Phenotypic variance is a function of genetic variability, environmental variation, and the ways in which genetic and environmental variation interact, i.e., VG×E. Reaction norms are a means of conceptually, graphically, and mathematically describing this total variance and are a powerful tool for decomposing it into its constituent parts (i.e., nature, nurture, and, critically, their interaction). A reaction norm is defined as the range of phenotypes expressed by a genotype along an environmental gradient. It is represented by a linear or nonlinear function which describes the value of a phenotypic trait for a particular genotype or group of genotypes in different environments. As such, it is closely related to the concept of phenotypic plasticity, which can be represented by a reaction norm with a non-zero slope (i.e., the phenotype varies with respect to the environment). While the term (which originated as Reaktionsnorm) has been in use for over one hundred years, there has been some debate about the most appropriate way to describe it mathematically. Nonetheless, there is general consensus that a reaction norm has multiple properties, each of which can be the target of selection. Reaction norms are typically described as consisting of: (1) an intercept, elevation, or offset, which describes the mean trait value across all environments, (2) a slope, which quantifies the degree of trait plasticity, and (3) shape or curvature (e.g., linear, quadratic, monotonic). Evidence that trait means and plasticities can evolve separately underscores the necessity of applying a reaction norm framework for studying ecological and evolutionary responses to the environment, because measuring phenotypes in a single environmental context does not necessarily reflect their relative values or diversities in a different context. These contextual differences are particularly important in a world of rapid anthropogenic change and increasing environmental variability. Therefore, in addition to being fundamental to ecological and evolutionary phenomena, reaction norm evolution is relevant for diverse biological fields, including behavior and psychology, conservation and natural resource management, global change biology, agriculture and breeding programs, and human health. Given that evolutionary change is defined by genetic change, we focus this article on variation among reaction norms from different genotypes (i.e., reaction norms that have potentially evolved to be divergent from one another) as well as the forces that promote and constrain reaction norm evolution. For an overview of the literature on plasticity itself (keeping in mind that reaction norms need not be plastic), see the separate Oxford Bibliographies in Evolutionary Biology article Phenotypic Plasticity.
General Overview: Books
There are currently no books dedicated exclusively to the topic of reaction norms or their evolution. However, there are several books on phenotypic plasticity or related evolutionary subfields that address reaction norms and provide a solid basis from which to pursue study on the topic. A newcomer to the topic might wish to start with Schlichting and Pigliucci 1998, which perhaps provides the most thorough introduction to reaction norm evolution, through the lens of phenotypic change in nature, and leaves the reader with the inspiring sense that much more remains to be discovered. There is some overlap between this book and Pigliucci 2001, which was published shortly after. The latter provides a deeper look at the history of phenotypic plasticity research and is perhaps less balanced in tone, and therefore more likely to spark lively debate. These books, as well as DeWitt and Scheiner 2004, make clear that theoretical research on plasticity and reaction norms has long outpaced empirical work, leading to a deeper understanding of what might occur compared to what actually does occur in nature. Those wishing to delve deeper into the foundations of mathematical modeling of reaction norms should consult Levins 1968. Aside from this seminal work, West-Eberhard 2003 is the most important and widely cited contribution to our understanding of phenotypic plasticity and reaction norms on this list. West-Eberhard pioneered the concept of genetic accommodation, which centres plasticity as a major force in evolution by enabling persistence of organisms in extreme or novel environments long enough to produce future generations and thereby the opportunity for evolutionary adaptation to occur (i.e., plasticity-first evolution, covered in depth in the Oxford Bibliographies in Evolutionary Biology article Phenotypic Plasticity. West-Eberhard 2003 also proved to be highly influential in integrating developmental perspectives in evolutionary thinking at all levels of biological organization. Recent books apply this developmental approach—Gilbert and Epel 2015, to evolutionary ecology (eco-evo-devo) and evolutionary medicine, and Sultan 2015, to eco-devo, niche construction, and eco-evolutionary dynamics—using taxonomically diverse, modern examples. The most recent addition by Hendry 2017 places the evolution of phenotypic plasticity into an eco-evolutionary context in an accessible way.
DeWitt, T. J., and S. M. Scheiner, eds. 2004. Phenotypic plasticity: Functional and conceptual approaches. Oxford: Oxford Univ. Press.
A broad survey of topics surrounding phenotypic plasticity, this edited volume ties in evolutionary theory throughout. Several chapters focus on the evolution of plasticity and reaction norms specifically, with more focus on theory and modeling and less on illustrative examples. Individual chapters may be useful jumping-off points into other literature on each of the subtopics and need not necessarily be read consecutively.
Gilbert, S. F., and D. Epel. 2015. Ecological developmental biology. 2d ed. Sunderland, MA: Sinauer.
Essentially a textbook for ecological, evolutionary, and developmental biology (eco-evo-devo) that incorporates plasticity throughout and reaction norms explicitly in developmental biological and historical contexts. Rich with fascinating ecological examples, it is suitable for upper-level undergraduates and above.
Hendry, A. P. 2017. Eco-evolutionary dynamics. Princeton, NJ: Princeton Univ. Press.
A contemporary perspective on how ecology influences evolution that considers the rapid speed at which evolution can occur and therefore influence ecology in turn. The chapter on plasticity is a concise overview of its role in eco-evolutionary dynamics using a reaction norm framework. The rest of the book provides useful context about population dynamics and connectivity and ecosystem structure and function that will be useful to a broadly thinking evolutionary ecologist at any career stage.
Levins, R. 1968. Evolution in changing environments: Some theoretical explorations. Princeton, NJ: Princeton Univ. Press.
Before the 1960s, adaptation was usually modeled by assuming the environment was constant. Levins, a mathematical geneticist, explored how ecology, genetics, and evolution are intertwined within a common theme of the consequences of environmental heterogeneity to adaptation. He introduced the concept of fitness sets to explore how adaptation depends on the range of environments experienced by a genotype.
Pigliucci, M. 2001. Phenotypic plasticity: Beyond nature and nurture. Baltimore: Johns Hopkins Univ. Press.
A broad, historically strong treatment of phenotypic plasticity and reaction norms. In places highly opinionated and idiosyncratic, this book nonetheless offers a conceptually rich review, discussion, and analysis of research on how genes and environments interact to produce organisms capable of highly variable responses to their surroundings.
Schlichting, C. D., and M. Pigliucci. 1998. Phenotypic evolution: A reaction norm perspective. Sunderland, MA: Sinauer.
This book is a highly readable starting point for understanding how reaction norms relate to evolutionary concepts. It includes a historical account of plasticity being incorporated into evolutionary thinking, a thorough overview of the field at the time of publication, and suggestions for specific research projects to move the field forward, which remain largely relevant today. Suitable for all graduate students and professionals studying evolutionary biology.
Sultan, S. E. 2015. Organism and environment. Oxford: Oxford Univ. Press.
This extensive volume broadly discusses the disciplines of ecological developmental biology (eco-devo) and eco-evolutionary dynamics of niche construction that have emerged over the last several decades. It is more up-to-date than most of the books on this list and rich with examples and detailed case studies from diverse taxa. Most suitable for graduate students and above.
West-Eberhard, M. J. 2003. Developmental plasticity and evolution. Oxford: Oxford Univ. Press.
This comprehensive book focuses on how developmental plasticity contributes to our understanding of evolution. It has served as the basis for many advances in our understanding of the evolution of plasticity and reaction norms since. A more advanced text for upper-level graduate students and professionals, but nonetheless an essential read for hardcore scholars working at the nexus of plasticity and evolution.
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