Tradeoffs
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© 1997
David H.A. Fitch
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Lecture notes

Adaptations:  Tradeoffs as a key to understanding the evolution of life history characteristics

I.  Fecundity (reproduction) as well as survivorship may change with age

A.  If older individuals contribute less to population growth, the coefficient of selection declines for genes that affect survival or fecundity at older ages

B.  Predictions under such conditions:
1.  Alleles that contribute to senescence and degenrative disease should be maintained, since there is less opportunity for selection once reproduction has ceased
a.  If genes are advantageous early in life, but have deleterious pleiotropic effects late in life, they will have a net selective advantage (and thus be maintained), even if the net effect is degeneraion, cessation of reproduction and death [note selection experiments in Drosophila cited in Futuyma]
b.  Senescence should have numerous causes (contributing genes), not just a single biochemical cause
2.  Organisms should reproduce as early as possible and all at once (semelparous reproduction), if progeny born earlier contribute more to population growth

II.  Tradeoffs often help explain deviations from semelparous reproduction

A.  Semelparous reproduction may result if adult survival is low, but iteroparous reproduction may result when probability of adult survival is high and population growth rate is low

B.  If fecundity is related to body size, reproductive effort may be delayed until an age with large size and high fecundity

C.  Brood size also results in a tradeoff between parental investment and number of offspring

III.  Dependence of fitness on population density

A.  Different genotypes may have different fitnesses at different population densities if these genotypes affect intrinsic rates of population increase (rm) (see Futuyma, Fig.11, Chapter 9)

B.  "K"-Selected populations
1.  If competition is intense on juveniles, but adults have longer survivorship, selection may favor:
a.  Iteroparous reproduction
b.  Reproductive delay
c.  Small brood sizes
d.  Large eggs
e.  Characters that result in a low intrinsic rate of increase
2.  Such populations tend to find an equilibrium near K

C.  "r"-Selected populations
1.  If populations tend to experience many periods of exponential growth (e.g., if limited by ephemeral resources), selection may favor:
a.  Semelparous (or at least early) reproduction
b.  Fast development
c.  Large brood sizes
2.  Such populations tend to maximize their intrinsic rate of increase, r

D.  "K" and "r" selection are extremes in a spectrum of possible patterns of life history adaptations that generally involve tradeoffs between the costs and benefits of particular strategies of survival and reproduction

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Exercises

  1. Salmon present an example of species that delay reproductive effort, but are semelparous.  Delay of reproductive effort is characteristic of "K"-selected populations but semelparous reproduction is typical for "r"-selected populations.  Under what conditions might such life history features have evolved?
     
  2. Under what conditions might you expect selection to result in everlasting life (i.e., no senescence)?  Can you think of any actual examples that approach this?
     

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 Examples of
Adaptations  Defining
Adaptation  Levels of
Selection  Optimal
Models  Tradeoffs  Sexual
Selection
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[Examples of Adaptations] [Defining Adaptation] [Levels of Selection] [Optimal Models] [Tradeoffs] [Sexual Selection]

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