Ecology 302: Lecture VI. Evolution of Life Histories

Transcription

1 Ecology 302: Lecture VI. Evolution of Life Histories (Ricklefs, Chapter 7; Life Histories, pp.46-48; Knowledge Project.) Spawning sockeye salmon (Oncorhyncus nerka). The adaptive significance (if any) of semelparity in Pacific salmon is yet to be determined (Photo by A. Hendry.) 1

2 Key Points. Age specific rates of fertility and reproduction determined jointly by environment and genetics. o Tremendous variation among even closely related species. 1. Iteroparity (repeated reproduction) vs. semelparity (reproduce once, then die). 2. Early vs. delayed reproduction. 3. Etc. To what extent is such variation adaptive? Cole s Paradox why are there so many perennials? o One of the first examples of the application of evolutionary thinking to life history variation. o Resolution focuses attention on the importance of distinguishing effective fecundity,, from the number of young produced,. Principle of allocation necessitates trade-offs between current fecundity and subsequent fecundity and survival. 2

3 Reproductive effort (RE) the fraction,, of available resources allocated to reproduction by an year-old, o An optimal life history maximizes max max, all o Criterion of optimality is maximization of see Lecture 5. o Evolution of senescence can be viewed from similar perspective. o Satisfaction of (*) can result in a single global optimum,,, or multiple optima. o Details depend on the shapes of the fertility and survivorship curves i.e., on the way they vary with reproductive expenditure. 3

4 Results from models without age structure: o Increasing reproductive success (includes juvenile survival) favors increased reproductive expenditure. o Increasing adult survival favors reduced reproductive expenditure. o Optimal response to density depends on whether increased crowding principally affects reproductive success (includes juvenile survival) or adult survival. o Increasing year-to-year variability in reproductive success (includes juvenile survival) favors reduced reproductive expenditure. o Increasing year-to-year variability in adult survival favors increased reproductive expenditure. Optimal effort per offspring,, maximizes,. 4

5 I. Life Histories. A. Age specific schedules of reproduction and mortality determined jointly by genes and environment. B. Enormous variation even among closely related species: 1. Annuals vs. perennials. Figure 1. Life history traits are molded by environment and selection. 2. Iteroparity (repeated reproduction) vs. semelparity (reproduce once, then die). 3. Early vs. delayed reproduction. 4. Long vs. short life expectancy. 5. Fecundity increasing with age vs. not 6. Large per offspring parental investment vs. small. 7. Etc. C. To what extent can this variability be explained by appealing to natural selection as an optimizing agent? 5

6 II. Why are there so Many Perennials? A. Look at the plants on campus. 1. Most are perennials. 2. Lamont Cole (1954) argued that this makes no sense when viewed from the perspective of evolution. B. Cole s argument. 1. Perennial expend energy on the structures support structures, storage organs, etc. that permit them to survive from one year to the next. 2. Why not forego these structures and instead produce more seeds? 3. The annual rate, λ p, at which a population of perennials multiplies is (1) where B p is the number of seeds produced by an individual and p is the probability of surviving from one growing season to the next. 6

7 4. A population of annuals multiplies at rate (2) 5. Then (3) 6. The maximum value of is 1. So a mutant that abandons the perennial habit and uses the energy saved to produce one or more additional seeds should outcompete perennial conspecifics. 7. But the world is filled with perennials. 7

8 C. Resolution of Cole s paradox. 1. Per the previous lecture, the B s in Eq 3 are effective fecundities a. Not the numbers of seeds produced, but b. The number that germinate and survive to flower the following year. 2. If and are the number of seeds produced by annuals and perennials, and is the probability that a seed survives to reproduce,, and. Then (4a) 3. Generally speaking, c and / Perennials generally increase in size from one year to the next. If perennial fertility multiplies by a factor of 1 per year, it can be shown that, and Eq 4a becomes (4b) 8

9 D. Annual perennial contrast a special case of iteroparity vs. semelparity. Figure 2. Degree of iteroparity as measured by average number of breeding seasons vs. ratio of juvenile to adult mortality. From Stearns (1976). According to Eqs 4, increasing juvenile to adult mortality should select for increasing iteroparity. 9

10 III. Reproductive Effort (RE). A. Principle of Allocation (PoA): Resources that an organism can allocate to different functions finite necessitates trade-offs. B. Idea dates to the poet, Goethe (Metamorphosis of Plants). C. Reproductive Effort: Let be the proportion of available resources allocated to reproduction by an i year-old. PoA Figure 3. Experimental demonstration of the tradeoff between current reproductive expenditure and subsequent survival I 0; 0; 0 0 (5) 10

11 D. In words: There is a tradeoff between current reproduction,, and subsequent survival,, and subsequent reproduction,. E. Let.. be the schedule of age-specific reproductive expenditures, and let,, be a schedule that maximizes. F. The following equivalences can be proved: max max, all max max, all (6) G. Because RV depends on all the s, one cannot simply step through the age classes and apply Eq 6 to each. 1. Instead, one computes and # corresponding to maxima and minima of. 2. Intersections are peaks, valleys and saddles of fitness in... space (Figure 4). 11

