Competition in structured zooplankton populations: Coupling population genetics and dynamics using theoretical and experimental approaches

Evolutionary ecologists have long recognized that population genetics and population dynamics are related through differential birth and death rates. Yet, ecologists have been reluctant to embrace this view because of the perception that natural selection operates on a slower time scale than ecological dynamics. However, recent evidence of rapid evolution suggests that the merger of genetics and ecology may improve our understanding of patterns in nature. The Daphnia-algal system is a good example of where this contemporary perspective may be important for resolving the unexpectedly high levels of genotypic diversity.; My thesis explores the interaction between population dynamics and population genetics in Daphnia. I use a combination of population modeling, including the development of new inverse methods, manipulative experiments, and a systematic scaling of ecological complexity to study this interaction. Using forced-food microcosm experiments, I demonstrate that selection among genotypes results from competition for shared food. I test the compelling hypothesis that unstable dynamics maintain diversity through fluctuating selection, and show that it is unlikely for Daphnia because of the narrow parameter region for coexistence. To test this mechanism in natural systems, I synthesize one of the first comprehensive data sets on both Daphnia dynamics and genetics. In agreement with the laboratory and modeling results, I found no evidence for fluctuating selection maintaining diversity.; To begin bridging the gap between microcosms and natural systems, I studied competition in more realistic coupled-algal environments. The results demonstrate that inter-cohort competition significantly slows the rate of selection among genotypes by erasing competitive differences during population cycles. This provides an alternative mechanism for the maintenance of diversity in natural systems.; To close the loop between the interactions of dynamics and genetics, I experimentally manipulated genotypic diversity and studied the effect on population dynamics. Populations with reduced genetic diversity displayed a pattern of high predator and low prey abundance that is governed by food quantity, whereas genetically diverse populations displayed a fundamentally different pattern of low predator and high prey abundance that is governed by food quality. This provides the opportunity for selection and dynamics to play off one another in an evolutionary context.