Phenotypic Evolution — A Reaction Norm Perspective

Abstract

Understanding the process of adaptive evolution of phenotypes is a fundamental problem in evolutionary biology. It has been approached from the point of view of population and quantitative genetics, optimality theory, or developmental biology. In the last decade, there has been an explosion of research on phenotypic plasticity (the environmentally induced production of different phenotypes by a single genotype) as well as on the molecular details of development, reflecting the increased recognition of their importance in shaping phenotypic evolution. However, the "hardening" of the neodarwinian synthesis in the 40s led to the largely independent investigation of genetic, developmental nad environmental bases of phenotypic expression. As a result, these different perspectives have not been integrated into a satisfying cohesive view of phenotypic evolution. Phenotypic Evolution explicitly recognizes organisms as complex genetic-epigenetic systems developing in response to changing internal and external environments. As a key to a better understanding of how phenotypes evolve, the authors have developed a framework that centers on the concept of the Developmental Reaction Norm. This encompasses their views: (1) that organisms are better considered as integrated units than as disconnected parts (allometry and phenotypic integration); (2) that an understanding of ontogeny is vital for evaluating evolution of adult forms (ontogenetic trajectories, epigenetics, and constraints); and (3) that environmental heterogeneity is ubiquitous and must be acknowledged for its pervasive role in phenotypic expression. Phenotypic Evolution: A Reaction Norm Perspective can serve as a text for graduate-level courses and seminars on phenotypic evolution or evolutionary developmental biology, and as a supplemental text for evolutionary biology. The extensive references provide links to a wide variety of studies examining the diversity of phenotypes. The book will also be of interest to organismal biologists in general, including ecologists, developmental biologists, and systematists. CONTENTS Preface 1. Phenotypic Evolution and Its Many Facets Fisher versus Wright? Open Questions on the Evolution of Phenotypes Theoretical Approaches Empirical Approaches A Different View: The Developmental Norm of Reaction What Will Follow 2. A Brief History of An Alternative Evolutionary Synthesis The Precursors: Baldwin, Woltereck and Johannsen The Theoretical Framework: Goldschmidt and Schmalhausen The Empiricists: Waddington and Clausen, Keck and Hiesey 3. Reaction Norms and Phenotypic Plasticity Adaptive Phenotypic Plasticity Recent Concepts of the Reaction Norm A Developmental Perspective on Reaction Norms A Genetic Perspective on Reaction Norms Genetic Control of Plastic Responses Gene Expression in Different Environments Phenotypic Plasticity: Target or By-Product of Selection? Why It Makes A Difference Conceptual Summary 4. Allometry On Size and Shape Classical and Recent Studies on Allometry Allometry and the Developmental Reaction Norm Conceptual Summary 5. Ontogeny History The Description of Ontogenetic Trajectories: Heterochrony and Sequence Models Models of Heterochrony The Mechanistic Bases of Ontogenetic Change Heterotopy Non-Heterochrony and Non-Heterotopy What Is This Thing Called Homology? Environmental Influences on Developmental Phenotypes Conceptual Summary 6. Constraints on Phenotypic Evolution: A Central Problem A Brief History of the Concept A Plethora of Constraints and an Attempt to Distill Them Selection and Constraint on the Developmental Reaction Norm Null Models and a priori Expectations Conceptual Summary 7. Phenotypic Integration The History: From Phenotypic Grids to the Sierra Nevada A Conceptual Framework: Evolutionary Quantitative Genetics Morphometrics: Measuring Phenotypic Integration "Classically and Revolutionarily" The Ontogenetic Component: Developmental Integration The Environmental Component: Plasticity Integration Conceptual Summary 8. Epigenetics The Hunt for Developments Master Switches: The Homeobox Genes The Fly: Drosophila The Worm: Caenorhabditis The Plant: Arabidopsis thaliana Epigenetics and Phenotypic Plasticity Limitations to a "Modern" Approach Conceptual Summary 9. Evolution of Developmental Reaction Norms and Phenotypes Integrating Allometry, Ontogeny and Plasticity Theory of Evolution in Heterogeneous Environments The Objects of Selection Building a Norm of Reaction Conceptual Summary 10. Implications and Projections Modes of Evolutionary Change The Origin of Phenotypic Novelty Evolutionary Futures A Research Program Research Projections Applying the Research Program