Evolution as a critical component of plankton dynamics

Abstract

Microevolution is typically ignored as a factor directly affecting ongoing population dynamics. We show here that density-dependent natural selection has a direct and measurable effect on a planktonic predatorprey interaction. We kept populations of Brachionus calyciflorus, a monogonont rotifer that exhibits cyclical parthenogenesis, in continuous flow-through cultures (chemostats) for more than 900 days. Initially, females frequently produced male offspring, especially at high population densities. We observed rapid evolution, however, towards low propensity to reproduce sexually, and by 750 days, reproduction had become entirely asexual. There was strong selection favouring asexual reproduction because, under the turbulent chemostat regime, males were unable to mate with females, produced no offspring, and so had zero fitness. In replicated chemostat experiments we found that this evolutionary process directly influenced the population dynamics. We observed very specific but reproducible plankton dynamics which are explained well by a mathematical model that explicitly includes evolution. This model accounts for both asexual and sexual reproduction and treats the propensity to reproduce sexually as a quantitative trait under selection. We suggest that a similar amalgam of ecological and evolutionary mechanisms may drive the dynamics of rapidly reproducing organisms in the wild.

Prey evolution on the time scale of predator–prey dynamics revealed by allele-specific quantitative PCR

“…The consumer in our system is an obligately asexual lineage of the rotifer Brachionus calyciflorus (24). Although these animals are herbivores consuming algae, we refer to them here as ''predators'' because they consume individual prey items (cells of the alga, C. vulgaris) whole.…”

Does the avoidance of sexual costs increase fitness in asexual invaders?

“…2). Transitions to obligate parthenogenesis have been observed by different research groups (59)(60)(61) and in one case the genetic mechanism has been elucidated (62,63): obligate parthenogens are homozygous for a recessive loss-of-function allele op (for obligate parthenogenesis). Clones with the genotype op/op are unresponsive to the mixis-inducing protein, a quorum-sensing factor that normally induces sex at high population densities (64).…”

Computer simulations of history of life: speciation, emergence of complex species from simpler organisms, and extinctions

“…However, over longer period of time, one would see not only extinction of some species but also the appearance of new ones. Besides, in many situations, macro-evolutionary changes occur at rates that are comparable to those of the ecological processes [10,11]. The artificial separation of this process into "ecological" time scales and "geological" time scales [12] has been made in many earlier theoretical works only for the convenience of modelling.…”