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area-effect speciation Speciation by the increased differentiation of two subspecies with incompatible gene complexes, so that selection is strongly against hybrids. The phenomenon is observed among slow-moving or sedentary organisms (e.g. snails and plants) occupying areas of habitat that remain unchanged for long periods (sometimes thousands of years) and within which stable local populations of particular polymorphs occur. As a result of strong selection for certain loci (see locus), particular alleles become more frequent, together with those genes that are compatible with them, thus forming gene complexes and consequent heterogeneity among the polymorphic populations. The fecundity of hybrids is low and a steep cline develops, sometimes over a distance of metres.