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BIO 140 SECTION QUIZ 10/14/99 Brian's section

1. (25 pts) Explain the process of natural selction, how it differs from typological thinking, and why it is considered a blind process. Include a discussion of the three modes of selection and how they affect the mean of the phenotype distribution in the population.

Natural selection is a process in which: (10 pts)

1. There is variation in the expression of a trait(s) in a population.

2. This variation is heritable.

3. The variation affects the ability of individuals to survive or reproduce, i.e. there is a consistent difference in fitness among individuals with varying levels of expression of the trait.

4. The differential fitness of individuals with variation in the heritable trait causes the population to evolve in the direction of expressing the most successful phenotypes for the trait.

N.S. differs from typological thinking in that typological thinking ignored variation within a species, considering it as noise around the one true and perfect form of the species. That perspective prevented practitioners of typological thinking from recognizing the importance of variation in evolution. Darwin's great contribution was that he recognized that variation is very important, since it forms the basis of differential survival and reproductive success between individuals. (5 pts) N.S. is a blind process because of the probabilistic nature of survial and reproduction. "Survival of the fittest" is based upon averages, and is not strictly deterministic. In addition, the source of all heritable variation is genetic mutation, which is by its very nature random in its occurance, in respect to both frequency of mutation and location of mutation. (4 pts)

Discusson and/or figures describing disruptive selection, stabilizing selection, and directional selection. (6 pts)

2. Answer one of the following (15 pts).

A. What are the effects of pre- and post-mating isolation mechanisms on the process of speciation? What are the effects on the fitnesses of individuals?

Premating isolation mechanisms: Can lead to speciation because N.S. can operate on assortative mating. As individuals with one morph recognize and prefer to mate with others of the same morph, as positive feedback loop is established, and reproductive isolation can evolve. The effect on individual fitness is that premating isolation mechanisms save an individual from mating with an "inappropriate" mate, thus allowing them to concentrate reproductive energy on potentially more fruitful matches. Therefore, premating isolation mechanisms are most effective in preventing a reduction in an individual's fitness due to mating doomed to not leave successful (or leave less successful) progeny. (7.5 pts)

Postmating isolation mechanisms: Cannot by themselves lead to speciation because the do not lead to assortative mating, without which nothing prevents individuals from choosing mates that will not give viable progeny. Since speciation requires reproductive isolation, postmating isolation mechanisms cannot lead to speciation. Instead, they often lead to a reduction in fitness of individuals due to their engaging in matings which result in unfit progeny. (7.5 pts)

B. You decide to do a senior thesis project studying the heritability of IQ. Your advisor suggests you look at correlations between sibs raised in different households. Why does she tell you to do that? What additional confounding factors does this design not remove from the heritability estimates?

Your advisor tells you this because heritability studies aim to determine which portion of a behavior or other trait is determined by additive genetic factors, as opposed to environmental or nonadditive genetic factors. (3 pts)

Studying sibs raised in separate houses removes much of the covariance that is due to environmental conditions, which would be shared by sibs brought up in similar circumstances in the same household. (6 pts)

This design does not remove maternal (womb) effects, nor does it remove dominance genetic (nonadditive) effects. (6 pts)