Chapter 10: Life Histories and Evolutionary Fitness

Journals related to ecology and evolution • Journal of Ecology • Journal of Molecular Ecology • Ecology • Oikos • Ecology Letters • Trends in Ecology and Evolution • Science/Nature (c) 2001 W.H. Freeman and Company

Chapter Opener A female sockeye salmon红鲑鱼, after swimming up to 5,000 km from her Pacific Ocean feeding ground to the mouth of a coastal river in British Columbia, faces another 1,000 km upriver journey to her spawning ground产卵地. There she lays thousands of eggs, and then promptly dies, her body wasted from the exertion（用尽）.

Figure 10.1

Life Histories • Consider the following remarkable differences in life history between two birds of similar size: • Thrushes（鸫鸟） • reproduce when 1 year old • produce several broods of 3-4 young per year • rarely live beyond 3 or 4 years • storm petrels (海燕） • do not reproduce until they are 4 to 5 years old • produce at most a single young per year • may live to be 30 to 40 years old (c) 2001 W.H. Freeman and Company

What is life history? • The life history is the schedule日程表 of an organism’s life, including: • age at maturity • number of reproductive events • allocation of energy to reproduction • number and size of offspring • life span（寿命） (c) 2001 W.H. Freeman and Company

What influences life histories? • Life histories are influenced by: • body plan体型 and life style生活型态 of the organism • evolutionary responses to many factors, including: • physical conditions • food supply • predators • other biotic factors, such as competition (c) 2001 W.H. Freeman and Company

A Classic Study • David Lack of Oxford University first placed life histories in an evolutionary context: • tropical songbirds（鸣禽） lay fewer eggs per clutch than their temperate counterparts • Lack speculated that this difference was based on different abilities to find food for the chicks: • birds nesting in temperate regions have longer days in which to find food during the breeding season (c) 2001 W.H. Freeman and Company

Figure 10.2

Lack’s Proposal • Lack made 3 key points, suggesting that life histories are shaped by natural selection: • because life history traits (such as number of eggs per clutch) contribute to reproductive success they also influence evolutionary fitness • life histories vary in a consistent way with respect to factors in the environment • hypotheses about life histories are subject to experimental tests (c) 2001 W.H. Freeman and Company

An Experimental Test • Lack suggested that one could artificially increase the number of eggs per clutch to show that the number of offspring is limited by food supply. • This proposal has been tested repeatedly: • Gören Hogstedt manipulated clutch size of European magpies（喜鹊）: • maximum number of chicks fledged corresponded to normal clutch size of seven (c) 2001 W.H. Freeman and Company

Figure 10.3

Life Histories: A Case of Trade-Offs • Organisms face a problem of allocation of scarce resources (time, energy, materials): • the trade-off: resources used for one function cannot be used for another function • Altering resource allocation affects fitness. • Consider the possibility that an oak tree might somehow produce more seed: • how does this change affect survival of seedlings? • how does this change affect survival of the adult? • how does this change affect future reproduction? (c) 2001 W.H. Freeman and Company

Figure 10.4

Components of Fitness • Fitness is ultimately dependent on producing successful offspring, so many life history attributes relate to reproduction: • maturity (age at first reproduction) • parity (number of reproductive episodes事件) • Fecundity结实力 (number of offspring per reproductive episode) • aging (total length of life) (c) 2001 W.H. Freeman and Company

Phenotypic plasticity allows an individual to adapt. • A reaction norm is the observed relationship between the phenotype and environment: • a given genotype gives rise to different phenotypes under different environments • Responsiveness（敏感度） of the phenotype to its surroundings is called phenotypic plasticity • example: the increased rate of larval development of swallowtail butterfly（燕尾蝴蝶） larvae at higher temperatures (c) 2001 W.H. Freeman and Company

Figure 10.5

Genotype-Environment Interaction • When populations have differing reaction norms across a range of environmental conditions, this is evidence of a genotype-environment interaction. • Such an interaction is evident in development of swallowtail larvae: • genotypes from Alaska and Michigan: each performs worse in the other’s habitat - the reaction norms for these genotypes cross (c) 2001 W.H. Freeman and Company

Figure 10.6

What is specialization? • Genotype-environment interactions are the basis for specialization（特化）. • Consider two populations exposed to different conditions over time: • different genotypes will predominate in each population • populations are thus differentiated with different reaction norms • each population performs best in its own environment (c) 2001 W.H. Freeman and Company

Figure 10.7

Reciprocal Transplant交互迁移 Experiments • Reciprocal transplant experiments involve switching（转换） of individuals between two localities: • in such experiments, we compare the observed phenotypes among individuals: • kept in their own environments • transplanted to a different environment • such experiments permit separating differences caused by genetic differences versus phenotypic plasticity (c) 2001 W.H. Freeman and Company

Figure 10.8

Figure 10.9

Food Supply and Timing of Metamorphosis（变态，形变） • Many organisms undergo metamorphosis from larval to adult forms. • A typical growth curve relates mass 重量 to age for a well-nourished individual, with metamorphosis occurring at a certain point on the mass-age curve. • How does the same genotype respond when nutrition varies? (c) 2001 W.H. Freeman and Company

