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Population Genetics Evolution by Natural Selection • Evolution is ______________________________ (through several generations). Darwin - Those individuals with ____________________ characteristics survive and pass on their traits to the next ____________________ - Population gradually includes more individuals with __________________ characteristics. Gene Variation in Nature • Polymorphism A gene with more than one allele is _______________________. - Natural populations tend to have more polymorphic genes than can be accounted for by ______________________. Before Hardy-Weinberg • Population Genetics - Study of properties of genes in populations. _______________________ inheritance was widely accepted. - New genetic variants would quickly be ______________________. ______________________ had not yet been discovered. SCIENTISTS COULD NOT ANSWER: Why don’t recessive traits ______________________after a while? Hardy-Weinberg Principle • Hardy-Weinberg – Two scientists came up with an explanation at the same time • Original proportions of genotypes in a population will remain _______________from generation to generation Five Assumptions: • Population is ___________________________ • Mating is __________________________ • No ___________________ • No ______________________ • No __________________ occurring Calculate Genotype Frequencies To find allele frequencies,let: T=frequency of _________________ allele t=frequency of __________________ allele T + t = 1 (100% of alleles in population) Frequency of homozygous recessive individuals (______) in the population is: t x t = t2 Frequency of ________________ allele (t) in the population is: t = √ t2 Frequency of ____________________ allele (T) in the population is: T=1-t Agents of Evolutionary Change 1. Mutation Mutation rates are generally so ________________ that they have _____________ effect on Hardy-Weinberg proportions of common alleles. - Ultimate source of genetic __________________. 2. Gene Flow ____________________ of alleles from one population to another. Tend to ___________________ allele frequencies. 3. Genetic Drift Frequencies of particular alleles may change by _____________________. - Important in _______________ populations. __________________ Effect - Few individuals found (start) new population (small allelic pool). __________________ Effect - Drastic reduction in population, and gene pool size. 4. Selection Artificial - Breeders exert ___________________. Natural - ___________________ exerts selection. - Variation must exist among individuals. - Variation must result in differences in numbers of viable offspring produced. - Variation must be genetically inherited. Natural Selection is a process, and Evolution is an outcome. • Selection Pressures: Avoiding Predators Matching Climatic Condition Pesticide Resistance Evolutionary Forces Maintaining Polymorphism • Adaptive Selection Theory Heterogeneous environments produce many alleles. • Neutral Theory Balance between mutation and genetic drift. • Gene Flow versus Natural Selection Genetic drift may decrease frequency of an allele favored by selection. Forms of Selection • Disruptive Selection Selection eliminates intermediate types. • Directional Selection Selection eliminates one extreme from a phenotypic array. • Stabilizing Selection Selection acts to eliminate both extremes. - Fitness - Number of surviving offspring passed to the next generation.