1. The focus of the course is on the theoretical and conceptual underpinnings of evolution by natural selection and genetic drift

Transcription

1 L567 Evolution 2006 First meeting 1. The focus of the course is on the theoretical and conceptual underpinnings of evolution by natural selection and genetic drift 2. Exploration of the basic models in population genetics, quantitative genetics and evolutionary game theory, with the hope of showing the interface between the approaches and the advantages and disadvantages of each. 3. Format is lecture/discussion, with some work in groups. 4. Grading is based on two exams (100pts each). The second exam can be replaced by an original model. 1

2 Quiz If natural selection is the survival of the fittest (as Herbert Spencer suggests), then we must ask Who survives? A. Then we must ask, who are the most fit? A. Then we must conclude that natural selection is the survival of the A. As such, natural selection is a meaningless tautology*. See optional reading Peters (1976) from course web page. *True by the virtue of its logical structure alone. **Warning: I will ask you to formally define fitness in about 30mins 2

3 Following Mayr 1991 ( One long argument ) Darwin s theory had five parts. 1. Evolution as such 2. Common descent 3. Radiation of species 4. Gradualism (population thinking) 5. Natural selection Page 26. The origin was a superb treatment of a common descent and a great plea for the efficacy of natural selection*, but it was vague and contradictory both on the nature of species and on the mode of speciation * Even so, most of Darwin s contemporaries rejected Natural Selection. WHY?? 3

4 Reasons for resistance to Natural Selection (from Mayr 1991) 1. Religious grounds (see also Ruse s book) Active association in England between the universities and the church. 2. Essentialism (or typological thinking), ingrained since Plato. Things have essences, and they do not change essences. a) rocks do not turn into water b) triangles do not turn into squares Rev. Whewell (1840) Species have real essence in nature, and transition from one to another does not exist John Stuart Mill: Kinds (species) are classes between which there is an impassable barrier. Essentialism was extremely useful in math and physics. Darwin s greatest contribution may have been to get past typological thinking and covert to population thinking. Thinking about variance. (example using triangles here.) Note that saltation is a necessary consequence of essentialism. 4

5 3. Strict determinism a) Acceptance of NS requires accepting statistical generalizations b) Reliance on statistical generalizations was considered higgledy piggledy by physicists and philosophers of science. 4. Teleology Rejection of NS because it was not a teleological process 5. No mechanism for heredity. (see also Ruse p ). a) Darwin originally favored blending inheritance, but this clearly could not work since favorable variants would be blended out of existence. Consider a field of white flowers b) There was a need for a mechanism of inheritance that worked, leading Darwin to the Provisional Theory of Pangenesis. 5

6 Darwin s definition of Natural Selection Can it, then, be thought improbable, that variations useful in some way to each being in the great and complex battle of life, should sometime occur in the course of thousands of generations? If such do occur, can we doubt that individuals having any advantage, HOWEVER SLIGHT, would have the best CHANCE of surviving and of procreating their kind? On the other hand we may feel sure that any variation in the LEASE DEGREE injurious would be rigidly destroyed. This preservation of favorable variations and the rejection of injurious variations, I call Natural Selection. 6

7 From Endler s Natural Selection in the Wild NS as a syllogism* 1. If there is heritable variation (h 2 >0), and 2. the variants differ with respect to their abilities to survive and reproduce in the present environment ( S! 0 ), then 3. those heritable traits conferring enhanced success should tend to increase in frequency. (there will be an increase in the probability that those traits will increase in frequency.) Is this testable idea? If so, it becomes science. *Syllogism: a deductive scheme of a formal argument consisting of a major premise, a minor premise and a conclusion. Now, with the ideas in mind, write your definition of fitness. 7

8 Are you familiar with the following cross-section of concepts? Could you teach them to an advanced undergraduate course. 1. Stable, unstable and neutral equilibria 2. Geometric mean fitness 3. Additive genetic variance 4. epistasis, linkage disequilibrium 5. Covariance (e.g. Parent-offspring covariance) 6. Fisherian sex ratio 7. Frequency-dependent selection 8. Hard selection, soft selection 9. Can you find local maximum for fitness? 10. Local mate competition 11. Explain the Price equations 12. Mutation selection balance 13. Write an excel file that iterates p t+1 = some function of p t? 14. Local mate competition 15. formally define heritability 8