Metamorphosis Under Varied Environments • Poorly-nourished organisms grow more slowly and cannot reach the same mass（重量） at a given age. • When does metamorphosis occur? • fixed mass, different age? • fixed age, different mass? • different mass and different age? • Solution is typically a compromise between mass and age, depending on risks and rewards associated with each possible combination. (c) 2001 W.H. Freeman and Company

Figure 10.10

An Experiment with Tadpoles[ˈtædˌpoʊl]（蝌蚪） • Tadpoles fed different diets illustrate the complex relationship between size and age at metamorphosis: • individuals with limited food tend to metamorphose at a smaller size and later age than those with adequate food (compromise solution) • the relationship between age and size at metamorphosis is the reaction norm of metamorphosis with respect to age and size (c) 2001 W.H. Freeman and Company

Figure 10.11

The Slow-Fast Continuum 1 • Life histories vary widely among different species and among populations of the same species. • Several generalizations emerge: • life history traits often vary consistently with respect to habitat or environmental conditions • variation in one life history trait is often correlated with variation in another (c) 2001 W.H. Freeman and Company

The Slow-Fast Continuum 2 • Life history traits are generally organized along a continuum of values: • at the “slow” end of the continuum are organisms (such as elephants, giant tortoises[ˈtɔrtəs], and oak trees) with: • long life • slow development • delayed maturity • high parental investment • low reproductive rates • at the “fast” end of the continuum are organisms with the opposite traits (mice, fruit flies, weedy plants) (c) 2001 W.H. Freeman and Company

Grime’s Scheme体系 for Plants • English ecologist J.P. Grime envisioned相像 life history traits of plants as lying between three extremes: • stress tolerators (tend to grow under most stressful conditions) • Ruderals杂草 (occupy habitats that are disturbed) • competitors[kəmˈpetɪtər] (favored by increasing resources and stability) (c) 2001 W.H. Freeman and Company

Stress Tolerators • Stress tolerators（压力耐受者）: • grow under extreme environmental conditions • grow slowly • conserve resources • emphasize vegetative spread, rather than allocating resources to seeds (c) 2001 W.H. Freeman and Company

Ruderals • Ruderals杂草: • are weedy species that colonize disturbed habitats • typically exhibit • rapid growth • early maturation • high reproductive rates • easily dispersed seeds (c) 2001 W.H. Freeman and Company

Competitors • Competitors: • grow rapidly to large stature（体型） • emphasize vegetative spread, rather than allocating [ˈæləˌkeɪt] resources to seeds. • have long life spans (c) 2001 W.H. Freeman and Company

Life histories resolve conflicting demands (冲突的需求). • Life histories represent trade-offs among competing functions: • a typical trade-off involves the competing demands of adult survival and allocation of resources to reproduction: • Kestrels（红隼） with artificially reduced or enlarged broods exhibited enhanced or diminished adult survival, respectively (c) 2001 W.H. Freeman and Company

Life histories balance tradeoffs. • Issues concerning life histories may be phrased表述 in terms of three questions: • when should an individual begin to produce offspring? • how often should an individual breed? • how many offspring should an individual produce in each breeding episode? (c) 2001 W.H. Freeman and Company

Age at First Reproduction • At each age, the organism chooses between breeding and not breeding. • The choice to breed carries benefits: • increase in fecundity at that age • The choice to breed carries costs: • reduced survival • reduced fecundity at later ages (c) 2001 W.H. Freeman and Company

Fecundity versus Survival 1 • How do organisms optimize the trade-off between current fecundity and future growth? • Critical relationship is:  = S0B（幼体生育增加）+ SSR（成体存活率提高） where:  is the change in population growth S0 is the survival of offspring to one year B is the change in fecundity S is annual adult survival independent of reproduction SR is the change in adult survival related to reproduction (c) 2001 W.H. Freeman and Company

Growth versus Fecundity • Some species grow throughout their lives, exhibiting indeterminategrowth无限生长: • fecundity is related to body size • increased fecundity in one year reduces growth, thus reducing fecundity in a later year • for shorter-lived organisms, optimal strategy emphasizes fecundity over growth • for longer-lived organisms, optimal strategy emphasizes growth over fecundity (c) 2001 W.H. Freeman and Company

Semelparity一次繁殖 and Iteroparity多次繁殖 • Semelparous organisms breed only once during their lifetimes, allocating their stored resources to reproduction, then dying in a pattern of programmed death: • sometimes called “big-bang” reproduction • Iteroparous organisms breed multiple times during the life span. (c) 2001 W.H. Freeman and Company

Semelparity: Agaves龙舌兰 and Bamboos • Agaves are the century plants of deserts: • grow vegetatively for several years • produce a gigantic flowering stalk, draining plant’s stored reserves • Bamboos are woody tropical to warm-temperate grasses: • grow vegetatively for many years until the habitat is saturated（饱和） • exhibit synchronous（同时产生的） seed production followed by death of adults (c) 2001 W.H. Freeman and Company

Bet Hedging赌注保护 versus Timing定时 • Why semelparity versus iteroparity? • iteroparity might offer the advantage of bet hedging in variable environments • but semelparous organisms often exist in highly variable environments • this paradox悖论 may be resolved by considering the advantages of timing定时 reproduction to match occasionally good years (c) 2001 W.H. Freeman and Company

Chapter 10: Life Histories and Evolutionary Fitness