Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels

Problem 13b

Chapter 20, Problem 13b

Two populations of deer, one of them large and living in a mainland forest and the other small and inhabiting a forest on an island, regularly exchange members that migrate across a land bridge that connects the island to the mainland. An earthquake destroys the bridge between the island and the mainland, making migration impossible for the deer. What do you expect will happen to allele frequencies in the two populations over the following 10 generations?

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Understand the concept of gene flow: Gene flow refers to the transfer of alleles or genes from one population to another. In this scenario, the land bridge allowed gene flow between the mainland and island deer populations, maintaining genetic similarity between them.

Consider the impact of isolation: With the destruction of the land bridge, the two populations are now reproductively isolated. This means there will no longer be gene flow between the island and mainland populations, leading to independent evolution of allele frequencies in each population.

Analyze genetic drift in the island population: The island population is smaller, which makes it more susceptible to genetic drift. Genetic drift is the random fluctuation of allele frequencies due to chance events, and it can lead to significant changes in allele frequencies over generations in small populations.

Evaluate the role of natural selection: Natural selection may act differently on the two populations due to differences in environmental pressures on the island and mainland. Over time, this could lead to divergence in allele frequencies as each population adapts to its specific environment.

Predict long-term outcomes: Over 10 generations, the lack of gene flow, combined with genetic drift and natural selection, is likely to result in genetic divergence between the two populations. The island population may experience more pronounced changes due to its smaller size and potential for stronger environmental pressures.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Genetic Drift

Genetic drift refers to the random fluctuations in allele frequencies within a population, particularly in small populations. When the land bridge is destroyed, the island population becomes isolated, leading to genetic drift as it can no longer exchange alleles with the mainland population. Over generations, this can result in significant changes in allele frequencies due to chance events, especially in the smaller island population.

Gene Flow

Gene flow is the transfer of genetic material between populations through migration and interbreeding. In this scenario, the destruction of the land bridge halts gene flow between the mainland and island deer populations. This isolation can lead to divergent evolution, as each population adapts to its specific environment without the influence of the other, potentially resulting in distinct genetic traits over time.

Natural Selection

Natural selection is the process by which certain traits become more or less common in a population based on their advantages or disadvantages in a given environment. With the populations now isolated, different selective pressures may act on the mainland and island deer, leading to adaptations that enhance survival and reproduction in their respective habitats. Over generations, this can further differentiate the allele frequencies between the two populations.

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Natural Selection

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Textbook Question

The figure illustrates the effect of an ethanol-rich and an ethanol-free environment on the frequency of the Drosophila Adh^F allele in four populations in a 50-generation laboratory experiment. Population 1 and population 2 were reared for 50 generations in a high-ethanol environment, while control 1 and control 2 populations were reared for 50 generations in a zero-ethanol environment. Describe the effect of each environment on the populations, and state any conclusions you can reach about the role of any of the evolutionary processes in producing these effects.

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Textbook Question

Biologists have proposed that the use of antibiotics to treat human infectious disease has played a role in the evolution of widespread antibiotic resistance in several bacterial species, including Staphylococcus aureus and the bacteria causing gonorrhea, tuberculosis, and other infectious diseases. Explain how the evolutionary mechanisms mutation and natural selection may have contributed to the development of antibiotic resistance.

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Textbook Question

Two populations of deer, one of them large and living in a mainland forest and the other small and inhabiting a forest on an island, regularly exchange members that migrate across a land bridge that connects the island to the mainland. If you compared the allele frequencies in the two populations, what would you expect to find?

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Textbook Question

Two populations of deer, one of them large and living in a mainland forest and the other small and inhabiting a forest on an island, regularly exchange members that migrate across a land bridge that connects the island to the mainland. In which population do you expect to see the greatest allele frequency change? Why?

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Textbook Question

Directional selection presents an apparent paradox. By favoring one allele and disfavoring others, directional selection can lead to fixation (a frequency of 1.0) of the favored allele, after which there is no genetic variation at the locus, and its evolution stops. Explain why directional selection no longer operates in populations after the favored allele reaches fixation.

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Textbook Question

What is inbreeding depression? Why is inbreeding depression a serious concern for animal biologists involved in species-conservation breeding programs?

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