Bimodal hybrid zones and the scale of a snail

Schilthuizen, Menno (2000) Bimodal hybrid zones and the scale of a snail. Trends in Ecology & Evolution, 15 (11). p. 469. ISSN 0169-5347


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In their recent TREE perspective, Jiggins and Mallet discussed the intriguing distinction between unimodal hybrid zones (where the hybrid zone is made up largely of recombinant genotypes) and bimodal zones (where recombinants form the minority)1. They indicated that bimodality is associated with assortative mating, and that it might be the stage where parapatric speciation and reinforcement take place. Two aspects of uni- or bimodality remain unexplored in their paper. The first of these is the fact that deciding whether a zone is uni- or bimodal depends on the sampling scale. A unimodal hybrid zone would be classified as bimodal if the sampling area was larger than the zone itself, which is a real possibility in organisms where limited capacities for dispersal produce extremely narrow hybrid zones. The second aspect, which might warrant more attention, is the fact that a single hybrid zone might be unimodal in some places and bimodal in other places, depending on the environmental circumstances. Both these phenomena are present in the hybrid zone between the land snails Albinaria hippolyti aphrodite and Albinaria hippolyti harmonia in Crete. This zone runs for a distance of approximately 10 km along a cliff where rugged terrain changes into more gently undulating hills2. In places where the environmental transition is gradual, the hybrid zone is more than 300 m wide. Where the ecotone is more abrupt, the hybrid zone can be as narrow as a few metres. Using a standard sampling area of 10 3 10 metres (and based on morphological hybrid indices and allozymes3), we would have to classify the 300 m hybrid zone as unimodal and the narrow hybrid zone as bimodal. Choosing a larger sampling area would render both situations bimodal, whereas unimodality would appear at extremely small sampling areas. The histographical representation of any hybrid zone should not be used to infer speciation without reference to the ratio between the sampling area and the dispersal distance of the organism under study. As it happens, in Albinaria, dispersal is only of the order of one metre per year4, thus assortative mating and reinforcement are unlikely where the hybrid zone is wide because parental types are not within cruising range of one another, whereas they might be in the narrower sections of the zone.

Item Type: Article
Keyword: Ecology, Evolutionary biology, Genetics and heredity, Snail, Hybrid zones
Subjects: Q Science > QH Natural history > QH301-705.5 Biology (General) > QH540-549.5 Ecology
Q Science > QL Zoology > QL1-991 Zoology > QL360-599.82 Invertebrates
Department: INSTITUTE > Institute for Tropical Biology and Conservation
Depositing User: ADMIN ADMIN
Date Deposited: 19 Jan 2012 13:38
Last Modified: 19 Oct 2017 12:34

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