Résumé
In many plants and animal species, asexuals are more common in marginal habitats such as high latitudes or altitudes than their closely related sexual counterparts. Here we propose a new hypothesis to explain this pattern called "geographic parthenogenesis". In marginal habitats, populations may often exist as metapopulations with high degrees of subdivision and local extinction and recolonization, resulting in genetic bottlenecks during colonization. Our hypothesis states that such dynamics could play a key role in geographic parthenogenesis. Genetic bottlenecks and subsequent drift have stronger negative fitness consequences in sexuals than in asexuals because genetic drift leads to increased homozygosity and inbreeding depression in sexual but not in asexual populations. Migration, leading to inter-population hybridisation, may induce temporary fitness recovery in sexuals. Asexuals arising from such hybrids have an increased likelihood of invading sexual populations because they keep their high fitness, whereas the fitness of sexuals is doomed to decrease due to subsequent inbreeding and inbreeding depression. Therefore, asexuals may replace sexuals in subdivided habitats with local extinction and recolonization while they would not succeed in unstructured habitats without local turnover dynamics.