Further Thought

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Further Thought

Use the questions at the end of the chapter to explore concepts and connections in greater depth through application and synthesis.

1. Listed below are four possible causes of aging that were discussed in the text. As a review, name the theory that is associated with each cause, and describe whether or not selection for a longer life span is possible under each theory. What predictions does each theory make about the effect of ecological mortality (death due to external causes—predators, starvation, etc.) on aging rate?

"Wearing out" due to metabolic activity
Reduction in size of telomeres with each cell division
Mutations that have negative effects late in life
Mutations that have positive effects early and negative effects late in life [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

2. How is the replicative senescence theory related to the antagonistic pleiotropy theory? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

3. Most domestic female rabbits will get uterine cancer if they are not spayed. The uterine cancer usually appears sometime after the age of 2 years. Describe a hypothesis for why rabbits have not evolved better defenses against uterine cancer. What do you think the average life span of a wild female rabbit might be? What do you think is a typical cause of death in wild rabbits? Why? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

4. We have seen how aging can evolve due to two different phenomena: First, aging may evolve due to mutations that have deleterious effects only late in life. As a review, explain how such mutations could ever become common in a population. Second, aging may evolve due to mutations with pleiotropic effects that cause "trade-offs"—positive effects early and negative effects late. What would happen if a mutation arose with a reverse trade-off, that is, a mutation with negative effects early and positive effects late in life? Could such a mutation ever be selected for? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

5. Look again at Figure 12.9a (page 466 in your textbook), which shows life history trade-offs for a hypothetical species. Suppose you have a large captive population of these (hypothetical) organisms, and you notice a new mutation that causes its carriers to have two offspring per year instead of just one. The new mutation does not alter the age of maturation, which still occurs at three years. Your preliminary observations indicate that the new mutation may cause an early death, but you are not certain exactly how early the deaths occur. You do notice, however, that the new mutation is increasing in frequency and the wild-type allele is decreasing. Make a prediction about the minimum possible age of death of organisms that carry this mutation, and explain your reasoning. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

6. Assuming for the moment that the grandmother hypothesis of menopause is correct, speculate on what aspects of a species' behavior and sociality may make menopause likely to evolve. For instance, is it important whether or not the species is highly social, or whether or not the species lives in kin groups? Might the age of independence of the young be important? Could menopause ever evolve in a species without parental care, such as aphids or willow trees? As fuel for thought, consider the likelihood of evolution of menopause in (1) orangutans, who live in small groups consisting simply of a female and her dependent young; (2) lions, in which females are very social and remain with their female kin for most of their lives; and (3) Arabian oryx, who live in small family groups in the desert, and must sometimes find distant waterholes known only to the older oryx. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

7. The examples of the chinook salmon and seed beetles indicate that females probably cannot produce many large eggs; instead, they must choose between producing many small eggs or producing a few large eggs (and sometimes, in unfortunate cases, just a few small eggs). Explain, then, how it is possible for a queen honeybee to produce a very large number of relatively large eggs. (Hint: Consider what the other bees are doing.) Does this suggest a general way in which a female can escape from the size/number trade-off? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

8. The 1998 movie Godzilla and the 1986 movie Aliens each depict a fictional large female carnivore. The Godzilla female lives off a large prey population of humans and fishes, but has no assistance from others of her kind. In a few days she produces hundreds of 7-ft-tall eggs, enough to fill Madison Square Garden. The Aliens female lives off a small prey population of a few dozen humans, is assisted by nonreproducing workers, and produces hundreds of large eggs in a few weeks. Comment on what is realistic and unrealistic about the life history traits and egg production abilities of each of these fictional animals. If they were real, would they have long or short life spans? Why? [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

9. Dairy farmers are sometimes frustrated in their attempts to breed a better milk cow because heritability values for milk production and reproductive traits are low—generally below 0.10. In addition, those cows that produce the most milk tend to have longer intervals between birth of successive calves, and require more breedings to a bull before the cow will conceive. Do these patterns make sense in light of evolutionary life history theory? Explain. See:

T. E. Aitchison, "Genetic Improvement" page at the University of Maryland's "Dairy Cattle Genetics and Selection" Website. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.




