Young, Rebecca L., and Alexander V Badyaev. “
Evolution of ontogeny: linking epigenetic remodeling and genetic adaptation in skeletal structures.” Integrative and Comparative Biology 47 (2): 234 - 244.
AbstractEvolutionary diversifications are commonly attributed to the continued modifications of a conserved genetic toolkit of developmental pathways, such that complexity and convergence in organismal forms are assumed to be due to similarity in genetic mechanisms or environmental conditions. This approach, however, confounds the causes of organismal development with the causes of organismal differences and, as such, has only limited utility for addressing the cause of evolutionary change. Molecular mechanisms that are closely involved in both developmental response to environmental signals and major evolutionary innovations and diversifications are uniquely suited to bridge this gap by connecting explicitly the causes of within-generation variation with the causes of divergence of taxa. Developmental pathways of bone formation and a common role for bone morphogenetic proteins (BMPs) in both epigenetic bone remodeling and the evolution of major adaptive diversifications provide such opportunity. We show that variation in timing of ossification can result in similar phenotypic patterns through epigenetically induced changes in gene expression and propose that both genetic accommodation of environmentally induced developmental pathways and flexibility in development across environments evolve through heterochronic shifts in bone maturation relative to exposure to unpredictable environments. We suggest that such heterochronic shifts in ossification can not only buffer development under fluctuating environments while maintaining epigenetic sensitivity critical for normal skeletal formation, but also enable epigenetically induced gene expression to generate specialized morphological adaptations. We review studies of environmental sensitivity of BMP pathways and their regulation of formation, remodeling, and repair of cartilage and bone to examine the hypothesis that BMP-mediated skeletal adaptations are facilitated by evolved reactivity of BMPs to external signals. Surprisingly, no empirical study to date has identified the molecular mechanism behind developmental plasticity in skeletal traits. We outline a conceptual framework for future studies that focus on mediation of phenotypic plasticity in skeletal development by the patterns of BMP expression.
Young, Rebecca L., Tamara S Haselkorn, and Alexander V Badyaev. “
Functional equivalence of morphologies enables morphological and ecological diversity.” Evolution 61: 2480 - 2492.
AbstractDiversity in organismal forms among taxa is thought to reflect distinct selection pressures across environments. The central assumption underlying this expectation is that taxa experiencing similar selection have similar response to that selection. However, because selection acts on trait function, taxa similarity in selection response depends crucially on the relationship between function and morphology. Further, when a trait consists of multiple parts, changes in function in response to selection can result from modification of different parts, and adaptation to the same environment might result in functional but not morphological similarity. Here, we address the extent to which functional and morphological diversity in masticatory apparatus of soricid shrews reflects a shared ecological characteristic of their diet type. We examine the factors limiting morphological variation across shrew species by assessing the relative contribution of trait function (biomechanics of the jaw), ecology, and phylogeny to species similarity in mandibular traits. We found that species that shared diet type were functionally but not morphologically similar. The presence of multiple semi-independently varying traits enabled functional equivalence of composite foraging morphologies and resulted in variable response to selection exerted by similar diet. We show that functional equivalence of multiple morphologies enabled persistence of differences in habitat use (e.g., habitat moisture and coverage) among species that specialize on the same diet. We discuss the importance of developmental and functional integration among traits for evolutionary diversification of morphological structures that generate equivalent functions.