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Ch. 26 - Population and Evolutionary Genetics
Chapter 25, Problem 25

List the barriers that prevent interbreeding, and give an example of each.

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Identify the concept of reproductive isolation, which prevents different species from interbreeding and producing fertile offspring.
Discuss prezygotic barriers, which occur before fertilization, preventing mating or fertilization between species.
Provide an example of a prezygotic barrier: Temporal isolation, where species breed at different times of the day or year.
Discuss postzygotic barriers, which occur after fertilization, affecting the viability or fertility of the hybrid offspring.
Provide an example of a postzygotic barrier: Hybrid sterility, where hybrid offspring are sterile, such as a mule, which is a cross between a horse and a donkey.

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Key Concepts

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

Prezygotic Barriers

Prezygotic barriers are reproductive obstacles that occur before fertilization, preventing different species from mating or fertilizing eggs. These barriers can include temporal isolation, where species breed at different times, and behavioral isolation, where differences in mating rituals prevent interbreeding. For example, two species of frogs may live in the same area but breed at different times of the year.
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Postzygotic Barriers

Postzygotic barriers occur after fertilization and affect the viability or fertility of the hybrid offspring. These barriers include hybrid inviability, where the hybrid does not develop properly or dies early, and hybrid sterility, where the hybrid is sterile and cannot reproduce. A classic example is the mule, a hybrid of a horse and a donkey, which is typically sterile.
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Ecological Isolation

Ecological isolation occurs when two species occupy different habitats within the same area, leading to limited interaction and mating opportunities. Even if they are not physically separated, their preferences for different environments can prevent them from interbreeding. For instance, two species of lizards may live in the same geographic region but prefer different types of vegetation, reducing their chances of encountering each other.
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Related Practice
Textbook Question

To increase genetic diversity in the bighorn sheep population described in Problem 23, ten sheep are introduced from a population where the c allele is absent. Assuming that random mating occurs between the original and the introduced sheep, and that the c allele is selectively neutral, what will be the frequency of c in the next generation?

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Textbook Question

What genetic changes take place during speciation?

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Textbook Question

Some critics have warned that the use of gene therapy to correct genetic disorders will affect the course of human evolution. Evaluate this criticism in light of what you know about population genetics and evolution, distinguishing between somatic gene therapy and germ-line gene therapy.

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Textbook Question

What are the two groups of reproductive isolating mechanisms? Which of these is regarded as more efficient, and why?

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Textbook Question

A form of dwarfism known as Ellis–van Creveld syndrome was first discovered in the late 1930s, when Richard Ellis and Simon van Creveld shared a train compartment on the way to a pediatrics meeting. In the course of conversation, they discovered that they each had a patient with this syndrome. They published a description of the syndrome in 1940. Affected individuals have a short-limbed form of dwarfism and often have defects of the lips and teeth, and polydactyly (extra fingers). The largest pedigree for the condition was reported in an Old Order Amish population in eastern Pennsylvania by Victor McKusick and his colleagues (1964). In that community, about 5 per 1000 births are affected, and in the population of 8000, the observed frequency is 2 per 1000. All affected individuals have unaffected parents, and all affected cases can trace their ancestry to Samuel King and his wife, who arrived in the area in 1774. It is known that neither King nor his wife was affected with the disorder. There are no cases of the disorder in other Amish communities, such as those in Ohio or Indiana.

What is the most likely explanation for the high frequency of the disorder in the Pennsylvania Amish community and its absence in other Amish communities?

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Textbook Question

A form of dwarfism known as Ellis–van Creveld syndrome was first discovered in the late 1930s, when Richard Ellis and Simon van Creveld shared a train compartment on the way to a pediatrics meeting. In the course of conversation, they discovered that they each had a patient with this syndrome. They published a description of the syndrome in 1940. Affected individuals have a short-limbed form of dwarfism and often have defects of the lips and teeth, and polydactyly (extra fingers). The largest pedigree for the condition was reported in an Old Order Amish population in eastern Pennsylvania by Victor McKusick and his colleagues (1964). In that community, about 5 per 1000 births are affected, and in the population of 8000, the observed frequency is 2 per 1000. All affected individuals have unaffected parents, and all affected cases can trace their ancestry to Samuel King and his wife, who arrived in the area in 1774. It is known that neither King nor his wife was affected with the disorder. There are no cases of the disorder in other Amish communities, such as those in Ohio or Indiana.

From the information provided, derive the most likely mode of inheritance of this disorder. Using the Hardy–Weinberg law, calculate the frequency of the mutant allele in the population and the frequency of heterozygotes, assuming Hardy–Weinberg conditions.

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