Mechanisms of divergent natural selection at a brittlebrush hybrid zone
Ecologically based divergent natural selection is presumed to be involved in most speciation events. However, the magnitude of natural selection required in many models of parapatric or sympatric speciation may be unrealistically high, and the specific ecological mechanisms are rarely elucidated. Additionally, most model systems exhibit multiple intrinsic and extrinsic barriers to gene flow making it difficult to disentangle the direct effects of adaptation from other isolating barriers.
Here I show using reciprocal transplant and resource addition field experiments between highly divergent, fully interfertile dune and desert shrubs that selection coefficients of 0.10-0.70 against hybrids and 0.76-0.98 against parental migrants can occur over very short geographic distances despite an interspecific hybridization rate of 17.9%. These patterns were further shown to be driven by a gradient in water availability such that adding water eliminated divergent selective pressures and caused one species to perform best in all habitats. The lack of species fusion at this site thus appears to represent an example of extreme phenotypic divergence maintained almost entirely by adaptation to different habitats.