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Life-history Characteristics. All organisms have been selected to maximize reproductive success over the course of their lifetimes. There is, however, tremendous variation in how organisms achieve this. Life-history Characteristics.
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Life-history Characteristics • All organisms have been selected to maximize reproductive success over the course of their lifetimes. • There is, however, tremendous variation in how organisms achieve this.
Life-history Characteristics • Some organisms produce many offspring once, but live only a short time. • Others produce a few offspring over the course of a long life.
Life-history Characteristics • There is also enormous variation in the size of offspring. Oysters produce 10-50 million tiny eggs whereas whales produce a single large calf. • What explains the variation we see?
Life-history Characteristics • Clearly, there are constraints and trade-offs in the strategies that organisms can employ. • The best strategies are determined by the availability of energy and an organisms’ prospects of survival.
Sample life history • Consider a female opossum’s life history. • Born and nursed by mother for about 3 months. • Becomes independent and grows to maturity. • Age 10 months has first litter of 8 pups. Age 15months has second litter of 7 pups • Killed by predator at 20 months.
Life-history Characteristics • Female’s energy came from different sources and was allocated in different directions over course of her life. • For first 3 months received energy from her mother. After that had to obtain her own.
Life-history Characteristics • As a juvenile she devoted energy to growth, metabolism and repair. • After reaching maturity she devoted energy to metabolism, repair and reproduction.
Life-history Characteristics • Fundamentally, differences in how and when energy is allocated affect life history strategies. • A different opossum might have matured earlier at a smaller size, and produced babies earlier, but perhaps fewer or smaller ones. • Alternatively, more energy might be allocated to repair and less to reproduction perhaps resulting in a longer life.
Differential energy allocation by sand crickets • Sand crickets occur in both long-wined and short-winged forms (papers by Zhao and Zera 2002, 2003). • Long-winged forms have well developed flight muscles and fuel to power them. This enables them to disperse if conditions are poor. • Short-winged forms cannot disperse, but can develop eggs more quickly. • There is a trade-off between dispersal ability and early reproduction.
Issues in life-history analysis • Analyzing life history decisions involves cost-benefit analysis and an examination of fitness trade-offs as it relates to the following questions: • Why do organisms age and die? • How many offspring should an individual produce in a given year? • How big should each offspring be?
Why do organisms age and die? • Senesence is a late-life decline in an individual’s fertility and probability of reproducing. • Same pattern found in many organisms.
If senesence reduces reproductive success we would expect it to be opposed by selection.
Hypotheses explaining senesence • Two major hypotheses on why aging persists: • Rate-of-living theory • Evolutionary trade-off theory
Rate-of-living Hypothesis • This hypothesis suggests that aging is caused by accumulation of cellular damage caused by accumulation of toxins and accumulation of errors during replication, transcription and translation of DNA. • Hypothesis suggests organisms have reached limit of biological repair and no more genetic variation exists for improved repair mechanisms.
Rate-of-living Hypothesis • Hypothesis makes two predictions. • 1. Cell and tissue damage caused by metabolism so aging rate should be correlated with metabolic rate. • 2. Species should not be able to evolve longer life spans.
Rate-of-living Hypothesis • Austad and Fisher (1991) tested prediction 1. • Calculated amount of energy expended per gram of tissue per lifetime for 164 mammal species. Theory predicts rate should be similar across groups. • Found range from 39 kcal/g/lifetime in elephant shrews to 1,102 kcal/g/lifetime in a bat.
Rate-of-living Hypothesis • Also found bats have rates similar to those of many other mammals but life spans that are 3 times as long. • These patterns don’t fit rate-of-living predictions.
Rate-of-living Hypothesis • Luckinbill et al. (1984) tested prediction 2 by artificially selecting for longevity in fruit flies. • Lineages in which they selected for late reproduction showed greatly increased longevity over the course of 13 generations of selection. Average lifespan increased from 35 to 60 days.
Rate-of-living Hypothesis • Results of tests thus do not support the rate-of-living hypothesis.
