WPopulation Genetics Translating what we ve learned about genetics into changes in species In the beginning there was a mutation And so allele frequency changed The first force of evolution is mutation and it increases variation in a species Variants different varieties of a single species are affected differently by the environment depending upon their fitness in that environment Some survive and pass on their new alleles Some don t The frequency of some alleles decreases because some are eliminated Consequently the frequency of others increases Because some new ones and some old ones are eliminated there are fewer variants The second force of evolution is natural selection and it decreases variation in a species Humans have always migrated traveled traded made war and generally made contact with other humans Where there is human contact there is sex Thus alleles never before seen in one population may be introduced through sex from another population As a population has sexual contact with others new alleles are acquired and increase the number of different alleles in the first population The faster this population assimilates others the faster the first population will come to look like others with which they ve had contact Conversely other populations will also incorporate features found in the first population The third force of evolution is gene migration and it increases variation in a species Sometimes you can do everything right but wind up in the wrong place at the wrong time Bad luck can wipe your group out When it does the alleles you carried are eliminated The fourth force of evolution is genetic drift and it decreases variation in a species Take a coin out of your pocket How many sides are there What do you think are your odds of getting heads on the first flip A series of 10 flips will rarely achieve the same number of heads and tails the true odds because of a phenomenon known as sampling error This is analogous to what happens when a small population is created from a large population for any reason To demonstrate how these forces change populations over generations open up the Micro program First click on this Computing Allele Frequencies 3 genotypes There are 50 people who There are 50 people who are homozygous A 100 are homozygous A 100 who are heterozygous who are heterozygous and 50 who are and 50 who are homozygous a homozygous a The Hardy Weinberg Theorem What it says If you know the frequencies of genotypes and by extension alleles today in any population it is possible to predict what their frequencies will be in the future in that population The conditions This is possible only if The population s size approaches infinity in practice at least 10 000 individuals There is only completely random mating No evolution is occurring The Hardy Weinberg Equilibrium The three possible genotypes homozygous A heterozygous Aa and homozygous aa will remain in the same proportion to each other over generations as long as the three conditions previous slide are satisfied This proportion is AA 2Aa aa or p2 2pq q2 Natural Selection We re interested in seeing how the frequency of the A allele changes over 100 generations Of all the genotypes the AA genotype has the least likelihood of surviving and producing fertile offspring 70 Of all the alleles A represents 25 That means 75 are a Heterozygote Advantage Further explanation coming soon tonight The Effect of Genetic Drift Genetic Drift N 200 N 5 000 Four small groups split off from the same parent population and each goes its own way Gene Migration AKA gene flow In every generation 20 of one population migrates to the other population Gene Migration with Small Populations Summary The four forces of evolution are 1 Mutation 2 Natural Selection 3 Gene migration 4 Genetic drift What happens to a population over generations is dependent on its size its initial genetic makeup the influences of evolution and luck