Bio1b Summer 2008 Evolution Lecture 5 Eric Harris Page 1 of 2 Updated on 06/17/08 EVOLUTION LECTURE 5: NATURAL SELECTION Reading: 7th ed., 462–471; 8th ed., 479–486. A. Natural selection How much variation is there in natural populations? How is variability maintained? Diploidy and balanced polymorphisms What can maintain a balanced polymorphism? - Heterozygote advantage - Frequency dependent selection - Patchy environment Fitness: success of an organism in reproducing; the contribution to the next generation. Fitness is a relative term. The most fit genotype is assigned a fitness value of 1. The difference between any two fitness values gives us the selection coefficient. B. Types of selection – assuming some distribution (for example a normal distribution) for a given phenotype i) Stabilizing selection – the intermediate phenotype is favored ii) Directional selection – one of the extreme phenotypes is favored iii) Disruptive selection – both of the extreme phenotypes are favored. C. Sexual selection – a type of natural selection acting on traits directly involved in obtaining mates. Will tend to enhance sexual dimorphism. Isogamy vs. Anisogamy Sexual modes of reproduction persist despite the two-fold cost of sex Intrasexual selection e.g., male-male competition for mates Intersexual selection e.g., female mate choice. Epigamic traits are those selected for by opposite sex. D. Adaptation – a trait that functions to enhance fitness of its bearer i) Adaptations to the physical environment – leading to geographical variation ii) Adaptations to the biological environment – may lead to coevolutionBio1b Summer 2008 Evolution Lecture 5 Eric Harris Page 2 of 2 Updated on 06/17/08 E - Why evolution does not result in perfectly adapted organisms. i) Evolution does not mean necessarily mean improvement, it just means change. The environment is constantly changing and species are all continuously evolving. The Red Queen hypothesis ii) Historical constraints – the Panda’s thumb ii) Compromise – finding mates vs. avoiding predators iii) Time and energy are limiting – bone strength iv) Selection only selects from the variation that is
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