Life History Traits Relation to the “Life Table” Life History Variation in
Brief review of strategies we’ve seen so far: Imagine following a cohort of individuals in a population Pacific Salmon •Foraging strategies Fecundity and Age at Maturation, etc. •Territorial behaviors •Sockeye •Signalling behaviors •Pink •Reproductive behaviors •Chum •Mating systems Time •Chinook (years) New Offspring Non-reproductive Reproductive AdultsDead Juveniles •Coho We investigated these different behaviors and traits in •Steelhead terms of how they affect fitness of the individual 0
Another class of traits have a clear and direct bearing on Intraspecific Variation in Fecundity in Chum Salmon fitness. These are the life-history traits. The major life 1 Chum salmon from the Amur River Siberia: history traits are: •Age at first reproduction Summer run: 39.8 eggs/cm fork length •Number and size of offspring 2 •Reproductive lifespan and ageing Autumn run: 53.5 eggs/cm fork length 3
All of these affect the so-called “Life Table” of an organism 4
Note, there are more formal representations using matrix algebra
3. Correlation of traits between relatives Goals of life history theory Evidence for Trade-offs •Quantitative genetics approach •Maybe useful for demonstrating short-term Four main ways people have tried to demonstrate To Answer, “Why all this variation?” constraints and trade-offs that these trade-off exist: •Long term selection may not be constrained by genetic correlations detected in such studies and 1. Phenotypic Correlations within or among populations •Example: egg size and egg number between salmon Ultimately wish to construct models which populations of the same species 4. Correlated responses to selection predict what sorts of traits will be favored in •Doesn’t really demonstrate direct trade-offs •Artificially select for a trait and see what else what sorts of environments. changes along the way. 2. Experimental Manipulations •Famous experiments by Rose and Charlesworth •Trade-offs between clutch size and offspring survival. •Squashed fruit flies at an unnaturally young age Without constraints, the answer is easy, for •Inspired greatly by the “Lack Clutch” hypothesis of 30
highest fitness one would. . . . the 1940’s 25 •Hypothesis regarding latitudinal clutch-size 20 trends 15 The CORNERSTONE (as in other optimality models): 10 •Hogsted (1980) Magpie clutch augmentation/ 5 Assumption of limited time and resources reduction experiments. 0 0 5 10 15 20 25 and the need to allocate these to particular -5 -10 4 -15 traits Difference in Eggs per Day 3 -20 2 -25 Adult Life Span (days) Trade-offs 1
Chicks Fledged 0 56789 Selection for both shorter life and higher fecundity Number of Eggs Another Model Simple Mathematical Models Relationship between reproductive life-span and Phenotypic Plasticity reproductive investment. Perennial vs. Annual Life Histories: Noted in many other types of traits as well, S0 = Probability of survival in the first year So = Probability of surviving the first year (to but has received much attention in the study of life SP= Probability of survival in later years reproductive age) history traits. B = number of offspring produced in a season Ba = Seed production rate of annual plant Back to our first day: Bp = Seed production rate of perennial Sp = probability of surviving adult years Environment Partitioned into: Genotype ------> Phenotype Sr = factor of survival probability that is L = rate of increase affected by your reproductive investment S = factor of survival probability unaffected by Reaction Norms. L (annual) = (Ba)(S ) reproductive investment 0 Selection for phenotypic plasticity itself L (perennial) = (Bp)(S ) + S 0 P L = (S)(Sr) + (So)(B)
What values of B and S lead to the annuals Now, imagine changing B (and hence Sr). Our common increasing faster than the perennials? trade-off assumption would say that Sr would become smaller as B became larger. Annuals will be favored when S is high relative to S 0 P ∆L = S(∆Sr) + So(∆B) If not, annual seed production must be high to compete High adult survival favors lower yearly investment in with perennials reproduction.
Reaction norms
Genetically determined---may be an item under selective pressure.