Chapter 23 Wednesday January 27 2016 2 06 PM Chapter 23 Evolution of Populations Genes Mutations and Inheritance Phenotypic variation is mostly genetic o But environment can influence expression creating non heritable variation o Discrete genetic variation Single gene locus Genes can have 2 or more alternatives 2 or more alleles Genotype determines the phenotype Expressed as complete or incomplete dominance or codominance Phenotype in heterozygote determines which is dominant Incomplete dominance intermediate middle Codominance have both phenotypes showing up at the same time o Continuous variation Phenotypes produced by combined effects of 2 or more genes 14 13 23 3 skin color variation appears continuous bell curve Sexual recombination promotes genetic diversity among offspring o Crossovers o Independent assortment o Random fertilization any sperm can combine with any egg at random New alleles arise from mutations in DNA o In cells that ultimately makes gametes o Point mutations 1 to 3 base pairs o Chromosomal alterations Due to unequal crossovers Include deletions duplications translocations Deletion one chromosome is lost Duplication duplicated Translocation repositioning of genes can change their function o Gene duplications can later mutate into new alleles expanding size of genome Mutations forming new alleles Most new alleles are harmful o But harmful effects may be hidden in heterozygotes o But some new alleles are neutral with respect to selection phenotype does not affect the likelihood of leaving offspring o But if environment changes harmful or neutral alleles may become adaptive increases likelihood of leaving offspring However most DNA variability does NOT affect phenotype o Because protein translation is not affected Hardy Weinberg and Microevolution The Population Population a group of individuals living in the same area interbreeding to produce fertile offspring But somewhat isolated from other groups Occasionally a centipede may float to the other side but normally would mate on same side Populations differ in genetic makeup Gene pool all the alleles of all the genes in a population Many genes have fixed alleles homozygous in all individuals Other genes 2 or more alleles Genotypic frequency proportion of each genotype in the population AA Aa and aa Allelic frequency A allele and a allele Population 1 12 25 48 0 48 Aa 9 25 36 0 36 aa 30 50 60 0 6 a Population 2 Population 3 Population 4 Microevolution any change in population genetic makeup over time Smallest fundamental unit of evolution May be due to random events May be due to natural selection on individual phenotypes o Hardy Weinberg principle The H W equilibrium IF a large population reproduces sexually at random THEN the genetic makeup should not change in the next generation remains in equilibrium The H W conditions No mutation Mating is random No selection equal survival Very large population size No gene flow in or out messes up the condition of randomness IF this population meets H W random conditions then every generation over time will have the same ratios 64 64 4 4 32 32 Note that allelic frequency are the same too o 128 32 200 0 8 If p frequency of dominant allele Q freqq Recessive allele And p q 1 Then in any generation P2 P2 freq of homozygous dominant genotype Freq of heterozygous genotype Freq Of homozygous recessive genotype Using the H W equation If you KNOW or CAN ASSUME a HW equilibrium then use the equation to determine population genetic makeup Mechanisms of Microevolution Natural Selection o Acts non randomly on phenotypes of individuals o Changes allelic and genotypic frequencies of populations non randomly o Always leads to adaptation of population to current environment Ex resistance to DDT o No did not expose to DDT itself to create DDT R allele Genetic Drif o think random changes in gene frequency due to random events o Small population size leads to dramatic drifts 23 9 like sampling errors in statistics Will only come back due to mutation or Allele frequencies may vary wildly and randomly each generation whether adaptive or not Often reduces diversity One allele may become fixed all other alleles lost o The founder effect and genetic drift A few founders start new isolated population Founder gene pool differs from original source Small population size less diverse population will go through more drift as a result will end up with a much less diverse population in the island Random less diverse founder population more genetic drift some adaptive alleles are lost Ex High rate of inherited blindness on Tristan da Cunha Maladaptive allele frequencies increased o The bottleneck effect and genetic drift An event drastically cuts population size Wipes out most of the population with few survivors Surviving population will look different than what they did before Gene pool of survivors is random some alleles are lost in surviving population more genetic drift Ex Overhunting of elephant seals and cheetahs reduced genetic variation Ex Prairie chicken habitat loss o o o Gene flow o alleles move In out of population o Transferring of alleles from one population to another Includes Migration of adults Dispersal of gametes seeds larvae Ex Africanized Honey Bees Enhanced to collect nectar Interbred with European Honey Bees Great at collecting nectar but very aggressive Results of gene flow May add diversity to population ex Kansas prairie chicken bred with Illinois prairie chickens May reduce differences between populations Understanding Natural Selection Relative Fitness o Fitness is relative to other individuals in the population fittest best phenotypes to survive and leave alleles to next generation best reproductive success Darwinian fitness relative fitness would be zero all relatives get killed so nothing left for reproduction Fitness includes survival finding mates and the number of healthy fertile offspring Forms of Natural Selection o Directional selection Shifts character s mean value to one direction Bell curve with original population curve shifted as evolved population Ex Bug beak length o Disruptive selection Intermediates are less fit more light and more dark less intermediates Maintains diversity against the mean towards two extremes Ex Seedcracker finch 2 size beaks Seeds available mostly in one of two sizes medium are not fit o Stabilizing selection Intermediate types more fit than extremes Bell curve is sharper Variation is reduced Sexual Selection o success based on traits