Using clines to estimate selection

The consequence of this equilibrial relationship between cline width, selection, and average dispersal distance is that evolutionary ecologists can utilize clinal theory to disentangle the relative strengths of selection and dispersal. For example, the strength of selection acting on loci within a cline of known width can be inferred using formula [1] when empirical estimates of dispersal are generated from direct censuses. However, selection estimates are probably far lower than the actual levels. This is largely because direct methods of measuring dispersal regularly underestimate the frequency of long-distance dispersal.

Selection operates either because hybrids of the parental lines are generally less fit, or alternatively, parents or hybrids may be less fit in non-native environments. Those different modes of selection have been called endogenous and exogenous selection, respectively, and they lead to similar consequences when clines occur between differentially adapted species or populations. Most commonly, clines are the result of some mix of both modes of selection. The scenario by which hybrid zones are maintained from selection favoring hybrids within a narrow zone of intermediate habitat (termed bounded hybrid superiority) is thought to be extremely rare, but when present, does not apply to the theoretical expectations.

The distinction between endogenous and exogenous selection is crucial for understanding the potential mobility of the hybrid zone. For example, if endogenous selection operates (selection against hybrids), then the transition zone tends to shift toward any region that had low population densities. Alternatively, hybrid zones maintained by exogenous selection tend to remain stationary at a particular place on an ecological gradient, or shift in geographic position when the environment changes.

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