Investigating Contemporary Evolution via Size-Selective Harvest

Abstract

[ILLUSTRATION OMITTED] Evolution is commonly taught as a slow process that changes gene frequencies over long time periods. These genotypes are changed through natural selection on phenotypes with the fittest individuals spreading more genes into the environment than less-fit phenotypes. What is now well known to science, but still under- emphasized in textbooks and classroom lessons, is that humans can be one of the strongest selection agents on an organism's phenotype through several different pathways. Humans have perhaps the longest history of affecting organism phenotypes through size-selective harvest, or collection of the largest organism. This activity was designed to supplement traditional evolution units and allow students to discover that evolution can occur in relatively short time-periods and be directed by human action. After completing this activity, students will be able to: * predict the consequences of selective harvesting * understand how evolution and natural selection affect genotypes and phenotypes * design and critique experimental design * collect and analyze data * make conclusions based on research results. Students are likely most familiar with size-selective harvest in trophy hunting of large mammals, such as deer. However, marine fisheries are probably the most common arena for size-selective harvest based upon common industry practices. For centuries, fishermen have used large nets pulled through schools of fish to maximize their catch-per-unit effort. In modern times, large commercial fishing fleets use sophisticated technology, including global positioning satellites and sonar, in conjunction with traditional knowledge about behavior and biology to locate the largest concentrations of fish. Nets are still used to haul in the fish and require fish to be of a certain size to be captured. Even with smaller mesh sizes, some fish will escape, but this is less so in the modern era of commercial fisheries. Fish often mature at a genetically-determined size and/or age, which in turn is structured by environmental conditions. These types of traits are called Life History Traits. Growth is highly correlated with age, and older individuals are usually larger. Generally speaking, the larger the mother, the more offspring she will have with each reproductive event. If she is sufficiently large, her offspring will also be larger. These larger offspring often have higher survival rates than small offspring. Darwinian fitness is related to the proportion of the parents' genes in the next generation; fitness is generically the number of offspring you have that live to reproduce. Larger female fish have higher fitness than smaller female fish. Darwin (2004) used changes seen with selective breeding of animals as evidence of natural selection. Selective breeding is common in today's commercial agriculture and food animal production, but is seldom discussed with relevancy to evolutionary processes. Humans can also inadvertently affect the evolution of species. Using the marine fisheries example, fish that escape fishing nets to reproduce are often smaller and thus pass on genes that allow maturity at smaller sizes. If the harvest is strong enough, or repeated over many generations, this selection can result in fish being smaller at maturity (Figure 1). This has fitness consequences and reduces biomass in commercial harvests (Conover & Munch, 2002; Hutchings, 2005). Contemporary evolution is defined as heritable trait evolution observed in contemporary time (i.e., less then a few hundred years) (Stockwell et al., 2003). [FIGURE 1 OMITTED] A simple laboratory exercise using inexpensive and nonperishable materials can demonstrate contemporary evolution. The following activity will allow students to discover how humans can interfere with the natural selection of organisms and do so in relatively little time. …