Population variabilityThe goal of our previous discussions in this class has been to understand the inheritance of a single trait, a trait that may be controlled by one, a few, or many genes. The goal of population genetics is different. Rather than studying the inheritance of a trait, population genetics attempts to describe how the frequency of the alleles which control the trait change over time. To study frequency changes, we analyze populations rather than individuals. Furthermore, because changes in gene frequencies are at the heart of evolution and speciation, population and evolutionary genetics are often studied together.
For a population of individuals to succeed over evolutionary time, it must contain genetic variability. Because we do not know all the genetic variables that would predict evolutionary success, we study the variability of different phenotypes and genotypes to provide an overview of the population. The traits that are analyzed can be outward phenotypes that can be easily scored. More recently, biochemical and RFLP data has been used to assess population variability.
This image illustrates a key concern of population genetics --- the measurement of variability. This is a collection of snail shells. As you can see, many different patterns and colors are possible for the shells. Clearly many different alleles exist that control color and shape. This is an example of a polymorphic or variabile population.
The cheetah is a species that suffers from low genetic variability. Because of hunting and the narrowing of its habitat, fewer and fewer cheetahs are found. Those that are found are often related. A population of closely related individuals exhibits low variability. This is especially critical (and dangerous) if the environmental conditions change and the population does not have the variability to cope with the change. That population could rapidly move toward extinction.
One method of expressing variability is by analyzing the genetic data and expressing the data in terms of gene (or allelic) frequencies. Any gene will have at least two alleles. The summation of all the allelic frequencies, for all the genes that are analyzed in a specific population, can be considered a characterization of that population. Any population can have a wide range of allelic frequencies for each of the genes that are being considered. Furthermore, two populations do not necessarily have the same set of frequencies even though they are the same species.
Copyright © 1997. Phillip McClean