12 Figure 4. Adaptive topographies for a two stage (juvenile-adult) life history. The coordinate axes, and, are juvenile and adult reproductive effort. Reproductive effort (RE) pairs,,, are color coded by according to the visible spectrum with dark red denoting low fitness and dark violet, high. The thick white lines are curves of conditional optima, and. Effective fecundity and post-reproductive survival functions are decelerating functions of RE as in Figure 6b. Arrows indicate selection. 12

13 IV. Theory of Senescence. A. Differentiating the stable age equation (Lecture 5) with respect to yields l (7) where is the reproductive value of an year-old, and (8) is total reproductive value Figure 5. Ricklefs identifies the strength of selection with the probability of surviving to the age in question,. Eq 7 is a more accurate criterion. B. Conclusion: Fitness benefits consequent to delaying agerelated deterioration (senescence) vary inversely with reproductive value i.e., the young and the old are relatively expendable see Ricklefs p 144 ff. 13

14 V. Dependence of Fertility and Survivorship on RE. A. Imagine for all age classes 1. A single effort, E. 2. B(E) and p(e) the same functions. B. Then (9) C. Optimal expenditure depends on the shapes of and (Figure 6). 1. Accelerating curves 0 or Decelerating curves Sigmoidal curves two optima. D. With multiple age classes, potential for multiple optima increases (Figure 7). 14

15 Figure 6. Optimal reproductive expenditure depends on the functional dependence of fertility and survival on RE. Four cases are shown. a. fertility and survivorship accelerating functions of RE (positive second derivatives). b. Fertility and survivorship decelerating functions of RE (negative second derivatives). c. Fertility sigmoidal; survivorship linear. d. Fertility linear; survivorship a reverse sigmoid. The most biologically plausible shapes are fertility sigmoidal; survivorship a reverse sigmoid. 15

16 Figure 7. Figure 4. Adaptive topographies for a two stage (juvenileadult) life history corresponding to the four cases shown in Figure 6. See caption to Figure 4 for additional details. 16

17 VI. Is Semelparity Adaptive? A. Two well-known examples: 1. Salmon. 2. Yuccas and agaves. B. Both groups include iteroparous & semelparous species. C. In neither group is there good evidence for an adaptive explanation see Schaffer (pp ), Ricklefs (pp ). D. An alternative explanation in yuccas and agaves is that semelparity consequent to flower stalk development from apical vs. lateral meristems. E. Post-flowering rosette survival and post-flowering vegetative reproduction in Agave parviflora (Figure 8) consistent with the latter, non-adaptive explanation. Figure 8. A, parviflora rosette and developing flower stalk. After the stalk dies, the rosette will hang-on for years (post-flowering halflife 30 months) and reproduce vegetatively. PFHL is much shorter in other agaves, and vegetative reproduction, while common, generally precedes flowering. 17

18 VII. Response to Environmental Chamge. A. Imagine a change in environmental quality that changes fertility and / or survival per unit effort. B. Consequence to optimal expenditure depends on whether B(E) or p(e) principally affected (Figure 9). C. Optimal allocation varies in proportion to fitness returns. D. Since juvenile survival a component of B(E) = cb(e), optimal response to changing levels of exogneous mortality depends on age specificity of their effect. 18

19 Figure 9. Top. Reducing effective fecundity,, per unit effort reduces E *. Bottom. Reducing post-reproductive survival,,, per unit effort increases E *. 19

20 VIII. Response to Increasing Density. A. depends on N. B. depends on E. C. Regardless of the agespecificity of crowding effects, maximizes (Figure 10). D. So-called K-selected species typically characterized by 1. Reduced reproductive output; 2. increased expenditure per offspring, etc. Figure 10. Optimal response to increased density can entail increased or reduced reproductive output. E. Consequent to the fact that juveniles generally more sensitive to crowding than adults. 20

21 IX. Response to Fluctuating Environment. A. Imagine good years and bad where or, 1, 1. 1, 1 (12a) (12b) B. Assume good and bad years equally frequent. Then selection maximizes geometric mean (13) C. For the case of decelerating (negative second derivatives) and, setting 0, yields the following results 1 : 1. Increasing favors reduced in RE if principallly affected; 1 Optional reference: Schaffer, W. M Optimal reproductive effort in fluctuating environments. Amer. Natur. 108:

22 2. Increasing favors increased in RE if principally affeted. D. This is not bet hedging in the sense of minimizing risk. 22

23 X. Effort per Offsping. A. Clutch size problem. 1. Juvenile survival increases with parental expnditure, e, per offspring. 2. Let be the number of eggs laid. 3. Problem is to choose (E,e) (equivalenetly, (E,n) to maximize Figure 12. Experimental demonstration of optimal clutch size. Average unmanipulated clutch was seven., (15) B. Number chicks fledged /. Then optimal expenditure per offspring, e *, is the value of e at which c(e) is tangent to the line, of greatest k (Figure 14a). (16) 23

24 Figure 13. Fledgling survival in great tits increases with body weight. From Blakey and Perrins (1999). All else being equal, fledgling weight, and hence survival, varies inversely with clutch size. Hence, there is a trade-off between clutch size and recruitment. Fledgling survival also depends on environmental conditions. Higher survival rates at the right reflect the fact that 1995 was a beech mast year. 24

25 C. Since e* maximizes B(E,e), E*(e) maximal at e=e* (Figure 14b). D. Parent-offspring conflict (Ricklefs, p. 192) a complicating factor is Figure 14. Optimizing effort per offspring, e, and total effort, E. 25