The Role of Environmental Heterogeneity in Maintaining Reproductive Isolation between Hybridizing Passerina (Aves: Cardinalidae) Buntings

Figure 3

Response curves for generalized dissimilarity models of the proportion of pure P. amoena individuals, plus one half of the hybrid individuals, in a population. Results are shown for “80–20” and “90–10” criteria, and for full models (with both geographic distance and environmental variables) and models using only environmental variables. Response curves in GDM are nonlinear, but constrained to be monotonic. The slope of each function is indicative of the rate of change in the proportion of hybrids along the environmental gradient concerned. The maximum height of a response curve indicates the relative importance of that variable in explaining the observed variation in the similarity between populations in the proportion of hybrids. The 𝑦-axes indicate fitted functions 𝑓(𝑥), where 𝑥 denotes the environmental variable concerned. Each of the functions is fitted as a linear combination of I-spline basis functions [41].