Moreover, harvest need not be selective to cause genetic change uniformly increasing mortality independent of phenotype will select for earlier maturation ( 4). It has also been suggested that the greater difficulty of catching introduced brown trout ( Salmo trutta) than native North American species of trout is the result of angling for brown trout in Europe for hundreds of years before their introduction to North America ( 3). Similarly, trophy hunting for bighorn sheep ( Ovis canadensis) in Alberta, Canada caused a decrease in horn size because rams with larger horns had a greater probability of being removed from the population by hunting ( 2). For example, the frequency of elephants ( Loxodonta africana) without tusks increased from 10% to 38% in South Luangwa National Park, Zambia, apparently brought about of poaching of elephants for their ivory ( 1). Such selective removal will bring about genetic change in harvested populations if the selected phenotype has at least a partial genetic basis ( Table 1). That is, individuals of certain size, morphology, or behavior are more likely than others to be removed from the population by harvesting. Human harvest of wild populations is almost always nonrandom. Humans have exploited wild populations of animals for food, clothing, and tools since the origin of hominids. We strongly encourage those responsible for managing harvested wild populations to take into account possible selective effects of harvest management and to implement monitoring programs to detect exploitation-induced selection before it seriously impacts viability. Evolution brought about by human harvest might greatly increase the time required for over-harvested populations to recover once harvest is curtailed because harvesting often creates strong selection differentials, whereas curtailing harvest will often result in less intense selection in the opposing direction. Nevertheless, it is likely that some undesirable changes observed over time in exploited populations (e.g., reduced body size, earlier sexual maturity, reduced antler size, etc.) are due to selection against desirable phenotypes-a process we call “unnatural” selection.
Determining whether phenotypic changes in harvested populations are due to evolution, rather than phenotypic plasticity or environmental variation, has been problematic. We also consider how harvesting could affect the mating system and thereby modify sexual selection in a way that might affect recruitment. We consider the potential effects of harvest on the genetics and sustainability of wild populations.
Hunting and fishing contrast with agricultural and aquacultural practices in which the most desirable animals are typically bred with the specific goal of increasing the frequency of desirable phenotypes. Human harvest of phenotypically desirable animals from wild populations imposes selection that can reduce the frequencies of those desirable phenotypes.