Chapter 6: Population Genetics 2: Migration, Drift, Non-random MatingOverview |
Two hundred years ago, the state of Illinois was almost entirely covered with prairie and was home to millions of greater prairie chickens. In 1837, however, the steel plow arrived (Thomas 1998) and as the Illinois prairie shrank, the range of the Illinois greater prairie chicken dwindled with it. Efforts to save the Illinois greater prairie chicken began with a ban on hunting in 1933 (Thomas 1998). In 1962 and 1967, respectively, the habitats occupied by the Jasper County and Marion County populations were established as sanctuaries and as sites for the restoration and management of grasslands (Westemeier et al. 1998).
Why did the Jasper County prairie chicken population continue to decline from the mid-1970s to the mid-1990s, even though the amount of habitat available was increasing? And what did wildlife managers do to finally reverse the decline? The answers to these questions involve three phenomena introduced in Chapter 5.
6.1 Migration To evolutionary biologists, migration means gene flow, the transfer of alleles from the gene pool of one population to the gene pool of another population. Migration can be caused by anything that moves alleles far enough to go from one population to another. Mechanisms of gene flow range from the occasional long-distance dispersal of juvenile animals to the transport of pollen, seeds, or spores by wind, water, or animals. The actual amount of migration among populations in different species varies enormously, depending on how mobile individuals or populations are at various stages of the life cycle.
In section 6.1 of your textbook you will explore examples of migration and its effect on the evolution of a species.
6.2 Genetic Drift We are now in a position to revisit the role of chance in evolution. Arguably, the most important insight from population genetics is that natural selection is not the only mechanism of evolution.
Turn to section 6.2 to learn about the random event of genetic drift and how it contributes to the evolutionary process.
6.3 Genetic Drift and Molecular Evolution The field of molecular evolution was launched in the mid-1960s, when biochemists succeeded in determining the amino acid sequences of hemoglobin, cytochrome c, and other particularly abundant and well-studied proteins found in humans and other vertebrates. These data sets provided the first opportunity for evolutionary biologists to compare the amount and rate of molecular change among species.
In section 6.3 of your textbook you can learn about how early workers in the field made several striking observations about these data sets.
6.4 Nonrandom Mating We have so far considered what happens in populations when we relax the assumptions of the migration and no genetic drift. The final assumption of the Hardy–Weinberg analysis is that individuals in the population mate at random.
In section 6.4 of your textbook we relax that assumption and allow individuals to mate nonrandomly and explore the effect of inbreeding on the genetics of a population.
6.5 Conservation Genetics of the Illinois Greater Prairie Chicken We opened this chapter with the case of the Illinois greater prairie chicken a once-abundant bird that, in the mid-1990s, appeared to be destined for extinction. Like a great many other vulnerable and endangered species, the prairie chicken’s worst enemy is habitat destruction. Yet habitat destruction is not the prairie chicken’s only problem.
Ronald Westemeier and colleagues (1998) developed a hypothesis that will be explored in section 6.5 of your textbook.