Population Structure and Life Histories
I. Even with assumptions, logistic growth model is a good approximation
for many animals under certain conditions.
A. Populations do fluctuate.
B. Logistic growth curve doesn't describe everything about population
dynamics.
C. One of the most important assumptions is that all individuals in
the population are equal in terms of reproduction. Not all individuals
have an equal probability of surviving or reproducing.
D. An obvious way that individuals differ is that they are different
ages.
E. Different populations have different distributions of ages, and the
relative frequency of different ages in a population is known as age structure.
F. To predict change in a population, one needs knowledge of the number
of individuals of each age, probability of survival, and rates of fecundity.
G. We need some way of organizing information that is flexible enough
to account for differences among populations.
II. Life tables organize statistics on age, survivorship, and fecundity.
A. Life tables are typically based on females because it is hard to
ascertain paternity in many species.
B. x represents age, and age-specific variables are represented with
subscript x. Mortality within intervals, but also probability of survival
over many age intervals. lx is the probability that a newborn will be alive
at age x. Also information on reproduction--number of offspring produced
in an age interval, and proportion per individual surviving.
Life table for Phlox drummondii
| age (days) |
(Ax) |
(lx) |
(dx) |
(Fx) |
(mx) |
(lxmx) |
| 0-63 |
996 |
1.000 |
0.329 |
|
|
|
| 63-124 |
668 |
0.671 |
0.375 |
|
|
|
| 124-184 |
295 |
0.296 |
0.105 |
|
|
|
| 184-215 |
190 |
0.191 |
0.014 |
|
|
|
| 215-264 |
176 |
0.177 |
0.004 |
|
|
|
| 264-278 |
172 |
0.173 |
0.005 |
|
|
|
| 278-292 |
167 |
0.168 |
0.008 |
|
|
|
| 292-306 |
159 |
0.160 |
0.005 |
53.0 |
0.33 |
0.05 |
| 306-320 |
154 |
0.155 |
0.007 |
485.0 |
3.13 |
0.49 |
| 320-334 |
147 |
0.148 |
0.043 |
802.7 |
5.42 |
0.8 |
| 334-348 |
105 |
0.105 |
0.083 |
972.7 |
9.26 |
0.97 |
| 348-362 |
22 |
0.022 |
0.022 |
94.8 |
4.31 |
0.10 |
| 362- |
0 |
0 |
|
|
|
|
A(x) = number surviving to each age
l(x) = probability that a newborn will survive to age x
d(x) = death rate for a given age interval
F(x) = # of seeds produced for individuals of age x
m(x) = mean number of seeds produced per individual of age x
III. Survivorship curves can be calculated from life tables and compared
among populations.
A. Survivorship curves give us insight as to the age structure in a
population.
B. Plot log survivorship vs pop size.
C. Survivorship can be constant with age (Type II).
D. Two variations.
1. Low juvenile mortality, mortality results from old age (Type I).
2. High juvenile mortality, but once reach a certain point, chances
of survivorship good (Type III).
3. What types of organisms have these different survivorship curves?
4. Survivorship curves can vary among populations of the same species.
Ex. Impatiens pallida.
5. Fecundity curves can also vary among populations. Example: Poa
annua (a grass)
IV. Important differences between populations lie in differences in life
history traits, such as survivorship, fecundity, age at first reproduction.
A. Life table illustrates that different combinations of these traits
produce different numbers of offspring. How they change with age is an
organisms life history.
B. Contrast between two extreme life history traits. Humans vs. cod.
C. Why do life histories vary so dramatically among different types
of organisms? Why doesn't natural selection result in all characteristics
being at their maximum i.e., long survivorship, high fecundity, short generation
time, etc. The study of these questions is call life history evolution.
V. Two factors that limit life history evolution.
A. Constraints are limitations imposed by an organisms evolutionary
history.
B. Tradeoffs are benefits from one process that are bought at the expense
of another.
1. Consider seed size in plants.
a. Orchids invest little per seed. On average they are 0.000002g. The
other extreme is a coconut palm (27,000 g/seed). Seed size is related to
whether a see survives.
b. Why don't they all have big seeds?
2. Tradeoffs between growth and reproduction. Ex. Pine growth and reproduction.
3. Tradeoffs between offspring quantity and offspring quality lead to
an "optimal" solution (Ex. Great tits).
4. More investment in offspring does increase fitness, but death can
result. (Ex. red deer).
5. These examples suggest that you should reproduce as much as possible
but not so much that it kills you, clearly organisms do reproduce once
and die. Ex. salmon.
VI.Iteroparity vs. semelparity.
A. Iteroparity: reproduction occurring multiple times over an organism's
lifetime. Survivorship important for these organisms.
B. Semelparity: reproduction occurs once and then organisms dies.
C. Semelparity seems perplexing. Why not hold enough back to survive,
build reserves back up, and reproduce again.
D. Adding 1 to your clutch size is all it takes to make semelparity
equal to iteroparity. This is known as Cole's Law.
E. Now the question reverses: why are there any iteroparous organisms?