Dimensions of genotypic and phenotypic divergence in marine metapopulations: space, time, and ontogeny
The tussle between selection and gene flow has considerable implications for evolution. As directly opposing counterparts, divergent selection should drive differentiation among subpopulations, whilst gene flow homogenises subpopulations. The balance between these two forces determines the evolutionary trajectories within and among subpopulations. However, many different outcomes can manifest, contingent on eco-evolutionary conditions, which makes predicting the product of selection–gene flow interactions challenging. Two particularly intriguing considerations are adaptive evolution amidst high gene flow and variability in selection–gene flow interactions.
Under strong gene flow, considerable selection is required to overcome homogenisation. Successful adaptation is predicted to produce genotype-phenotype-environment associations. However, phenotypic plasticity might be favoured under high gene flow because it facilitates phenotype-environment matching when there is appreciable likelihood for genotype-environment mismatches. Species with extensive dispersal therefore represent interesting systems to investigate the conditions that local adaptation emerges, and contributions of directly heritable and plastic components to phenotypes, amidst homogenising gene flow.
Additionally, factors affecting the pattern of selection and gene flow are not static, and selection–gene flow interactions are potentially variable across different scales. Spatial and temporal variation might exist in both these forces. Moreover, dispersal might be limited to specific life stages, or selection may not be consistent across an individual’s lifetime. Collectively, the ecological dimensions of space, time, and ontogeny have potentially complex ways of structuring genotypic and phenotypic variation within and among subpopulations. Understanding when, where, and how this variation is structured can elucidate the effect of selection–gene flow interactions on biodiversity and genotype-phenotype-environment associations in natural settings.
Marine species are extremely useful for studying how space, time, and ontogeny structure biological variation in high gene flow systems. Many marine species occupy discrete habitat patches in metapopulation-like systems. Considerable dispersal potential affords high connectivity, yet dynamic marine environments cause significant spatial and temporal heterogeneity in processes that modulate gene flow and selection. A vast majority of marine organisms also have complex life cycles, which sets the stage for possible selective antagonism across ontogeny, and dispersal that is restricted to early life stages (particularly in benthic taxa).
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