Recent empirical work has documented substantial intraspecific genetic variation in plant growth responses to elevated C02> thus raising the issue of selective responses to elevated C02 in plant populations. In contrast to the well-studied case of selection for heavy-metal tolerance, selection under rising C02 is likely to be density dependent and contingent on local availability of other plant resources. The component processes of natural selection, namely, the expression of phenotypic variation in fitness, the degree to which this variation is heritable, and genetic covariance of other traits with fitness, may each respond in predictable ways to rising C02 conditions. Each of these parameters is examined, with specific reference to reaction norm experiment investigating the responses of genotypes of Abutilon theo-phrasti to population density and C02. Phenotypic variation within plant populations may be enhanced by elevated C02: for example, variation in size-related traits may increase due to accelerated divergence in growth and/or accelerated competitive interactions. Heritabilities of fitness-related traits may often be expected to be higher, because elevated C02 constitutes a novel environmental state not present in the recent selective histories of most plant species. Genetic correlation structure may also be altered, perhaps due to a disruption of functional integration of plant development. Existing data suggest that rising C02 levels will have profound selective consequences on plant populations. However, there is very little data available to suggest what phenotypic characteristics will ultimately be selected. The consequences of such selection on community and ecosystem level processes are also presently unclear, although selection under rising C02 will necessarily result in some loss of genetic variation among plant populations.

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