1Life-history evolution: why life’s a grind and then we dieBiol 3410, 27 Feb 09Why is death life’s only certainty (other than taxes, and declining vigor)?Why do the sexes often senesce at different rates?And why do species differ so much in life spans and schedules of reproduction?The rate-of-living theorycan’t explain it!The most popular theory is that organisms simply wear out, through inability to repair all oxidative damage, mutations, and the like.This theory predicts that the rate of aging should be directly tied to metabolic rate, and that species should not be able to evolve longer life spans without reducing their metabolic rate.batsrodentsinsectivoreselephant shrewsprimatesThe evolutionary theory of aging:selection weakens as life advancesSir Peter MedawarIn the early 1950s, Peter Medawar pointed out that selection against deleterious mutations with effects that appear only latein life will be much weaker than selection against equivalent mutations with effects earlyin life, if there is inescapable extrinsicmortality.Late-effect mutations will therefore accumulate at much higher rates than early-effect mutations, causing senescence.Model: assume 20% mortality per generation, first reproduction at age 32.385Late-onsetlethal mutationProfessor Emeritus George C. Williams(Ecology & Evolution, SUNY Stony Brook)And earlyreproduction is worth much morethan late reproductionIn the late 1950s, George Williams pointed out that mutations with harmful (even lethal) effects late in life could be positively favored by selection if they also increased earlyreproduction.Such “pleiotropic” tradeoffs between early and late fitness can explain the adaptiveevolution of shortened lifespans (that is, mortality).Model: begin reproducing at age 2, drop dead at age 10 if still alive.If the evolutionary theory is correct, life histories are shaped by (1) rates of extrinsic mortality, and (2) the costs of reproductionExtrinsic mortality rates vary greatly (e.g.,rodents > bats), and so do rates of senescence (e.g.,rodents > bats).Reproduction (especially early reproduction) is costly, as shown by these data for collared flycatchers (Ficedula albicollis).Even so, females that begin at age 1 have higher lifetime fitness (1.24 offspring) than those that begin at age 2 (0.90 offspring). (Gustafsson and Pärt, Nature347, 279-281, 1990)Females that begin at age 1Females that begin at age 2Females given extra eggs at age 1Ficedula albicollisRates of senescence should evolve downwardwhen extrinsic mortality is loweredSteven Austadwith his friend, a long-lived house mouseVirginia opossums on predator-free Sapelo Island age and reproduce more slowly than those on the neighboring South Carolina mainland, where many individuals are killed by bobcats and other predators.Collagen cross-linking in tendons even goes more slowly in the island ‘possums!2Selection favors the fittest phenotype, right?Yes, by definition.When it comes to offspring, more is better, right?Yes, other things being equal.So what’s not equal?Resources. Like food, for example.Oh. So the kids each get less in big families?Yes, something like that.OK, so what’s the fittest family size?The one that maximizes the parents’ fitness.That would be the sumof the kids’ fitnesses, right?Right!n*W(n)?Right!So we just take the derivative with respect to n (the number of offspring), set it equal to zero, and solve?Right! Do it!Offspring number (n)Offspring number (n)Offspring size and number: a fundamental tradeoffW (n)The theory:Smith & Fretwell (1974)The theory in action: evolution of salmon egg sizeHeath et al. (2003)Birds should lay the number of eggs that maximizes their number of surviving young.The great tits (Parus major) in Wytham Wood have been followed for decades by David Lack and his students.The most frequent clutches are much smaller than the most productive clutches.What’s going on? Don’t these birds care about fitness?Parus majorSolid bars: FrequencyHatched bars: Surviving youngTo address this question, Lack and his students manipulated clutch sizes.They added 1-4 eggs to some nests and removed 1-7 eggs from others, then followed the fates of the parents and chicks.Adding more than one or two eggs almost always resulted in a lowerreproductive success for the parents.This result suggests that parents know what they’re doing when they decide to lay a clutch of a certain size!Parents differ in ability to provide for offspring, and they adjust their clutch sizes to their current conditions.Typical (9-egg) parents are maximizing their expected reproductive success, under the circumstances, as are allparents.We infer that the evolved “function” of a clutch-size “decision” or “strategy” is to help the parents fledge as many healthy chicks as they can, given their current condition.Food-rich environment Food-limited environmentA “longevity gene” in Caenorhabditis elegans?The AGE-1 protein is a kinase (protein phosphorylatingenzyme) required for the regulation of metabolism, stress resistance, embryonic development, and life span.Mutations in the age-1gene increase lifespan by up to 80%.One of these mutations (hx546) is selectively neutral in a food-rich environment but deleteriousunder food limitation.Thus longevity comes at a price, and this explains why age-1remains unmutated in natural populations of C. elegans.Wet season Dry seasonDistinguished Professor Kristen Hawkes(Anthropology,Univ. of Utah)Why do human females live long after last reproduction?Few other animals do this, and it is not expected. (Why?)Kristen Hawkes and her colleagues have proposed that non-reproductive grandmothers help their daughters reproduce by helping to feed (and otherwise care for) the daughter’s older children.In support of this hypothesis, they have gathered data showing that older women are extremely productive as foragers in modern hunter-gatherer societies.A 65-year-old Hadza woman gathers