Extreme environments are closely associated with phenotypic evolution, yet the mechanisms behind this relationship are poorly understood. Several themes and approaches in recent studies significantly further our understanding of the importance that stress-induced variation plays in evolution. First, stressful environments modify (and often reduce) the integration of neuroendocrinological, morphological and behavioural regulatory systems. Second, such reduced integration and subsequent accommodation of stress-induced variation by developmental systems enables organismal ‘memory ’ of a stressful event as well as phenotypic and genetic assimilation of the response to a stressor. Third, in complex functional systems, a stress-induced increase in phenotypic and genetic variance is often directional, channelled by existing ontogenetic pathways. This accounts for similarity among individuals in stress-induced changes and thus significantly facilitates the rate of adaptive evolution. Fourth, accumulation of phenotypically neutral genetic variation might be a common property of locally adapted and complex organismal systems, and extreme environments facilitate the phenotypic expression of this variance. Finally, stress-induced effects and stress-resistance strategies often persist for several generations through maternal, ecological and cultural inheritance. These transgenerational effects, along with both the complexity of developmental systems and stressor recurrence, might facilitate genetic assimilation of stress-induced effects. Accumulation of phenotypically neutral genetic variance by developmental systems and phenotypic accommodation of stress-induced effects, together with the inheritance of stress-induced modifications, ensure the evolutionary persistence of stress–response strategies and provide a link between individual adaptability and evolutionary adaptation.

Developmental evolutionary biology has, in the past decade, started to move beyond simply adapting traditional population and quantitative geneticsmodels and has begun todevelopmathematical approaches that aredesignedspecifically to study theevolution of complex, nonadditive systems. This article first reviews some of these methods, discussing their strengths and shortcomings. The article then considers some of the principal questions to which these theoreticalmethods have been applied, including the evolution of canalization, modularity, and developmental associations between traits. I briefly discuss the kinds of data that could be used to test and apply the theories, as well as some consequences for other approaches to phenotypic evolution of discoveries from theoretical studies of developmental evolution. Key words: evolutionary theory; evolution of development; genetic architecture; canal-ization; modularity

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using metaphors and visualization techniques like the adaptive landscape and morphospace and their mathemat-ical descriptors for understanding phenotypic evolution. I visualizations relate to each other and how variation at each level of biological organization (e.g. genotype, developmental program, phenotype) is manifest at other levels. In particular, what is the nature of the adaptive and phenotypic landscapes and what is the distribution of mutational effects in those landscapes? Does the structure inherent at one level promote clustering of organisms at a higher (e.g. phenotypic) level? I prefer to view the relationships among genetic chan-ges, phenotypes, and the adaptive landscape a little differently than in the accompanying paper. The environ-