Chapter 12. Aging and other life history characters

Chapter 12. Aging and Other Life History Characters

Evolution of Senescence

Aging = senescence, a late-life decline in fertility and survival (fitness), Figure 12.4. Why doesn't natural selection postpone or eliminate senescence?
Rate of living theory of aging: live fast, die young or live slow die old.

Mechanism = irreversible accumulation of cellular damage.
Natural selection has resulted in maximum ability to repair damage, so no genetic variance for this trait remains, hence no further evolution possible.

Energy expenditure per gram per lifetime should be similar over broad range of taxa.
Life span should be resistant to selection.

Austad and Fisher (1991, Figure 12.5) found considerable variation in energy expenditure.

Bats: live 3X as long as other mammals with similar body masses and metabolic rates.
Marsupials: low metabolic rate, but live longer than other mammals with similar body masses.

Luckinbill (1984, Figure 12.6): artificial selection in Drosophila can double life span.
Molecular rescue of Rate of Living Theory:

With each nuclear/cell division bits of telomeres lost and this is associated with cell death.
Telomeres are repaired by telomerase, especially in germline and cancer cells.
In mammals, cell and organism life spans are correlated (Figure 12.7).
So, why hasn't natural selection increased telomerase activity? A trade-off with cancer?

Example: mutations of genes encoding DNA repair enzymes.
Hereditary nonpolyosis colon cancer -- median age of diagnosis is 48.
How to detect these mutations? Hughes (2002, Figure 12.10) created inbred lines of Drosophila, compared with outbred lines to detect inbreeding depression. This increases with age.

Antagonistic pleiotropy: mutations resulting in less energy devoted to repair and more to reproduction early in life.

Gustaffson and Part (1990, Figure 12.13): collared flycatchers that begin reproduction early have smaller clutches; when given additional eggs, senesce sooner.
Austad (1993, Figure 12.14): opossums exposed to reduced extrinsic mortality have delayed reproductive and physiological senescence.

Explanation: reduced extrinsic mortality results in more late acting mutations being exposed to natural selection and more individuals living long enough to experience late-life costs of early reproduction. Natural selection "sees" and removes alleles favoring senescence.

Evolution of Cutch Size

How many offspring should an individual produce in a given year?
Theoretical work in this area begins with Lack (1947): optimal clutch size is the one that maximizes number of surviving young. Elements of model (Figure 12.16):

But investigations generally show that mean clutch size in a bird population is "suboptimal" (e.g., Boyce and Perrins 1987, Figure 12.17). This could be because Lack’s model doesn't take into account:

Future reproduction by mothers.
Reproductive success of daughters.
Both could be enhanced by reduced clutch size (see Figure 12.18 for effect of clutch size on daughters.)

Evolution of Body Size

How big should offspring be?
Smith and Fretwell (1974, Figure 12.22): A trade-off between size and number of offspring should result in parental fitness being maximized when offspring of intermediate size are produced.
Heath (2003, Figure 12.23): tested the Smith and Fretwell model with salmon (egg size of hatchery fish). Optimal egg size for hatchery fish lower than optimal for wild fish. This can be explained by considering survival of hatchery and wild fish.

Some Ecological Considerations: Some plant examples.

In unstable ephemeral habitats (e.g. river banks, fields) extrinsic mortality is high and unpredictable. This may favor early development and reproduction, which results in small body size, which increases extrinsic mortality. These are characteristics of annual weeds and other pioneer species.
In physically extreme environments with short growing seasons (e.g. arctic tundra), there may not be enough time to complete a life cycle in the span of a growing season. This may favor evolution and maintenance of the perennial habit.
In any environment there are multiple life-history solutions to environmental challenges. For example in the desert some plants may be short-lived annuals, others may be long-lived shrubs with deep roots.