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Ch. 20 - Population Genetics and Evolution at the Population, Species, and Molecular Levels
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 20 - Population Genetics and Evolution at the Population, Species, and Molecular LevelsProblem 14
Chapter 20, Problem 14

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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Understand the concept of directional selection: Directional selection is a type of natural selection that favors one extreme phenotype over others, leading to a shift in allele frequencies in a population.
Recognize the outcome of directional selection: Over time, the favored allele increases in frequency, potentially reaching fixation, where its frequency becomes 1.0.
Define fixation: Fixation occurs when an allele's frequency in a population reaches 100%, meaning all individuals are homozygous for that allele.
Explain the lack of genetic variation: Once an allele is fixed, there is no genetic variation at that locus because all individuals have the same allele.
Discuss why directional selection stops: Without genetic variation, there is no alternative allele for selection to act upon, so directional selection can no longer operate at that locus.

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Key Concepts

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

Directional Selection

Directional selection is a mode of natural selection that favors one extreme phenotype over others, leading to a shift in allele frequencies in a population. This process enhances the reproductive success of individuals with advantageous traits, gradually increasing the frequency of the favored allele while decreasing that of others. Over time, this can result in the fixation of the favored allele, where it reaches a frequency of 1.0.
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Natural Selection

Fixation of Alleles

Fixation occurs when a particular allele becomes the only variant present at a genetic locus within a population, resulting in a frequency of 1.0. At this point, all individuals in the population carry the same allele, eliminating genetic diversity at that locus. This lack of variation means that there are no alternative alleles for selection to act upon, effectively halting the evolutionary process at that specific gene.
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New Alleles and Migration

Genetic Variation and Evolution

Genetic variation is essential for evolution, as it provides the raw material for natural selection to operate. Without variation, populations cannot adapt to changing environments or respond to selective pressures. Once an allele is fixed, the absence of alternative alleles means that directional selection can no longer occur at that locus, as there are no competing traits to favor or disfavor, leading to a stagnation in evolutionary change.
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Related Practice
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. 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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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?

Textbook Question

The mtDNA sequence of Neanderthals is more similar to that of modern humans than to that of Denisovans. However, analyses of nuclear DNA clearly indicate that Neandertals and Denisovans share a more recent common ancestor than either of these hominins shares with modern humans. Propose a hypothesis to resolve the discrepancy between the mtDNA and the nuclear genome.

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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?
Textbook Question

If you were to compare your genome sequence with that of your parents, how would it differ? If you were to compare your genome sequence with another student's in the class, how would it differ? What additional difference might you see if your genome was compared with that of a sub-Saharan African, or if you are of sub-Saharan African descent, with that of a non-African?

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