Module 8

Module 8—Populations, Individuals and Gene Pools

 

Explore

 

The male great frigate bird in the photo is demonstrating a courtship ritual that makes it clear that he is available for breeding and would be an excellent choice for a discriminating female seeking a mate.

 

His ability to advertise by puffing out his brightly coloured chest is probably due to a dominant allele. Assume that the frequency of this allele is 0.5—that is, it makes up half the gene pool in this particular population. Reflect on the factors that could increase the frequency of this allele in the gene pool.

 

Because microevolution is simply a change in the frequency of alleles, there are five factors that lead to population change and microevolution:

• an open population with gene flow in and out through immigration and emigration
  A lack of natural geographic barriers, such as mountain ranges, or barriers due to human intervention, such as highways, allow organisms to flow in and out of populations. When organisms move, they can bring with them new or different allele combinations, resulting in changes in allele frequencies. Organisms that swim or fly may have an advantage in gene flow.

• a small population whose frequencies can be greatly affected by chance events (genetic drift)

   

  Small and isolated populations suffer from inbreeding, where members become more alike as the frequency of one allele increases over the other. Isolated, inbred populations are prone to rapid evolutionary change as a result. Chance events, such as a situation where one individual or group doesn’t breed at all, can have major effects in tiny populations, changing allele frequencies drastically and causing microevolution of the gene pool. This situation is found in the founder effect and the bottleneck effect.

• mate selection (non-random mating)

   

  Most complex animals spend a lot of energy going through mating rituals and courtship displays designed to attract a certain phenotype of mate. The traits that attracted a mate will then appear in the next generation, increasing the frequency of the successful alleles. However, many organisms don’t select mates. Simple organisms like protists or wind- and water-pollinated plants are examples of organisms that mate randomly.

• changes in mutation rates (Random mutations of DNA can cause B alleles to become b, and vice versa.)
  If the rate of mutation from one to the other is the same, no change in frequency occurs. If, for some reason, the rate is different, the frequency of alleles changes and microevolution occurs.

• natural selection: In a given environment, individuals with certain phenotypes, and certain alleles, are more likely to survive and produce many offspring.
  
  The gene pool of the offspring generation will have a higher frequency of these successful alleles as a result. A phenotype that is naturally selected either increases fecundity (the number of offspring) or decreases mortality.

When environmental change happens, another phenotype becomes desirable, and adaptation occurs as allele frequencies shift. However, if the initial population has little variation and does not have any members that show the new desired phenotype, the population may become extinct. Evolution from one form to another can only occur if the new form is present in the initial population.

 

These are the five mechanisms by which population change or microevolution occurs. Notice that they are the exact opposite of the five conditions for Hardy-Weinberg equilibrium.

Read

Read “The Causes of Gene Pool Change” on pages 689 to 696 of your textbook to understand these processes. You may wish to make summary notes or a chart of the ideas presented.

 

Watch and Listen

 

To review and summarize the information of this lesson, watch “Gene frequencies, Natural Selection, and Speciation: The Burgess Ghosts.” Although you may wish to watch the entire video for review, pay particular attention to the following sections:

• “Bio Simulation: Hardy-Weinberg Principle”
• “Bio Reports: Mechanisms for Evolution”
• “Mechanisms and Rate of Speciation”

Self-Check

 

SC 1. Compare and contrast the following terms.

1. the founder effect and the bottleneck effect
2. natural selection and genetic drift
3. non-random mating and natural selection

 

Try This

 

TR 1. Answer question 19 on page 701 of the textbook. Provide an example of an advantage to the heterozygote condition.

TR 2. Provide three examples of impediments to gene flow that are also geographical barriers.

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

TR 4. “Launch Lab: Pick Your Plumage” on page 677 in the textbook allows you to select mate phenotypes/genotypes of sage grouse during their mating displays and see the results. Pick out three characteristics that the female may be using to select the best mate. Remember that mate selection is a cause of gene pool change (microevolution).

TR 5. Wildlife preserves can be notorious for causing genetic drift due to inbreeding in their small, isolated populations. Suggest at least one method wildlife managers could use to prevent the negative effects of genetic drift and lost biodiversity in wildlife preserves.

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