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Population Genetics & Evolution Population Genetics • The study of evolution from a genetic point of view Causes of Variation • Mutations • Recombination • Random fusion of gametes Allelic Frequency • Determined by dividing the number of a certain allele by the number of alleles of all types in the population. • Consider a population of ten individuals and a given locus with two possible alleles, A and a. Suppose that the genotypes of the individuals are as follows: – AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA • Then the allele frequencies of allele A and allele a are: – • so there is a 70% chance of the population getting that allele – • and there is a 30% chance of the population getting this allele Gene Pool • The total genetic information available in a population. Hardy-Weinberg Principle • Godfrey Hardy(18621937) = British mathematician • Wilhelm Weinberg(1877 – 1947) = German Physician The H-W Principle • Allele frequencies in a population tend to remain the same from generation to generation unless acted on by outside influences. • Genetic Equilibrium • Formula p+q=1 p2 + 2pq + q2 = 1 What do the letters mean? • • • • • p = dominant allele q = recessive allele p2 =homozygous dominant q2 = homozygous recessive 2pq = heterozygous • p+q=1 p2 + 2pq + q2 = 1 H-W Based on Assumptions • No net mutations occur • Individuals don’t enter or leave the population • The population is large • Individuals mate randomly • Selection does not occur Disruptions to Genetic Equilibrium • Mutations occur constantly at a slow rate • Some mutations will be bad and some will be good for the population Disruptions to Genetic Equilibrium • Individuals and their genes frequently move in and out of populations • Sometimes males will leave when they mature to form their own group Disruptions to Genetic Equilibrium • Genetic Drift – The changes of allele frequencies in small populations due to chance • Small populations can have genetic issues quickly Disruptions to Genetic Equilibrium • Nonrandom mating • Most organisms choose their mates • Females often pick “more fit” males to breed with. Disruptions to Genetic Equilibrium • Natural Selection is an ongoing process in nature • Traits are being selected for and against all the time. Species Evolution • A species is a population of organisms that can successfully interbreed but cannot breed with other groups Forming a New Species • Geographic Isolation when the habitat of a species is divided over a long period of time, two new species can arise. Forming a New Species • Reproductive Isolation Barriers to successful breeding between population groups in the same area Reproductive Isolation Prezygotic - before fertilization • Chromosomal problems • Seasonal mating differences Reproductive Isolation Postzygotic = after fertilization Creation of hybrids that will be born infertile. Rates of Speciation • Gradualism – idea that species originate through a gradual buildup of new adaptations. • Punctuated Equilibrium = idea that speciation occurs quickly in rapid bursts, with long periods of stability in between.