Abstract.A test of the niche width variation hypothesis was conducted in the three Mediterranean littoral species of the familyTrochidae: Monodonta turbinata, M. turbiformisandGibbula richardi.Allozymic variation encoded by 26 gene loci was analyzed, and the levels of polymorphism (P), heterozygosity (H) and allele diversity per locus (A) were estimated and appear to support the niche‐width variation hypothesis. The broad‐niche species,M. turbiformisandG. richardiare more polymorphic and heterozygous than the relatively narrow‐nicheM. turbinata(P = 0.571; 0.731 and 0.417; H = 0.108; 0.116 and 0.078; A = 2.025; 2.489 and 1.965, respecti
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