10.2.1 Microevolution by Genetic Drift

Question 1

Microevolution by Genetic Drift

Answer 1

• Populations that become greatly reduced in size may deviate from Hardy-Weinberg equilibrium.
• The changes in the gene pool (frequency of alleles) of a reduced population are called genetic drift.
• Two situations that can lead to genetic drift are bottlenecks and the founder effect.
- Review: The Hardy-Weinberg theory predicts gene
frequencies in a nonevolving population. One of the five conditions for maintaining Hardy-Weinberg equilibrium is a large population size.
- A population that becomes greatly reduced may experience genetic drift, changes in the gene pool because of chance. After genetic drift, the gene frequency of the reduced population will no longer be representative of the original population.
- To understand genetic drift, consider a population of flowers. In generation 1, the original population has flowers of three different genotypes. The population consists of 14 A alleles and 6 a alleles. These values can be converted to p and q values of 0.7 and 0.3, respectively. Only five of the plants produce seeds. In generation 2, the frequency of p and q has changed to 0.55 and 0.45, respectively. Only two of the generation 2 plants produce seeds. In generation 3, the frequencies of p and q are 1.0 and 0, respectively.
- Because the original population is so small, random
fluctuations in the gene pool can greatly change the allelic frequencies of the populations. Eventually, genetic drift may reduce variability as the gene pool loses rare alleles.
- Genetic drift can occur as the result of disasters such as floods, fires, and human impact, which may greatly reduce an existing population. By chance, certain alleles may no longer be present in the population, and some may be greatly overrepresented.
- The bottleneck effect occurs when a small surviving
population no longer represents the genetic frequency of the original population.
- The bottleneck effect has occurred in a population of Northern elephant seals (Mirounga angustirostris), a species that was hunted nearly to extinction. The population has since rebounded, but at the expense of genetic diversity.
- Genetic drift in a new colony is called the founder effect.
- For example, a few individuals from a large population may colonize a new habitat. The smaller the founding colony, the greater the likelihood that genetic drift will occur.
- In 1814, a group of 15 people formed a British colony on the islands of Tristan da Cunha. By chance, one of the founding members carried a rare recessive allele for retinitis pigmentosa, a disease that causes blindness. Of the 240 descendants of the original colony, four were afflicted with the disease and at least nine others were carriers. The frequency of the disease in the founding population was much greater than in the original population.

Question 2

Which of the following is an example of microevolution through genetic drift?

Answer 2

• A small population has drastic changes in allele frequencies over time.

Question 3

Under which condition would you expect a more rapid rate of evolution to occur?

Answer 3

• in a small population recently arriving on an island

Question 4

The founder effect

Answer 4

• is a cause of genetic drift.

Question 5

True or false?
In generation 1 in a small population, the frequency, p, of an allele is 0.7. In the next generation the frequency of the recessive homozygote for this gene is determined to be 0.20. This population is in Hardy–Weinberg equilibrium.

Answer 5

• false

Question 6

Which statement about genetic drift is incorrect?

Answer 6

• Genetic drift leads to improved adaptation.

Question 7

Which of the following best describes an evolutionary bottleneck?

Answer 7

• a loss of genetic variability during a period of reduced population size