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General Ecology

General Ecology. Adaptation and Evolution cont: Population Genetics. Evolution in Populations. Evolution is often a process of adaptation. Adaptation is not possible w/o genetic variation.

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General Ecology

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  1. General Ecology Adaptation and Evolution cont: Population Genetics

  2. Evolution in Populations • Evolution is often a process of adaptation. • Adaptation is not possible w/o genetic variation. • Organisms present a phenotype to the environment (selective regime). The phenotype is an expression of the genotype.

  3. Evolution in Populations • The phenotypic variation we see can be a consequence of a number of things: • Genotypic or genetic variation. • Environmental variation. • Error

  4. Basic Population Genetics • We have defined evolution as a change in allele frequencies over time. • The sum of all genes in a population is the gene pool. • We characterize the gene pool be measuring allele frequencies.

  5. Popuation Genetics • In pea plants, there are red flowers and white flowers. Flower color is controlled by a single gene with 2 alleles. • R is dominant and r is recessive. • RR and Rr individuals produce red flowers while rr individuals produce white flowers.

  6. Population Genetics • ‘RR’ is homozygous dominant • ‘Rr’ is heterozygous • ‘rr’ is homozygous recessive. • If there are 100 individuals in a population, there are 200 flower color alleles. • The number of those alleles that are ‘R’ is the allele frequency of ‘R’, and 1 minus this is the allele frequency of ‘r’.

  7. Population Genetics • If we note that the frequency of R has changed from .20 to .30 in 1 generation, then evolution has occurred. • Imagine we have 30 RR individuals, 20 Rr individuals, and 50 rr individuals. • Let p be the frequency of R and 1-p, or q, be the frequency of r. Note p + q = 1.

  8. Population Genetics • For our example,

  9. Population Genetics • Also,

  10. Population Genetics • Note that p+q=1. • Now, imagine that the individuals in this population mate panmictically. • What is the probability that a R allele will combine with a R allele? • This is simply p2. • This is also the expected frequency of RR individuals in the next generation.

  11. Population Genetics • The probability of rr will be q2, and the probability of Rr is 2pq. • Note: there are 2 ways of getting Rr. You can get Rr or rR. • Note also that p2+2pq+q2 = 1 • Finally, note that this is (p+q)2

  12. Population Genetics • What happens? If we repeat this for many generations, we find that the system reaches equilibrium: a point at which allele frequencies no longer change. This is called Hardy-Weinberg equilibrium. In other words, allele frequencies will not change unless something happens.

  13. Population Genetics • The system is dependent on a number of assumptions: • Panmictic breeding • Equal survival and reproduction of individuals. • The population is closed. • No mutation. • Neither of these is likely to be true. • Thus, evolution is inevitable.

  14. Mechanisms of Evolution • What can disrupt HW equilibrium? • Forms of selection already discussed. • Genetic drift. • Here, changes in allele frequencies can occur by chance alone. This is a small population size phenomenon. Effective population size is critical. • Gene flow • This can wash out changes in allele frequencies, introduce new alleles, or promote change.

  15. Mechnisms of Evolution • Modeling natural selection • Imagine the homozygous recessive genotype does not survive as well as the heterozygote or homozygous dominant. We can denote this with a selction coefficient ‘s’. • If the fitness of the homozygous dominant and heterozygote are 1, then the fitness of the homozygous recessive is 1-s.

  16. Mechanisms of Evolution • We can use algebra to model changes in allele frequency. Let Dp = change in frequency of p from original generation, to generation after selection has occurred. So, Dp = p – p’ • We want to compute the new frequency of p, p’. We do this just as before, but know that some alleles were removed by selection.

  17. Mechanisms of Evolution • The portion removed by selection is q2 * their selective disadvantage ‘s’. Thus, the number of alleles in the next generation is 1-sq2, • since (p2+2pq+q2)-sq2. • Also, the total number of alleles is 2(1-sq2)

  18. Mechanism of Evolution

  19. Mechanism of Evolution • Note that the rate of change in allele frequency is a consequence of the strength of selection (s) and the initial allele frequency (p and q).

  20. Finally • We often characterize selection in a number of ways:

  21. Finally • Convergent evolution • ESS: Evolutionary stable strategy. • Wrights adaptive landscape.

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