Review Article

Plasticity-Mediated Persistence in New and Changing Environments

Figure 2

Fisher’s geometric model of adaptation used to explain evolutionary rescue and plasticity-mediated population persistence (PMP). In both instances, a population encounters a novel environment with phenotypic optimum O. The distance from the population’s mean phenotype (A) to O is a proxy for the strength of selection and therefore for the likelihood of persistence. (a) Evolutionary rescue. The population begins far from the optimum; their phenotypic state in the new environment is shown by A. Mutations of small effect (small arrows, small dashed circle) are just as likely to bring the population closer to the phenotypic optimum (the space denoted by the solid circle) than they are away from the phenotypic optimum. Mutations of large effect (long arrows, large dashed circle) are more likely to move the population away from the optimum than towards the optimum. Upon encountering the novel environment, the probability that a population persists depends on the strength of selection and the likelihood of mutations arising that will move the population towards the optimum. Similarly, standing genetic variation could move the population to its optimum rapidly, with the rare mutations of large effect that had persisted in the old environment suddenly being favoured, or numerous small effect variants shifting in frequency (as per [128]). (b) Plasticity-mediated persistence (PMP). In this case A represents the phenotype in the old environment, and the dashed arrows represent how the phenotype can change upon exposure to a new environment. (1) Plasticity is perfect and the population is under stabilizing selection. (2) Plasticity moves the population away from the optimum, reducing its likelihood of persistence. (3) Plasticity is imperfect but moves the population towards its optimum. (4) Imperfect plasticity permits the population to survive long enough for evolutionary rescue (solid arrows) to occur. (2) and (3) combined could represent the random nature of plasticity revealed in the new environment through the uncovering of cryptic genetic variation. Figures adapted from [97, 129, 130].
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