Biology 301 Life History ‘Strategies’

A. Quantitative traits

1. Traits which are essentially continuous

2. Polygenic = influenced by alleles of multiple genes

3. Influenced by the environment

4. Described by a mean and variance

5. Phenotypic variance = genetic variance plus environmental variance plus variance due to the interaction of the environment and the genotype

6. Heritability = h2 = (genetic variance) / (phenotypic variance)

7. Determination of environmental effects typically by holding genotype constant and changing the environment

8. Traits that affect fitness are often quantitative

9. Fisher’s Fundamental Theorem of Natural Selection

Rate of change in fitness is approximately equal to the additive genetic variation in fitness divided by the mean fitness of the parental generation


B. Life history traits

1. Age at first reproduction = maturity

2. Parity = number of reproductive episodes

semelparity = breed only once

iteroparity = breed more than once

3. Fecundity = number of offspring per reproductive episode

4. Senescence time

[Note these are age specific traits, age specific survival, age specific fecundity]

C. Life history theory

1. Relates to the ‘evolutionary choices’ maintained in individuals regarding reproduction and its success

2. All such traits are typically multifactorial with numerous contributing genes as well as environmental components

3. Assume

a. Organisms are not equally good at all tasks i.e. are costs

b. Individuals have limited energy so that when energy is diverted to a task, energy is reduced for other tasks e.g. reproduction vs adult survival

c. Reproduction has a metabolic cost and has a cost in future survival – e.g. mutant nematode males which do not produce sperm live longer than those which produce sperm, unmated males also live longer

4. Implication is that these traits are potentially linked

5. Measures of costs and allocations to those costs are often informative as to the differences between organisms

6. Why breed more than once? Need to determine current vs future reproductive value (RV)

RV = reproduction discounted by the probability of survival = relative contribution of an individual of age ‘x’ to population growth

If semelparous: RV = current RV + future RV so if probability of future RV is 0 then selection should favor current reproduction and if probability of adult survival is less than the probability of juvenile survival, then selection should favor current reproduction e.g. Pacific salmon

If iteroparous: RV = current RV + future RV so if probability of future RV > current RV then selection should favor future reproduction; if probability of adult survival is greater than the probability of juvenile survival, then selection should favor future reproduction.

7.      Cole’s result for why Iteroparity:

·        Annual:  Nt+1 = baNt + 0 Nt

·        All adults die after reproduction

·        Perennial:  Nt+1 = bpNt + Nt

·        All adults survive reproduction

·        How many more seeds must an annual make to equal a perennial?

·        Solve annual = perennial:   ba = bp + 1

·        Annual that makes 1 more seed is equivalent to an immortal perennial!

8.      Charnov’s result for why iteroparity:

·        Annual N = (ba N) (c)

·        Where c is the survival of the juvenile

·        b is the number of offspring per parent

·        Perennial N = (bp N) (c) + N (a)

·        Where a is the survival of an adult

·        Solve annual = perennial:    ba = bp + a/c

·        Annual can equal perennial’s growth by adding a/c offspring

·        Thus relative success depends on the ratio of adult survival to juvenile survival