The Evolution of Plant Ecophysiological Traits: Recent Advances and Future Directions
Plants exhibit enormous ecophysiological and functional diversity, which underlies variation in growth rates, productivity, population and community dynamics, and ecosystem function. The broad congruence of these variations with climatic and environmental conditions on local, regional, and global scales has fostered the concept that plant ecophysiological characteristics are well adapted to their local circumstances. For example, the repeated occurrence of plants with CAM (Crassulacean Acid Metabolism) photosynthesis and succulent leaves or stems in severely water-limited environments, and the independent evolution of these traits in numerous plant lineages, provides compelling evidence of the physiological evolution of these water-conserving traits under the influence of natural selection (Ehleringer and Monson 1993). Similarly, studies of the evolution of heavy metal tolerance confirm that natural selection may cause rapid ecophysiological evolution in just a few generations, leading to local adaptation in populations just a few meters apart (Antonovics et al. 1971). Many ecophysiological traits—considered here as all aspects of resource uptake and utilization, including biochemistry, metabolism, gas exchange, leaf structure and function, nutrient and biomass allocation, canopy structure, and growth—are likely to influence fitness and undergo adaptive evolution. Traits affecting the assimilation and use of resources such as carbon, water, and nutrients directly influence plant growth. Patterns of resource allocation to growth, reproduction, defense, and stress tolerance are also likely to be under strong selection. Phenotypic plasticity, the expression of different phenotypes by the same genotype in response to environmental variation, also affects plant function and, hence, fitness in diverse environments. However, only recently have plant biologists directly studied selection in natural populations. Moreover, until the last few years, little was known about the genetic basis for evolutionary change in ecophysiological traits, or about genetic, developmental, or phylogenetic constraints on the evolutionary response to natural selection. The authors of this article presented research findings and discussed future directions in evolutionary plant ecophysiology in a symposium at the 1998 annual meeting of the Ecological Society of America. This article presents an overview of advances in the field and addresses a number of questions regarding the evolution of ecophysiological traits: How are ecophysiological traits related to fitness? How do patterns of natural selection for these traits vary with the environment? What is the genetic basis for ecophysiological traits? How much genetic variation for these traits exists in natural populations? How do genetic and developmental constraints influence the adaptive evolution of ecophysiological traits? What is the evidence for adaptive evolution of ecophysiological traits in natural populations? Do closely related populations or species demonstrate ecophysiological divergence, and do their differences reflect patterns of environmental variation as expected on functional grounds? What are the macroevolutionary patterns in ecophysiological traits, viewed from a phylogenetic perspective? Do these patterns suggest constraints on ecophysiological evolution or do they suggest high evolutionary lability and frequent convergence? An ecophysiological trait can be considered adaptive if it has a direct impact on fitness in natural environments. Here we examine the successes and limitations of varied perspectives from which this problem has been addressed. The most straightforward evidence for the adaptiveness of ecophysiological traits is the observation of correlations between traits and fitness in natural populations, but this approach has proven problematic. For example, direct correlations between photosynthetic rates and fitness are rarely observed in natural populations (e.g., Farris and Lechowicz 1990 and references). Moreover, even when correlations are observed, it is difficult to determine whether individual traits contribute directly to variation in fitness or whether these relationships reflect indirect selection via correlated traits. For example, a study of Plantago lanceolata found a significant positive correlation between photosynthetic capacity and reproductive dry weight, but correlations were also observed for corm diameter, number of leaves, leaf leaf weight, and and it is to that variation in photosynthetic directly to individual these when traits may influence selection and provides a for selection on ecophysiological traits. 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