species with large range areas, with discontinuities and marginal populations. We herein compare the genetic patterns of central populations of Coccinella septempunctata L. with those of two phenotypically differentiated populations considered as rear‐edge populations and subspecies based on phenotype (Algeria and Japan). According to the central‐marginal model and expected characteristics of rear‐edge populations, we hypothesize that these rear‐edge populations have (1) a reduced genetic diversity, resulting from their relative isolation over long periods of time, (2) a higher population genetic differentiation, explained by low contemporary gene flow levels, and (3) a relationship between genetic diversity characteristics and phenotypes, due to historical isolation and/or local adaptation. Based on genotyping of 28 populations for 18 microsatellite markers, several levels of regional genetic diversity and differentiation are observed between and within populations, according to their localization: low within‐population genetic diversity and higher genetic differentiation of rear‐edge populations. The genetic structuring clearly dissociates the Algerian and Eastern Asia populations from the others. Geographical patterns of genetic diversity and differentiation support the hypothesis of the central‐marginal model. The pattern observed is in agreement with the phenotypic structure across species range. A clear genetic break between populations of Algeria, the Eastern Asia, and the remaining populations is a dominant feature of the data. Differential local adaptations, absence of gene flow between marginal and central populations, and/or incapacity to mate after colonization, have contributed to their distinct genotypic and phenotypic characteristics.
Highlights
Processes such as genetic drift, gene flow, and natural selection impact the distribution of the genetic diversity and structuring across a species range
Ecology and Evolution published by John Wiley & Sons Ltd
Our results suggest the presence of three main clusters with the populations from Algeria and Eastern Asia (Japan and China) highly genetically separated from the rest of the populations sampled
Summary
Processes such as genetic drift, gene flow, and natural selection impact the distribution of the genetic diversity and structuring across a species range. These processes may be strongly affected by both the species evolutionary history and its present demographic characteristics, such as population size, biotic and abiotic factors it might experience, or habitat fragmentation. Marginal populations are expected to be less genetically diverse and to present a potentially higher genetic differentiation, relative to central populations These patterns are supported by numerous empirical studies, the decline in genetic diversity at range limits is not an ubiquitous trend (see Rajora et al 1998; Gapare et al 2005; Eckert et al 2008; Neiva et al 2012).
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