CHAPTER 7: LIFE HISTORIES AND EVOLUTIONARY FITNESS

1. CHAPTER 7: LIFE HISTORIES AND EVOLUTIONARY FITNESS

2. Life Histories • Consider the following remarkable differences in life history between two birds of similar size: • thrushes • reproduce when 1 year old • produce several broods of 3-4 young per year • rarely live beyond 3 or 4 years • storm petrels • 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

3. Parental investment

4. What is life history? • The life history is the schedule 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

5. What influences life histories? • Life histories are influenced by: • body plan and life style of the organism • evolutionary responses to many factors, including: • physical conditions • food supply • predators • other biotic factors, such as competition

6. A Classic Study • David Lack of Oxford University first placed life histories in an evolutionary context: • tropical songbirds 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 to find food during the breeding season

7. 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

8. 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: • maximum number of chicks fledged corresponded to normal clutch size of seven

9. 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?

10. 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) • fecundity (number of offspring per reproductive episode) • aging (total length of life)

11. Life history: set of rules and choices influencing survival and reproduction

13. 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

14. 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, 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)

15. Grime’s Scheme for Plants • English ecologist J.P. Grime envisioned life history traits of plants as lying between three extremes: • stress tolerators (tend to grow under most stressful conditions) • ruderals (occupy habitats that are disturbed) • competitors (favored by increasing resources and stability)

16. Grime’s Scheme for Plants

17. Stress Tolerators • Stress tolerators: • grow under extreme environmental conditions • grow slowly • conserve resources • emphasize vegetative spread, rather than allocating resources to seeds

18. Ruderals • Ruderals: • are weedy species that colonize disturbed habitats • typically exhibit • rapid growth • early maturation • high reproductive rates • easily dispersed seeds

19. Competitors • Competitors: • grow rapidly to large stature • emphasize vegetative spread, rather than allocating resources to seeds • have long life spans

20. Life histories resolve conflicting demands. • 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 with artificially reduced or enlarged broods exhibited enhanced or diminished adult survival, respectively

21. Parental investment affects parental survival

22. Life histories balance tradeoffs. • Issues concerning life histories may be phrased 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?

23. 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

24. Long-lived organisms mature later than short-lived ones

25. Fecundity versus Survival 1 • How do organisms optimize the trade-off between current fecundity and future growth? • Critical relationship is:  = S0B + SSR 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

26. Fecundity versus Survival 2 • When the previous relationship is rearranged, the following points emerge: • changes in fecundity (positive) and adult survival (negative) are favored when net effects on population growth are positive • effects of enhanced fecundity and reduced survival depend on the relationship between S and S0 • one thus expects to find high parental involvement associated with low adult survival and vice versa

27. In other words… • The number of offspring produced today can reduce the number produced tomorrow • Natural selection should optimize the trade-off between present and future reproduction • What factors influence the resolution of this conflict? • High mortality rates for adults… ? • Long adult life span… ?

30. Fecundity and mortality rates for 33 species of birds: vary together

31. Growth versus Fecundity • Some species grow throughout their lives, exhibiting indeterminategrowth: • 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

32. Semelparity and Iteroparity • 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.

33. Semelparity: Agaves 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 • exhibit synchronous seed production followed by death of adults

34. Agaves: semelparous

36. 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 may be resolved by considering the advantages of timing reproduction to match occasionally good years

37. More on Semelparity in Plants • Semelparity seems favored when adult survival is good and interval between favorable years is long. • Advantages of semelparity: • timing reproductive effort to match favorable years • attraction of pollinators to massive floral display • saturation of seed predators

38. Senescence is a decline in function with age • Senescence is an inevitable decline in physiological function with age. • Many functions deteriorate: • most physiological indicators (e.g., nerve conduction, kidney function) • immune system and other repair mechanisms • Other processes lead to greater mortality: • incidence of tumors and cardiovascular disease

39. Senescence… (males in English population in 1980s)

40. Why does senescence occur? • Senescence may be the inevitable wearing out of the organism, the accumulation of molecular defects: • ionizing radiation and reactive forms of oxygen break chemical bonds • macromolecules become cross-linked • DNA accumulates mutations • In this sense the body is like an automobile, which eventually wears out and has to be junked.

42. Why does aging vary? • Not all organisms senescence at the same rate, suggesting that aging may be subject to natural selection: • organisms with inherently shorter life spans may experience weaker selection for mechanisms that prolong life • repair and maintenance are costly; investment in these processes reduces investment in current fecundity

43. Life histories respond to variation in the environment • Storage of food and buildup of reserves • Dormancy  physiologically inactive states • Hibernation  spending winter in a dormant state • Diapause  (insects) – water is chemically bound or reduced in quantity to prevent freezing and metabolism drops so low to become barely detectable

44. What are the stimuli for change • Proximate factors (day length, for example) – an organism can assess the state of the environment but these factors do not directly affect its fitness • Ultimate factors (food supplies, for example) – environmental features that have direct consequences on the fitness of the organism • Photoperiod: the length of daylight: proximate factor to virtually all organisms

45. Relationships between age and size at maturation may differ when growth rates differ

47. Food Supply and Timing of Metamorphosis • Many organisms undergo metamorphosis from larval to adult forms. • A typical growth curve relates mass 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?

48. Metamorphosis Under Varied Environments • Poorly-nourished organisms grow more slowly and cannot reach the same mass 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.

49. An Experiment with Tadpoles • 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