Abstract
Local environmental features can shape hybrid zone dynamics when hybrids are bounded by ecotones or when patchily distributed habitat types lead to a corresponding mosaic of genotypes. We investigated the role of marsh-level characteristics in shaping a hybrid zone between two recently diverged avian taxa - Saltmarsh (Ammodramus caudacutus) and Nelson's (A.nelsoni) sparrows. These species occupy different niches where allopatric, with caudacutus restricted to coastal marshes and nelsoni found in a broader array of wetland and grassland habitats and co-occur in tidal marshes in sympatry. We determined the influence of habitat types on the distribution of pure and hybrid sparrows and assessed the degree of overlap in the ecological niche of each taxon. To do this, we sampled and genotyped 305 sparrows from 34 marshes across the hybrid zone and from adjacent regions. We used linear regression to test for associations between marsh characteristics and the distribution of pure and admixed sparrows. We found a positive correlation between genotype and environmental variables with a patchy distribution of genotypes and habitats across the hybrid zone. Ecological niche models suggest that the hybrid niche was more similar to that of A.nelsoni and habitat suitability was influenced strongly by distance from coastline. Our results support a mosaic model of hybrid zone maintenance, suggesting a role for local environmental features in shaping the distribution and frequency of pure species and hybrids across space.
Highlights
Hybrid zones are considered windows onto the evolutionary process (Harrison 1990), providing unique environments for investigating the mechanisms driving reproductive isolation and the role of these processes in generating and preserving biodiversity
We investigated the relationship between habitat and genotype across a naturally occurring hybrid zone between Saltmarsh (Ammodramus caudacutus) and Nelson’s sparrows (A. nelsoni)
We developed ecological niche models for each parental species and hybrids using a maximum entropy method implemented in the program Maxent v.3.3.2 (Phillips et al 2006; Phillips and Dudik 2008)
Summary
Hybrid zones are considered windows onto the evolutionary process (Harrison 1990), providing unique environments for investigating the mechanisms driving reproductive isolation and the role of these processes in generating and preserving biodiversity. Stable hybrid zones are maintained by a balance between dispersal of parental taxa into a zone and selection against hybrids (Haldane 1948; Barton and Hewitt 1985). The selective forces responsible for shaping zone dynamics within a stable hybrid zone, can vary. Selection against hybrids can take many forms, but is often broadly categorized as either environment-independent (endogenous) or environment-dependent (exogenous). Often these forces are not mutually exclusive, and a range of factors, including habitat affinity, behavior, and fitness can shape hybrid zone dynamics within a natural system. Identifying the relative influence of these selective forces can provide new insights into the role and function of isolating mechanisms and their relative predominance across taxa and systems (e.g., Bronson et al 2003; Hamilton et al 2013; Tarroso et al 2014)
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