However, more recently it has been shown that the relationship between spatial and genetic distance is more complex than a simple function of the probability of gene flow.

The model was originally developed for a continuously distributed population ( Wright 1940), but was later extended to a stepping stone model with discrete populations ( Kimura and Weiss 1964 Malécot 1955 Wright 1943). This is the basis of Sewall Wright's isolation-by-distance model, whereby proximal individuals tend to be more similar genetically than distant ones ( Wright 1938, 1940). Thus the probability of gene flow decreases with spatial distance. In most organisms, movements of individuals are constrained relative to the entire range of the species. We hypothesize that isolation-by-distance at this smaller scale is a function of restricted gene flow via seed dispersal. At this scale we observed a positive relationship between genetic and spatial distance along two vertical transects, one at each site.

At two of our sites we used transects (two horizontal and two vertical with respect to slope at each site) to sample at distances ranging from 10 m to more than 5000 m. This pattern could have resulted from recent and rapid postglacial colonization, from more recent human disturbance, or as a function of frequent and random “natural” long-distance colonization. This suggests that genetic drift and gene flow are not in equilibrium for these populations. At these scales we detected no association between geographic distance and genetic distance between populations. We sampled 17 sites with distances of 4 km to more than 1000 km between them. We examined relationships between genetic and geographic distance, using 115 dominant amplified fragment length polymorphism (AFLP) markers to assess genetic structure over a wide range of spatial scales. (Ericaceae) is a subalpine shrub found throughout the Pyrenees and Alps at elevations of 1600–2200 m.

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