Mathematical models are used to explore the evolutionary and demographic consequences of size-selective harvesting and natural selection in populations with discrete, nonoverlapping generations. We specify the harvesting intensity and size selectivity in terms of both the phenotypic criterion for harvesting and the demographic goal of harvesting. The phenotypic criterion is defined by a threshold that specifies the minimum size of individuals harvested, while the demographic goal defines the desired catch or escapement from the harvest. Models use either a constant phenotypic threshold, or a variable phenotypic threshold that moves each year to remain a fixed distance from the mean size prior to harvest. Demographic goals are defined as a desired yield or escapement, measured in abundance or biomass. We find that all combinations of phenotypic criteria and demographic goals can produce evolutionary changes in equilibrium mean size and abundance when harvesting is size selective. Reductions in mean size are more severe under harvesting scenarios for which it is not possible to fully satisfy the demographic goals of harvesting at equilibrium. Models with a variable phenotypic threshold always produce a characteristic equilibrium for mean size and abundance for a given set of harvesting and natural selection parameters, regardless of the initial population characteristics. For some combinations of parameters, constant-threshold models produce alternative equilibria for the mean size and abundance. Our results suggest that the evolutionary effects of selective harvesting warrant consideration in long-term conservation and management planning.
