**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 = h^{2}
= (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: N_{t+1} = b_{a}N_{t}
+ 0 N_{t}

·
All adults die
after reproduction

·
Perennial: N_{t+1} = b_{p}N_{t}
+ N_{t}

·
All adults
survive reproduction

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

·
Solve annual =
perennial: b_{a}
= b_{p}_{ }+ 1

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

8. Charnov’s result for why iteroparity:

·
Annual N = (b_{a} N) (c)

·
Where c is the
survival of the juvenile

·
b is the number
of offspring per parent

·
Perennial N = (b_{p} N) (c) + N (a)

·
Where a is the
survival of an adult

·
Solve annual =
perennial: b_{a}
= b_{p} + 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