4 Natural selection - BIO 370: Evolution Instructor: Mark...

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BIO 370: Evolution Instructor: Mark Kirkpatrick p. 1 4. Natural selection A look at Darwin’s idea of natural selection in detail Organisms can produce far more individuals than can survive and reproduce This guarantees a “struggle for existence” The theorem of evolution by selection If: 1) There is a correlation between a trait and the number of descendants an individual leaves, and 2) Some variation for that trait is inherited Then: Evolution by selection must occur – it’s logically inevitable Evolution caused by natural selection can be directly observed Darwin’s medium ground finch in the Galapagos Islands Observation: The mean beak depth of individuals that survived the drought of 1977 was significantly larger than the mean beak depth before the drought We have observed natural selection! Why did selection occur? No seeds produced during drought, all the small seeds eaten, only birds with big bills could feed on the remaining seeds and avoid starvation Did evolution occur? Yes. Beak size is heritable Mean beak size was larger in the following generation Selection on shell color in a snail Snails with pink shells tend to be found in woodlands, yellow shells in open habitat Snails moved between habitats. Yellow shells have high mortality in woodland, pink shells high mortality in the open Body size, color, and life histories of guppies Some guppies live in high predation streams, some in low Guppies transplanted from high to low predation environments Over 11 years, large genetic changes in body size, coloration, and life histories evolved in the predicted direction
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BIO 370: Evolution Instructor: Mark Kirkpatrick p. 2 Quantifying fitness Fitness: The number of descendant zygotes produced by an individual, phenotype, or genotype Fitness can be decomposed into components: survival, mating success, . .. Key quantities W Absolute fitness of an individual W The mean fitness of the population w = W / W The relative fitness of an individual Selection acts on differences in relative fitness How fast does a population evolve under natural selection?: One locus Consider a case where the relative fitnesses are:
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4 Natural selection - BIO 370: Evolution Instructor: Mark...

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