Module 8 Population Genetics

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What mechanisms lead to gene pool change and micro-evolution?

If the conditions of Hardy Weinberg equilibrium are the conditions that keep allele frequencies from changing and prevent evolution, then were we to reverse the list, we would get the conditions that lead to changing allele frequencies and evolution.

 

The conditions that lead to micro-evolution include the following:

• An open population with gene flow in and out through immigration and emigration.
• A small population whose frequencies can be greatly affected by chance events (genetic drift).
• Mate selection – choosing mates on the basis of preferred genotypes/phenotypes.
• Changes in mutation rates – creation of either new alleles or switching from one allele to the other.
• Natural Selection – In a given environment, individuals with certain alleles are better suited for survival and reproduction than others.

These 5 mechanisms for changes in allele frequencies are therefore the tools used for species survival on an ever-changing earth.

 

Random copying and transcription/translation errors commonly cause dominant alleles to mutate to recessive alleles and vice versa. However, a net change in direction results in microevolution. If one begins to occur more than the other, then this net change in mutation rate will cause a change in allele frequencies and thus microevolution. This can either increase or decrease diversity in the population, depending on which is favored. The gene flow that results from immigration and emigration decreases variation between populations, but increases diversity within populations. In most species, the process of mate selection is anything but random. The complex courtship rituals, dances, and displays of animals result in sexual selection. If one phenotype/genotype is very popular with mates, the frequency of the involved alleles will increase, decreasing diversity. If the taste in mate changes, so will the allele frequencies.

 

The genetic drift that occurs when small populations experience major genetic change due to chance events occur where sub-populations become isolated either by migration (Founder effect) or natural disaster (Bottleneck effect). Both result in the increased frequency of a rare allele, so diversity is increased. The controlled breeding done in agriculture (crop and livestock) purposely sets up conditions of genetic drift to increase the frequency of rare but desirable alleles. Natural selection is entirely dependent on the environment of the time—those with suitable alleles survive to reproduce. If competition is fierce under conditions of rapid environmental change, the frequency of a suitable but rare allele will increase, giving its owner a survival advantage, increasing diversity in the gene pool. If there are no variants in the population with suitable alleles, extinction occurs.

 

Read

Read p. 689-695 of your text to understand these processes. You may wish to make summary notes, or a chart of the ideas presented.

 

Self-Check

 

Compare and contrast.

 

Complete the following and file in your course folder for study purposes.

 

Compare and contrast the following terms:

1. The founder effect and the bottleneck effect (as causes of genetic drift)
2. Natural selection and genetic drift
3. Non-random mating and natural selection

 

Self-Check

1. Question 19 on page 695. Provide an example.

2. Impediments to gene flow are often geographical barriers. Provide 3.

3. The bottleneck effect can occur after a natural disaster. Give 3 examples of a natural disaster that could result in this type of genetic drift.

4. The Launch Lab on page 676-7 allows you to select mate phenotypes/genotypes of sage grouse during their mating displays, and see the results. Pick out 3 characteristics that the female may be using to select the best mate.

5. How can wildlife preserves prevent the negative effects of genetic drift and lost biodiversity in wildlife populations.