Niche divergence of Abies grandis–Abies concolor hybrids

Abstract

Homoploid hybridization has increasingly been recognized as an important source of genetic variation to support adaptation to changing environmental conditions, the invasion of novel ecological niches and, ultimately, speciation. Although ecological selection pressures have been theorized to play a key role in the stabilization of homoploid hybrid lineages, surprisingly few detailed quantitative studies compare the realized ecological niches of hybrids and their progenitors. In this study, we compare twenty bioclimatic variables (Worldclim) at representative occurrence sites of Abies grandis, Abies concolor, and two classes of their natural hybrids, which are variably intergradient between their progenitors, to assess the potential role of ecological selection in the evolution of the observed morphological patterns of introgression. We find that both hybrid classes occupy distinct habitats that are variably intermediate between those associated with A. grandis and A. concolor for a majority of environmental variables, but with lower precipitation in the summertime in the southern Cascade, Siskiyou, and Sierra Nevada Mountains than associated with either progenitor. We hypothesize that moisture stress and high levels of solar insolation in the summertime associated with hybrid populations have played an important role in the observed patterns of introgression of key traits: leaf angle, specific leaf area, and abundance of adaxial stomates. Phenotypic plasticity in two of these traits may allow hybrids to adapt to an environment with a higher degree of variability in moisture stress. Moreover, in the context of moisture stress associated with predicted climate change, hybrids may have an adaptive advantage over A. grandis.