Lecture 19

Topics

• Two more "age-specific" variables
  1. Reproductive Value

  2. Age structure

• Phenotypic Frequency Distributions

• Frequency distributions and Natural Selection
  1. Effects of Stabilizing Selection

  2. Effects of Directional Selection

  3. Effects of Disruptive Selection

Study Sources

• Ricklefs: Some history of Reproductive Value, pp.661-662.

• Ricklefs: Chapter 15 (especially Fig. 15-8)

• Study Guide: Biology of Single Populations

• Study Guide: Basic Genetics and Selection

The Main Points

• Reproductive Value is an age-specific variable that depicts the relative contribution a female, at a given age, would be expected to make to future generations. The "value" is a balance of the values of present offspring and the expectation of future offspring.
  1. At birth an individual has a certain value based mainly upon the expectation of living to reproductive age. A newly hatched herring in the Atlantic Ocean has a very low "value" because the expectation of survival is extremely low. On the other hand, a human female has a relatively high "reproductive value" because the probability of reaching reproductive age is high.

  2. In either case, on the second day of life, the reproductive values of each has risen and will continue to rise until the first offspring are being "born". That is, each day, the probability of reaching reproductive age gets higher.

  3. After the first reproduction, the female has fewer reproductive periods left in her lifetime so her relative value is now less than it was before the first reproduction. Therefore, her "reproductive value" declines as she ages and expends her eggs. The "value" reaches zero upon her last reproduction.
  

• We may also characterize a population by the relative abundance of the different phenotypes. The frequency distributions below show how the relative abundances of the phenotypes can change as the environment imposes new conditions with each generation.

  The frequency distribution is maintained by selection that favors the present modal phenotypes. New phenotypes may be generated by the population but they are generally lost to selection or could be maintained at very low frequencies. If the frequency distribution retains the same shape from year to year, we generally consider that the population is undergoing stabilizing selection.

  Selection favoring phenotypes to the right of the original modal phenotype causes the frequency distribution to skew. At this point, no phenotypes have been lost and we see simply a change in the relative abundance of the phenotypes. New phenotypes that are generated to the right may eventually become more numerous and phenotypes may be lost on the left.

  Disruptive selection is the result of an environment that favors two rather distinct groups of phenotypes in the population. The result is the development of a bimodal distribution as depicted here. One could imagine that sexual dimorphisms might arise in this way.