Module 8 Lesson 1 - 3
Lesson 1 — The Gene Pool
Hardy-Weinberg Equilibrium
Can allele frequencies in a gene pool change?
The Amish people are the descendants of Swiss founders who immigrated to the United States in the 1800s in an effort to renounce technological progress. One of the founders of the Amish had Ellis-Van Crevald syndrome, which causes short stature and polydactyly (extra fingers and toes). Although the allele for Ellis-Van Crevald syndrome is very rare in the Swiss gene pool, most Amish marry within their group. Therefore, the frequency of the polydactyly allele has increased substantially in the Amish population today.
On the other hand, some populations have allele frequencies that have stayed in relative equilibrium over time. For example, the 23 species of crocodiles have experienced great stability in their allele frequencies in the 200 million years of their existence
to the degree that not one of the 23 species has been forced into extinction in that time.
Why is it important for biologists to keep track of allele frequencies?
The definition of micro-evolution
is a change in allele frequencies. A relative increase or decrease in allele frequencies indicates to population geneticists that the population is experiencing some kind of pressure and is adapting in response.
An increase in the frequency of the lighter coat colour allele versus the darker in ground squirrels might indicate that only the lighter squirrels can remain camouflaged and survive in the dry yellow grasses that are becoming prevalent under drought conditions. This could be an indicator of the effects of global warming.
That populations are evolving (or allele frequencies are changing) is not necessarily a bad thing. Populations under pressure have two options. Either they evolve to adapt to the new conditions or they become extinct. Clearly, evolution is the more favourable
choice.
Hardy-Weinberg Equilibrium
Logically, if each breeding couple removes two alleles from the gene pool to reproduce and then returns them to the gene pool in the form of offspring, the relative frequencies of each allele in a gene pool should not change from generation to generation.
In other words, the population should stay in equilibrium.
Godfrey Hardy and Wilhelm Weinberg made the observation that, as long as certain conditions are met, allele frequencies should stay the same generation after generation. This situation is known as the Hardy-Weinberg equilibrium.