Adaptive and Demographic Responses of Plankton Populations to Environmental Change

Because of their large population sizes, short generation times, and clonal mode of propagation, microorganisms should often be the first members of a community to respond evolutionarily to temporal changes in the environment. Because the planktonic microbial community directly or indirectly influences all other members of aquatic ecosystems, it is useful to have a general theory for the magnitude and limits of such response. Models are presented for the expected dynamics of evolutionary change for the mean and variance of a quantitative character under natural selection toward a fixed or a moving optimum. It is also shown how the rate of population growth is related to the phenotypic composition of the population and the selective aspects of the environment. These models, which lead to the identification of extinction thresholds for the rate of environmental change beyond which a population cannot maintain itself, provide a heuristic basis for understanding the response of ecosystems to environmental perturbations. The analyses also indicate that clones of microorganisms isolated into novel laboratory environments are likely to undergo substantial evolutionary change over periods of a few hundred days, which raises questions about the utility of such cultures for infemng ecological properties of natural populations. Models for the growth of natural microbial populations typically assume that the species involved are static evolutionary entities that do not adapt genetically to biotic or abiotic sources of environmental change. However, it is well known that relatively small populations confined to laboratory chemostats and initiated as clones can often evolve new variants in response to selective challenges. In nature, it is common for local population sizes of planktonic organisms to be in the neighborhood of loL2 or many orders of magnitude larger. The mutation rate per locus is on the order of per generation and the characters upon which natural selection operates are often encoded by tens to hundreds of loci (Falconer 198 1; Lande 1981), so the opportunities for fa-