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Body Position Review: Humans Body Position Review: Animals Natural Selection and Fitness What is Fitness • Fitness = determined by reproductive success • Not necessarily bigger, faster, stronger • Fitness is not determined by one trait but by the totality of traits in the organism What is Natural Selection? • Certain phenotypes are more successful • Particular environments “select” certain phenotypes • Adaptive Process Natural Selection in a nut shell Natural Selection in a nut shell Natural Selection in a nut shell Modes of Selection Stabilizing selection – acts against extreme phenotypes and favors intermediate Modes of Selection Directional Selection – shifts the overall makeup of the population by favoring variants at one extreme of the distribution Modes of Selection • Diversifying Selection – favors variants at both ends of the distribution. Mechanism : Natural Selection • Fact 1: Over-reproduction occurs in nature • Fact 2: Populations do not increase exponentially • Fact 3: There are limited natural resources (food, shelter) Mechanism: Natural Selection • Fact 4: Variation exists in populations • Fact 5: Much of the variation is heritable – Fact 4 was physically observed. Darwin’s weakness was the 5th fact Inferences • Inference 1: – struggle for survival ensues • Inference 2: – Organisms with the best variations survive the struggle for life • Inference 3: – Unequal survival of organisms with different variations leads to favorable variations accumulating over time Examples of Natural Selection • Darwin’s Finches • Peppered Moths • Antibiotic resistance in bacteria • Arms Race Environmental Influence • Environmental plays a huge role on selective pressure. • Organisms adapt to their environment • Ex. Eye sockets in blind salamanders (Vestigial structures) Vestigial structures Eye sockets in blind salamanders Key Concepts • Natural Selection happens because of heritable variations • Overtime, natural selection results in changes in the inherited characteristics of a population. • These changes increase a species fitness (survival rate). Natural Selection Does Not Design Perfect Organisms • Organisms are locked into historical constraints • Adaptations are often compromises as with seals that need to swim and walk • Not all evolution is adaptive • Organisms can’t get a trait just because they “need” it.