This presentation was originally prepared by C. William Birky, Jr. Department of Ecology and Evolutionary Biology The University of Arizona It may be used with or without modification for educational purposes but not commercially or for profit. The author does not guarantee accuracy and will not update the lectures, which were written when the course was given during the Spring 2007 semester.

Section 18 divided into 18a, b, c. 18a and b on web. Another homework assignment, on population and evolutionary genetics, will be posted soon, hopefully on Friday.

Drosophila experimental results usually not as neat as those cases we use in class. Did three-factor cross in genetics lab course. All mutant genotypes (e.g. white eye) present in fewer than the expected numbers. All visible mutants, all at least slightly detrimental. If they were not detrimental, they would be more common in nature. If they were advantageous, they would have become the wild type.

F = (1 – e-2Nes/N)/ (1 – e-4Nes) Combined Effects of Selection and Drift Probability of fixation of a new mutant allele with selection coefficient s in population with effective size Ne is given by Kimura’s equation: F = (1 – e-2Nes/N)/ (1 – e-4Nes) Solving equation for various values of N, Ne, and s leads to the following conclusions: Even detrimental mutations can be fixed by drift. Even advantageous mutations can be lost by drift. Relative strength of selection and drift depends on the product Ne|s|: Ne|s| >> 1 selection dominates Ne|s| << 1 drift dominates (mutation is effectively neutral) If Ne|s| >> 1, then either Ne is very large so drift works very slowly, or |s| is very large so selection is very strong (or both). If Ne|s| << 1, then either Ne is very small so drift is strong, or |s| is very small so selection is very weak (or both). Analogy: selection is signal, drift is noise.

One Implication for Conservation Endangered species have small N. This means small Ne and Ne|s|, which means less effective selection, which includes more accumulation of detrimental mutations, leading to reduced fitness and further reduction in N. Vicious circle: smaller N smaller Ne and Ne|s| reduced fitness less effective selection