Life Histories - Part 2 – Chapter 7 Concept 7.3 There are trade-offs between life history traits. Life History Theory... ...assumes that observed combinations of traits represent adaptive solutions to selection pressures constraints limited options are available to organisms limited range of adaptation is possible allometric constraints (size of organisms) larger organisms often produce more offspring example: larger female crabs are physically able to carry more eggs How can animals get larger? -grow quickly to a large size -grow slowly, but live long enough to be a large size What are the trade-offs between these options? Why might one species grow faster than another? Other allometric constraints: body mass vs. time to reproductive maturity body mass vs. lifespan diameter of tree vs. tree height allometric constraints relate to physics phylogenetic constraints evolutionary history differences in genetic information example: variation in the life history traits of 24 mammal species For 24 mammal species, relative age at first reproduction, relative life expectancy, relative period of maternal investment, and relative annual fecundity – “relative” means that size effects have been removed (e.g. age per body mass) Studies of life histories require a comparative approach study adaptations by comparing closely related species or variation within a single species e.g., within snails instead of snails vs. mammals? life history theory is most appropriate within a single species -- variants within or among populations removes most phylogenetic constraints; some allometric constraints may remain Reproductive value: an individual’s contribution to the future population current reproduction + future reproduction growth now may influence reproduction now and in the future Costs of Reproduction Delay of reproduction can yield faster growth or longer survival Example: Why don’t birds put more eggs into a nest? Costs of Reproduction Delay of reproduction can yield faster growth or longer survival Example: Why don’t birds put more eggs into a nest? Increased reproductive effort can cause reduced growth of parent reduced survival of parent reduced future reproduction of parent Optimality Models – Quantifying Trade-Offs survival Offspring survival Parental survival Amount of Defense by Parents Optimality Models – Quantifying Trade-Offs Offspring survival survival What other factors could influence brood size? Parental survival Amount of Defense by Parents How much energy should be spent on each offspring? Trade-offs between current and future fecundity Reznick and Endler (1982): guppies in Trinidad Populations of guppies in streams and pools with different predators display different life-history traits that can also be manipulated in aquaria. Trade-offs between current and future fecundity Reznick and Endler (1982): guppies in Trinidad Predator Predation on Mortality Time of Reproduction Reproductive effort Offspring Individual growth rate Cichlid Fish Large, mature High in adults Earlier Killifish Small, juvenile High in juveniles Later (Delayed) Greater Less More, smaller Low Fewer, larger High Trade-offs between current and future fecundity Reznick and Endler (1982): guppies in Trinidad Predator Predation on Mortality Time of Reproduction Reproductive effort Offspring Individual growth rate Cichlid Fish Large, mature High in adults Earlier Killifish Small, juvenile High in juveniles Later (Delayed) Greater Less More, smaller Low Fewer, larger High Are these traits independent? Concept 7.4 Organisms face different selection pressures at different life cycle stages. Concept 7.4 Life Cycle Evolution Different morphologies and behaviors are adaptive at different life cycle stages. Differences in selection pressures over the course of the life cycle result in distinctive patterns in life histories. Concept 7.4 Life Cycle Evolution Parental Investment: • Provisioning eggs or embryos—yolk and protective coverings for eggs, nutrient-rich endosperm in plant seeds • Parental care—invest time and energy to feed and protect offspring Figure 7.21 Kiwi Parental Investment Concept 7.4 Life Cycle Evolution Dispersal and Dormancy: • Small offspring are well-suited for dispersal. • Dispersal can reduce competition among close relatives and allow colonization of new areas. • Dispersal can allow escape from areas with diseases or high predation. Concept 7.4 Life Cycle Evolution Dormancy: State of suspended growth and development in which an organism can survive unfavorable conditions. Small seeds, spores, eggs, and embryos are best suited to dormancy—less metabolic energy is needed to stay alive. But some larger animals also enter dormancy. Concept 7.4 Life Cycle Evolution Functional specialization of stages is common in complex life cycles. In many insects the larval stage stays in one small area, such as on a single plant. The larvae are specialized for feeding and growth. The adult is specialized for dispersal and reproduction. Concept 7.4 Life Cycle Evolution In marine invertebrates, larvae are specialized for both feeding and dispersal in ocean currents. Many larvae have specialized ciliated bands for feeding, covering most of the body. They may also have spines, bristles, or other structures to deter predators. Figure 7.11 The Pervasiveness of Complex Life Cycles (Part 2) Concept 7.4 Life Cycle Evolution Complex life cycles may result from stage-specific selection pressures and help minimize the drawbacks of small, vulnerable early stages. Separate life history stages can evolve independently in response to size- and habitat-specific selection pressures. Chapter 7 Life History CONCEPT 7.1 Life history patterns vary within and among species. CONCEPT 7.2 Reproductive patterns can be classified along several continua. CONCEPT 7.3 There are trade-offs between life history traits. CONCEPT 7.4 Organisms face different selection pressures at different life cycle stages.