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Three Possible Outcomes of Selection Disruptive selection (3) types of selection: Directional Directional Frequency # offspring small Body size (or any range of phenotypes) large Environmental change = selection for optimal phenotype, Results in a directional shift of gene frequencies Odocoileus virginianus white-tailed deer http://animaldiversity.ummz.umich.edu Bergmans Law = large bodies are beneficial in colder climates (3) types of selection: Stabilizing Directional Stabilizing Frequency # offspring small Body size large Selection for the mean or median gene frequency the mean increases, the outliers decrease = overall decrease in genetic diversity Percentage survival to age 4 weeks Birth weight in humans: Stabilizing selection? Birth weight in pounds (Karn and Penrose, 1951) (3) types of selection: Disruptive Directional Stabilizing Disruptive (selectional dimorphis m) No selection Frequency # offspring small Body size large Individuals at both extremes of a range of phenotypes are favored over those in the middle Accentuates or preserves discontinuities in populations Time--------------> Fossils: Speciation and the phylogenetic “tree” Extinction Speciation event Lineage <---------Morphology -------> Recognizing species in fossil records New Time Extinction: loss of species, loss of lineage Lineage Speciation e vent/origination Old Morphology • Difficult to detect • Does morphology imply breeding barrier? • How to recognize species in the fossil record? Adaptive radiation (after Futuyma 1998; based on Purves et al., 1997; Grant, 1986) Premating Mechanism: seasonal isolation late spring breeding Bufo fowleri Fowler's Toad Bufo americanus American Toad early spring breeding Character displacement: Finch variation (Lack, 1947)