Eco-evolutionary dynamics in predator-prey systems 捕食者-被食者系における生態-進化動態
Eco-evolutionary dynamics in predator-prey systems
SummaryRecent studies have revealed that ecological and evolutionary dynamics have close interactions. Not only ecological dynamics affect adaptive evolution, evolution can occur as rapidly as ecological dynamics (i.e., rapid evolution) and can also affect ecology including population dynamics, community structures, and even ecosystem functions. Ecological settings cause adaptive evolution, and then trait evolution modifies its surrounding environments and thereby changes selection pressure: such feedbacks between ecology and evolution are called as eco-evolutionary dynamics. Recently it was cautioned that predicting future biological dynamics would be difficult ignoring eco-evolutionary feedbacks. Understanding eco-evolutionary dynamics is crucial not only for the consilience of basic ecology and evolutionary biology but also for applied ecology: conservation and management of the wildlife. Here I theoretically investigated eco-evolutionary dynamics in one of the most common interspecific interactions, predator-prey systems. Because predation is tightly related to organisms’ fitness, eco-evolutionary dynamics is widespread in predator-prey systems and important to predict future dynamics.In chapter 2, I focused on eco-evolutionary dynamics of phenotypic plasticity and population dynamics. Understanding causes and consequences of population cycles has been an important research focus as cycles can cause extinction of populations and one third of population dynamics in the wild shows periodic dynamics (cycles). Plankton predator-prey systems in chemostats (continuously flowing microcosms) are ideal experimental systems to investigate the effects of rapid evolution and phenotypic plasticity (induced defense) of prey species on population dynamics in detail. Based on the chemostat models, I confirmed that phenotypic plasticity is better at stabilizing population dynamics whereas a plastic genotype has higher fitness in fluctuating environments than in stable environments. Combining these two characteristics that have been studied separately in population and evolutionary ecology, I found a dilemma of plasticity: the plastic genotype is better in fluctuating environments, but it stabilizes the fluctuation and thereby decreases its fitness by itself. By decreasing the plastic genotype, the system again begins to oscillate. The dilemma results in a novel phenomenon in which phenotypic plasticity evolve rapidly causing intermittent cycles. I proposed to call this as ‘eco-evolutionary bursting.’In chapter 3, I focused on ecological speciation via functional pleiotropy, in which evolution of the speciation gene contributes not only to reproductive isolation, but also to anti-predatory adaptation. Classically it was believed that single-gene speciation is almost impossible, because the first mutant is strongly selected against. However, there are some empirical evidences of single-gene speciation in snails. Recent studies proposed a ‘right-handed’ predator hypothesis, in which specialized predation of snakes on dextral (clockwise coiling) snails can elevate relative survival rate of sinistral (counter-clockwise coiling) snails and thereby promote fixation of a sinistral mutant allele. I theoretically revealed that functional pleiotropy and the maternal effect (i.e., delayed inheritance, in which an individual’s phenotype is determined by its mother’s genotype) of the speciation gene can promote single-gene speciation. In small populations, indeed, I found that a recessive mutant has higher fixation probabilities without pleiotropy, whereas a dominant mutant has higher one with pleiotropy. In large populations, the dominant and recessive mutant alleles have the same fixation probability without pleiotropy. This theoretical prediction would be testable by examining allele dominance of the speciation gene in snails living within or outside the snake range.As future perspectives of studies on eco-evolutionary dynamics, I propose four important topics: (1) space and time, (2) combining theoretical and empirical approaches, (3) genomics and eco-evolutionary studies, and (4) eco-evolutionary conservation and management. This thesis did not consider macroscale dynamics of space (e.g., metacommunity) or time (e.g., macro evolution), but it would be interesting to consider eco-evolutionary dynamics in these scales. Second, here I focused on theoretical modeling to understand dynamics, but combining theoretical and empirical approaches with a sophisticated statistical framework is crucial to understand real biological systems. Especially, in this post-genomic era, it will be possible to understand eco-evolutionary dynamics from the genomic scale to the ecological scale. Therefore, future researches are needed to directly connect evolution in the genomic level to ecological dynamics. Finally, conservation and management studies should incorporate perspectives from eco-evolutionary dynamics, as evolution can drastically alter ecological dynamics of nearly extinct populations (e.g., evolutionary rescue) or heavily exploited populations (e.g., fisheries-induced evolution). With eco-evolutionary dynamics, it will be possible to conserve and manage wild populations better.