Evolutionary Hypothesis for aging • If selection can produce longer life spans why does it not do so? • Under evolutionary hypothesis for aging, organisms age because the body fails to repair cell and tissue damage rather than because it cannot do so.
Evolutionary Hypothesis for aging • Failure to repair may be due to (i) accumulation of deleterious mutations or (ii) trade-offs between repair and reproduction.
Evolution of senesence in a hypothetical population. • Population has annual probability of survival each year of 0.8 (death by accident, predation, etc.). Population declines exponentially over time. • Individuals with wild-type genotype mature at age 3 and die at age 16 (if not killed). Have one offspring a year. • Population expected lifetime Reproductive success of 2.419.
Evolution of senesence in a hypothetical population. • New mutation occurs which causes death at age 14. Rest of life history unchanged. • Expected lifetime RS reduced to 2.34 offspring, a small reduction and 96% of the lifetime RS of the wildtype. • Few individuals live beyond 14 in wildtype population so effect is small.
Evolution of senesence in a hypothetical population. • In general, mutations that cause death late in natural life will be only weakly selected against. • Mutation that causes death at a young age, of course, will be strongly selected against. • Such mutations may be maintained in population by mutation-selection balance.
Evolution of senesence in a hypothetical population. • An example of the kind of mutation that could cause death only late in life might be one that causes cells to not repair themselves as well as is possible. • For example, in humans, a DNA mismatch repair mutation causes a form of colon cancer. Median age of diagnosis is 48 (range 17 to 92) well after reproduction has begun.
Evolution of senesence in a hypothetical population. • In our hypothetical population a second mutation occurs that causes reproduction to begin at age 2 and death at age 10. • There is thus a trade-off between age of first reproduction and longevity. • Expected lifetime RS of individuals with mutation is 2.66, which is 1.1 times the wildtype’s RS.
Evolution of senesence in a hypothetical population. • Most individuals reap benefit of early reproduction, bur few pay cost of earlier death. • This mutant allele should spread rapidly. • A gene that causes less energy to be devoted to cellular damage repair and more to be devoted to reproduction would fit profile of such a mutant. Several have been identified in fruit flies and nematodes.
Evidence of a trade-off caused by early reproduction. • In a study of Collared Flycatchers individuals that bred at age 1 had smaller clutches at ages 2-4 than individuals who don’t first breed until age 2.
Evidence of a trade-off caused by early reproduction. • Also, females whose clutches were artificially enlarged in year 1 had progressively smaller clutches in years 2-4.
Evidence of a trade-off caused by early reproduction. • Conclusion is that there is a trade-off between early and late reproduction in Collared Flycatchers. • First year breeders do have higher life time RS than second year breeders.
Evolution of ageing in Opossums • We expect populations with low rates of mortality due to factors such as predation to evolve delayed senesence. • Under these circumstances mutations that cause senesence are more likely to make themselves felt because animals live to be older and so will be selected against.
Evolution of ageing in Opossums • Austad (1993) studied two populations of Opossums one on Georgia mainland, the other on Sapelo Island off the coast. • Opposums on mainland have high mortality rates from predators (>50% of all deaths). • No mammalian predators on Sapelo Island.
Evolution of ageing in Opossums • Austad followed life histories of radio-collared opossums on both sites. • Island populations aged more slowly than mainland populations on several measures including rate of survival, reproductive performance, and connective tissue physiology.
How many offspring should an individual produce in a year? • In life history decisions a fundamental choice is how many offspring to produce in a year. • The more offspring produced in a year, the less each can be cared for and additional offspring affect the parents prospects for survival.
Clutch size in birds • The question of how many young is optimal has been extensively studied in birds. • David Lack 1947 suggested that selection would favor the clutch size that produced the most surviving offspring.
Clutch size in birds • If probability of average offspring surviving falls with increasing clutch size then we can calculate optimal clutch size by multiplying clutch size by probability of survival. • An intermediate clutch size is thus optimal.
Clutch size in birds • There have been numerous field studies that have tested Lack’s hypothesis. • Many studies (including ones in which additional eggs are added to the brood) have found that the most productive clutch is often several eggs larger than that laid by the birds.
