Genetic Variation makes evolution possible 01 26 2015 Evolutionary Processes Natural selection not only evolutionary mechanism Each of four processes listed below has different consequences o Natural and sexual selection o Genetic drift o Gene flow o Mutation Sources of Genetic Variation Genetic variation sets stage for natural selection Sources o Formation of new alleles genes mutations o Altering gene number of location chromosomal mutations o Rapid reproduction mutation rates constant generation times not constant o Sexual reproduction recombination of chromosomes Set of equations for determining if populations evolving at particular Hardy Weinberg Equilibrium gene What is a population o Group of interbreeding individual of the same species o gene pool start simply gene with two alleles A 1 and A 2 A 1 p A 2 q P q 1 always true when only two alleles o First important equation Three qenotypes possible A 1 A 1 A 1 A 2 A 2 A 2 o Frequency A 1 A 1 p 2 homozygote A 1 A 2 2pq heterozygote A 2 A 2 q 2 homozygote There are only three types of genotypes in one population o P 2 2pq q 2 1 Genotype frequencies must equal 1 Second hardy weinberg equation Hardy Weinberg as Null Mode Biologists want to test whether o 1 natural selection acting on a particular gene o 2 nonrandom mating occurred o 3 one of other evolutionary mechanisms at work In such situations Hardy Weinberg principle functions as null hypothesis When genotype frequencies of allele do not conform to Hardy Weinberg proportions o Evolution o Non random mating o NOT EVERY GENE IS BEING EFFECTED Occurring in that population Hardy Weinberg Equilibrium Populations in H W equilibrium for specific genes Five conditions for populations to be in H W o No natural selection no evolution o No genetic drift random allele frequency changes no evolution o No gene flow individuals groups entering or leaving populations gene pools no evolution o No mutation no evolution one thing you can not control o Random mating The Hardy Weinberg equation can be used to test whether a population is evolving 01 26 2015 Hardy Weinberg Equilibrium Set of equations for determining if populations evolving at particular gene What is a population o Group of interbreeding individual of the same species o gene pool start simply gene with two alleles A 1 and A 2 o A 1 p A 2 q o P q 1 always true when only two alleles First important equation Three qenotypes possible A 1 A 1 A 1 A 2 A 2 A 2 o Frequency A 1 A 1 p 2 homozygote A 1 A 2 2pq heterozygote A 2 A 2 q 2 homozygote There are only three types of genotypes in one population o P 2 2pq q 2 1 Genotype frequencies must equal 1 Second hardy weinberg equation Hardy Weinberg as Null Mode Biologists want to test whether o 1 natural selection acting on a particular gene o 2 nonrandom mating occurred o 3 one of other evolutionary mechanisms at work In such situations Hardy Weinberg principle functions as null hypothesis When genotype frequencies of allele do not conform to Hardy Weinberg proportions o Evolution o Non random mating NOT EVERY GENE IS BEING EFFECTED Occurring in that population Hardy Weinberg Equilibrium Populations in H W equilibrium for specific genes Five conditions for populations to be in H W o No natural selection no evolution o No genetic drift random allele frequency changes no evolution o No gene flow individuals groups entering or leaving populations gene pools no evolution o No mutation no evolution one thing you can not control o Random mating Natural Selection Genetic drift and gene flow can alter allele frequencies in a population 01 26 2015 Natural Selection Differential reproductive success higher fitness o Certain alleles passes on to next generation in higher proportions o Lead to adaption of organisms to environment better fit Natural selection NOT chance event o Environment selects for those with criteria for reproductive success Genetic Drift Describes how allele frequencies fluctuate randomly from one generation to the next Disproportionately affects SMALL populations Usually neutral alleles little or no effect on fitness fluctuate at random o Not effected by natural selection Genetic drift change in allele frequency due to chance blind luck o Does not result in adaption Occurs in every population and every generation Causes allele frequencies to drift up down randomly over time Consider tracing allele in group o In which allele frequency changes from higher lower with equal probability o Process continues over and over with time o Since no stabilizing force returns frequency to original frequency Frequency drifts to 0 or 1 Cannot change further unless mutation occurs Genetic Drift in Nature Big concern for conservation biologists o Small populations o Nature reserves and zoos cause padlock Impact of genetic drift increased by events that reduced genetic variation no evolution population will suffer o Founder effect o Population bottleneck Founder Effect Occurs when few individuals start new populations in new area o Most likely results in change in allele frequency founder Founder population usually smaller than source population o Only small number move don t represent genetic variation of effect population o Disproportionate effect of genetic drift Colonization of isolated habitats Population Bottleneck Sudden decrease in population size o Disease outbreak o Natural catastrophes Gene Flow Like founder effects bottlenecks allow for disproportionate effect of genetic drift o Ex Cheetah s leading to reduced genetic diversity and endangering their long term survival Every cheetah in Africa sampled nearly identical DNA Occurs when individuals gametes pollen move from one population to another o Increases genetic variation o Reduced genetic differences among populations over time Can lead to reduced fitness in some populations o Immigration from mainland brings alleles reducing fitness of central population o Populations without high migration rates have higher fitness Natural selection is the only mechanism that consistently causes adaptive evolution 01 26 2015 Mutations restores genetic diversity o creates new alleles o random with respect to fitness o slow compared to other evolutionary processes o essential for evolution most mutations result in deleterious alleles o lower fitness eliminated by natural selection Rarely beneficial alleles produced o Increased fitness o Increases frequency in populations due to natural selection Random mating In nature mating may not be random Two