1.	Listed below are four possible causes of aging that were discussed in the text. As a review, name the theory that is associated with each cause, and describe whether or not selection for a longer life span is possible under each theory. What predictions does each theory make about the effect of ecological mortality (death due to external causes—predators, starvation, etc.) on aging rate? "Wearing out" due to metabolic activity Reduction in size of telomeres with each cell division Mutations that have negative effects late in life Mutations that have positive effects early and negative effects late in life [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	How is the replicative senescence theory related to the antagonistic pleiotropy theory? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



3.	Most domestic female rabbits will get uterine cancer if they are not spayed. The uterine cancer usually appears sometime after the age of 2 years. Describe a hypothesis for why rabbits have not evolved better defenses against uterine cancer. What do you think the average life span of a wild female rabbit might be? What do you think is a typical cause of death in wild rabbits? Why? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



4.	We have seen how aging can evolve due to two different phenomena: First, aging may evolve due to mutations that have deleterious effects only late in life. As a review, explain how such mutations could ever become common in a population. Second, aging may evolve due to mutations with pleiotropic effects that cause "trade-offs"—positive effects early and negative effects late. What would happen if a mutation arose with a reverse trade-off, that is, a mutation with negative effects early and positive effects late in life? Could such a mutation ever be selected for? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



5.	Look again at Figure 12.9a (page 466 in your textbook), which shows life history trade-offs for a hypothetical species. Suppose you have a large captive population of these (hypothetical) organisms, and you notice a new mutation that causes its carriers to have two offspring per year instead of just one. The new mutation does not alter the age of maturation, which still occurs at three years. Your preliminary observations indicate that the new mutation may cause an early death, but you are not certain exactly how early the deaths occur. You do notice, however, that the new mutation is increasing in frequency and the wild-type allele is decreasing. Make a prediction about the minimum possible age of death of organisms that carry this mutation, and explain your reasoning. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



6.	Assuming for the moment that the grandmother hypothesis of menopause is correct, speculate on what aspects of a species' behavior and sociality may make menopause likely to evolve. For instance, is it important whether or not the species is highly social, or whether or not the species lives in kin groups? Might the age of independence of the young be important? Could menopause ever evolve in a species without parental care, such as aphids or willow trees? As fuel for thought, consider the likelihood of evolution of menopause in (1) orangutans, who live in small groups consisting simply of a female and her dependent young; (2) lions, in which females are very social and remain with their female kin for most of their lives; and (3) Arabian oryx, who live in small family groups in the desert, and must sometimes find distant waterholes known only to the older oryx. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



7.	The examples of the chinook salmon and seed beetles indicate that females probably cannot produce many large eggs; instead, they must choose between producing many small eggs or producing a few large eggs (and sometimes, in unfortunate cases, just a few small eggs). Explain, then, how it is possible for a queen honeybee to produce a very large number of relatively large eggs. (Hint: Consider what the other bees are doing.) Does this suggest a general way in which a female can escape from the size/number trade-off? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



8.	The 1998 movie Godzilla and the 1986 movie Aliens each depict a fictional large female carnivore. The Godzilla female lives off a large prey population of humans and fishes, but has no assistance from others of her kind. In a few days she produces hundreds of 7-ft-tall eggs, enough to fill Madison Square Garden. The Aliens female lives off a small prey population of a few dozen humans, is assisted by nonreproducing workers, and produces hundreds of large eggs in a few weeks. Comment on what is realistic and unrealistic about the life history traits and egg production abilities of each of these fictional animals. If they were real, would they have long or short life spans? Why? [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



9.	Dairy farmers are sometimes frustrated in their attempts to breed a better milk cow because heritability values for milk production and reproductive traits are low—generally below 0.10. In addition, those cows that produce the most milk tend to have longer intervals between birth of successive calves, and require more breedings to a bull before the cow will conceive. Do these patterns make sense in light of evolutionary life history theory? Explain. See: T. E. Aitchison, "Genetic Improvement" page at the University of Maryland's "Dairy Cattle Genetics and Selection" Website. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

Chapter 12: Aging and Other Life History Characters

Further Thought