Ch. 26 - Population and Evolutionary Genetics

Problem 26

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Chapter 25, Problem 26

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

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span>Identify the two main groups of reproductive isolating mechanisms: prezygotic and postzygotic.</span

span>Explain prezygotic isolating mechanisms, which prevent mating or fertilization between species. Examples include temporal isolation, habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation.</span

span>Explain postzygotic isolating mechanisms, which occur after fertilization and result in hybrid inviability, hybrid sterility, or hybrid breakdown.</span

span>Discuss why prezygotic mechanisms are generally regarded as more efficient. They prevent the wastage of gametes and energy by stopping the formation of hybrids altogether, thus conserving resources.</span

span>Conclude by emphasizing that prezygotic mechanisms are more efficient because they prevent the occurrence of unsuccessful mating attempts or the production of non-viable or sterile offspring.</span

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Key Concepts

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

Reproductive Isolating Mechanisms

Reproductive isolating mechanisms are biological features that prevent different species from interbreeding. They are crucial for maintaining species boundaries and can be classified into two main groups: prezygotic and postzygotic mechanisms. Prezygotic mechanisms occur before fertilization, while postzygotic mechanisms take effect after fertilization has occurred.

Prezygotic Mechanisms

Prezygotic mechanisms prevent mating or fertilization between species. They include temporal isolation (different mating seasons), behavioral isolation (different mating rituals), and mechanical isolation (incompatible reproductive structures). These mechanisms are often considered more efficient because they stop the formation of hybrid zygotes, thus conserving energy and resources for the species involved.

Postzygotic Mechanisms

Postzygotic mechanisms occur after fertilization and affect the viability or reproductive capacity of hybrid offspring. Examples include hybrid inviability (offspring do not develop properly) and hybrid sterility (offspring are sterile, like mules). While these mechanisms can prevent gene flow, they are generally less efficient than prezygotic mechanisms because they allow for the initial formation of hybrids, which can still have ecological impacts.

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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

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

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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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Textbook Question

The original source of new alleles, upon which selection operates, is mutation, a random event that occurs without regard to selectional value in the organism. Although many model organisms have been used to study mutational events in populations, some investigators have developed abiotic molecular models. Soll et al. (2006. Genetics 175: 267-275) examined one such model to study the relationship between both deleterious and advantageous mutations and population size in a ligase molecule composed of RNA (a ribozyme). Soll found that the smaller the population of molecules, the more likely it was that not only deleterious mutations but also advantageous mutations would disappear. Why would population size influence the survival of both types of mutations (deleterious and advantageous) in populations?

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