Red Queen Dynamics Research Articles

On evolution under asymmetric competition

The evolutionary consequences of asymmetric competition between species are poorly understood in comparison with symmetric competition. A model for evolution of body size under asymmetric competition within and between species is described. The model links processes operating at the scale of the individual to that of macroscopic evolution through a stochastic mutation–selection process. Phase portraits of evolution in a phenotype space characteristically show character convergence and parallel character shifts, with character divergence being relatively uncommon. The asymptotic states of evolution depend very much on the properties of asymmetric competition. Given relatively weak asymmetries between species, a single equilibrium point exists; this is a local attractor, and its position is determined by the intra- and interspecific asymmetries. When the asymmetries are made stronger, several fixed points may come about, creating further equilibrium points which are local attractors. It is also possible for periodic attractors to occur; such attractors comprise Red Queen dynamics with phenotype values that continue to change without ever settling down to constant values. From certain initial conditions, evolution leading to extinction of one of the species is also a likely outcome.

THE GEOGRAPHY OF COEVOLUTION: COMPARATIVE POPULATION STRUCTURES FOR A SNAIL AND ITS TREMATODE PARASITE.

Gene flow and the genetic structure of host and parasite populations are critical to the coevolutionary process, including the conditions under which antagonistic coevolution favors sexual reproduction. Here we compare the genetic structures of different populations of a freshwater New Zealand snail (Potamopyrgus antipodarum) with its trematode parasite (Microphallus sp.) using allozyme frequency data. Allozyme variation among snail populations was found to be highly structured among lakes; but for the parasite there was little allozyme structure among lake populations, suggesting much higher levels of parasite gene flow. The overall pattern of variation was confirmed with principal component analysis, which also showed that the organization of genetic differentiation for the snail (but not the parasite) was strongly related to the geographic arrangement of lakes. Some snail populations from different sides of the Alps near mountain passes were more similar to each other than to other snail populations on the same side of the Alps. Furthermore, genetic distances among parasite populations were correlated with the genetic distances among host populations, and genetic distances among both host and parasite populations were correlated with "stepping-stone" distances among lakes. Hence, the host snail and its trematode parasite seem to be dispersing to adjacent lakes in a stepping-stone fashion, although parasite dispersal among lakes is clearly greater. High parasite gene flow should help to continuously reintroduce genetic diversity within local populations where strong selection might otherwise isolate "host races." Parasite gene flow can thereby facilitate the coevolutionary (Red Queen) dynamics that confer an advantage to sexual reproduction by restoring lost genetic variation.

Host–parasite interactions: infection of common clones in natural populations of a freshwater snail ( Potamopyrgus antipodarum )

The Red Queen hypothesis for the maintenance of cross-fertilization requires that parasites evolve to infect disproportionately the most common of local host genotypes and prevent their spread. Here we test the idea that digenetic trematodes are non-randomly infecting the common clonal genotypes in four populations of the freshwater snail Potamopyrgus antipodarum. The results show that the most common clone was significantly over-infected in one population, significantly under-infected in two other populations, and proportionately infected in the fourth population by the predominant castrating parasite. We show that, given the expectation of time lags in the system, these results are consistent with expectation under the Red Queen theory.

The coevolution of predator—prey interactions : ESSS and Red Queen dynamics

A model for the coevolution of body size of predators and their prey is described. Body sizes are assumed to affect the interactions between individuals, and the Lotka-Volterra population dynamics arising from these interactions provide the driving force for evolutionary change. The space of phenotypes of predator and prey contains a region, oval in shape, in which the predator and prey species coexist. Within this region, evolutionarily stable strategies (ESSS) and evolutionary saddles may be found, and coevolution may tend to an ESS, develop a Red Queen dynamic, or move to predator extinction. Ten qualitatively distinct kinds of phenotype space are described, depending mainly on the number of ESSS and evolutionary saddles. These varied outcomes are in part due to the range of ways in which density-dependent selection within the prey interacts with density- and frequency-dependent selection on the prey due to the predator. The results point to a ‘loser wins’ principle, in which the evolution leads to a weakening of the interaction between predator and prey. The results also illustrate the deterioration of the environment associated with each evolutionary step of the species and the lack of a net improvement in their mean fitness.