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Evolutionary Analysis

Fourth Edition

Chapter 13

Aging and Other Life

History Characters

Scott Freeman • Jon C. Herron

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After female sockeye salmon (Onchorhynchus nerka) lay their eggs, they defend their nests against disruption by later-spawning salmon for as long as they have sufficient energy to stay alive

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Andrew Hendry and colleagues (2004) found that early spawners reserve more energy for nest defense, and live longer, than late spawners.

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Darwin’s Demon

An organism truly perfected for reproduction would mature at birth, continuously produce high-quality offspring in large numbers, and live forever (Richard Law 1979)

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Extreme reproductive strategies

(a) Having devoured their mother from the inside, three thrips egg mites (Adactylidium sp.) prepare to depart her empty cuticle. The mother's legs are visible at lower right (180x).

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Figure 13.1b Extreme reproductive strategies

(b) An X ray of a female brown kiwi (Apteryx australis mantelli) ready to lay an egg.

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Trade-offs

Organisms face fundamental trade-offs in their use of energy and time.

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Evolution of Life History

1. Why do organisms age (senescence) and die?

2. How many offspring should an individual produce in a given year?

3. How big should each offspring be?

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13.1 Basic Issues in Life History Analysis

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Figure 13.2 The life history of a hypothetical female Virginia opossum (Didelphis virginiana)

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Life History

Change in life history are caused by changes in the allocation of energy.

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Short-winged and long-winged morphs of female sand crickets (Gryllus firmus)

Trade-off between dispersal vs. early reproduction

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11.2 Why Do Organisms Age & Die?

Aging should be opposed by natural selection.

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Aging in three animals

Figure 13.4 Aging in three animals

(a) Aging in a bird. For females in a wild population of collared flycatchers (Ficedula albicollis), the number of young fledged each year declines after age 3 (age-related differences in number fledged are significant at P < 0.001). The probability of survival from one year to the next declines slightly, but not significantly, after age 2. Sample sizes in parentheses. From Gustafsson and Part (1990). (b) Aging in a mammal. For males in a wild population of red deer (Cervus elaphus), the number of calves fathered each year declines sharply after a peak at about age nine; for females, the number of calves produced each year declines gradually starting at about age 13. For both sexes, the probability of surviving from one year to the next is nearly 100% from age 2 to 9. After age 9, the probability of surviving plummets. From Clutton-Brock et al. (1988). (c) Aging in an insect. For females in a laboratory population of fruit flies (Drosophila melanogaster), the average number of eggs laid per day declines after the age of about 12 days. In three laboratory populations, the probability of surviving from one day to the next falls at the age of about 20 days. Modified from Rose (1984).

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Why Do Organisms Age & Die?

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Hypothesis (1) The Rate-of-Living Theory of Aging

Aging is caused by the accumulation of irreparable damage to cells and tissues.

One theory holds that aging is a function of metabolic rate…

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Predictions of ROL Theory

1. Because cell & tissue damage is caused in part by the by-products of metabolism, aging rate should be correlated with metabolic rate.
2. Because organisms have been selected to resist and repair damage to the maximum extent possible, species should not be able to evolve longer life span, whether subject to natural or artificial selection.

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Predictions of ROL Theory

Test prediction 1: According to the ROL theory, all species should expend the same amount of energy per gram per life time?

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The Rate-of-Living Theory

… but data on variation in metabolic rate and aging among mammals refute this theory.

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Testing prediction 2: Species cannot evolve longer life spans by artificically selecting for longevity.

Artificial selection increases life span in fruit flies:

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Nematodes engineered to have longer telomeres live longer

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Genetic Variation of Longevity

Many population harbor genetic variation for longevity, yet longer life spans have not evolved.

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Hypothesis (2) The Evolutionary Theory of Aging

Natural selection varies as a function of an individual’s age.

Under the evolutionary theory of aging, the failure to completely repair damage is ultimately caused by either:

1) deleterious mutation, or 2) trade-offs between repair & reproduction (The Antagonistic Pleiotropy Hypothesis).

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A simple genetic model reveals two mechanisms through which aging can evolve�

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Population 1

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1) Deleterious Mutations and Aging: The Mutation Accumulation Hypothesis

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Population 2

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The Mutation Accumulation Hypothesis

Natural selection is weak late in life, so alleles that cause aging are only mildly deleterious.
They may persist in mutation-selection balance or rise to high frequency by drift.

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An increase in inbreeding depression with age consistent with the mutation accumulation hypothesis of senescence

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Evolution of shorter life spans consistent with the accumulation of deleterious mutations

Some of these alleles will rise to high frequency by drift, causing a decline in natural longevity

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Trade-Offs and �Aging: The Antagonistic Pleiotropy Hypothesis

A mutation that affects two different life history characters - pleiotropic

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The Antagonistic Pleiotropy Hypothesis

Because natural selection is weaker late in life, alleles that enhance early-life reproduction may be favored even if they also hasten death.

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Nematode worm C. elegans

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Under semi-natural conditions, the hx546 allele of age-1 gene, which causes increased longevity, in the nematode worm C. elegans exhibits antagonistic pleiotropy

Periodic bouts of starvation

(selectively against)

Normal (selectively neutral)

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Fruit Fly

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The methuselah gene controls a trade-off between reproductive success versus longevity and stress resistance

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Flycatchers Ficedula albicollis

In collared flycatchers (Ficedula albicollis), natural variation and experimental manipulations demonstrate a trade-off between reproduction early in life versus reproduction late in life

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(a) Females that first breed at age 1 (blue boxes) have smaller clutches at ages 2, 3, and 4 than females that do not breed until age 2 (red boxes).

(b) Females given extra eggs at age 1 (red boxes) have progressively smaller clutches each year at ages 2, 3, and 4. In contrast, control females (blue boxes) do not begin to show a decline in clutch size until age 4

1st year breeder has higher life time reproductive success

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Box 13.2 Is there an evolutionary explanation for Menopause?

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(a) Reproductive senescence in women and men. These data are from the Ache, hunter-gatherers of Paraguay. The graph shows the probability that women and men will have a child born during the year that they are any given age.

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(b) This graph shows the functional capacity of various physiological systems in women as a function of age.

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(c) Reproductive capacity as a function of age in captive rats.

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(d) Fraction of individuals surviving as a function of age in three hunter-gatherer cultures.

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(e) Time spent foraging each day during the wet season (left) and the dry season (right) by Hadza women of different ages: I = women who have reached puberty, but not yet married or begun to have children; II = women who are pregnant or have young children; III = women who are past childbearing age and have no children younger than 15. Blue circles represent women nursing young children

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(f) This Hadza woman is approximately 65 years old. She is using a digging stick and muscle power to dig tubers from underneath large rocks. Digging tubers requires knowledge, skill, patience, strength, and experience, making Hadza grandmothers the most productive foragers.

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End