Chapter 23Wednesday, January 27, 20162:06 PM Chapter 23: Evolution of Populations Genes, Mutations, and Inheritance-Phenotypic variation is mostly geneticoBut environment can influence expression, creating non-heritable variationoDiscrete 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 timeoContinuous 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 offspringoCrossoversoIndependent assortmentoRandom fertilization - any sperm can combine with any egg at random-New alleles arise from mutations in DNAoIn cells that ultimately makes gametesoPoint mutations-1 to 3 base-pairsoChromosomal alterations-Due to unequal crossovers-Include deletions, duplications, translocationsDeletion - one chromosome is lostDuplication - duplicatedTranslocation - repositioning of genes can change their functionoGene duplications can later mutate into new alleles, expanding size of genome-Mutations forming new alleles-Most new alleles are harmfuloBut harmful effects may be "hidden" in heterozygotesoBut some new alleles are neutral with respect to selection-= phenotype does not affect the likelihood of leaving offspringoBut if environment changes, harmful or neutral alleles may become adaptive-= increases likelihood of leaving offspring-However, most DNA variability does NOT affect phenotypeoBecause protein translation is not affectedHardy-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 phenotypesoHardy-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-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 genotypeoUsing 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 SelectionoActs non-randomly on phenotypes of individualsoChanges allelic and genotypic frequencies of populations non-randomlyoAlways leads to adaptation of population to current environment-Ex: resistance to DDToNo, did not expose to DDT itself to create DDT-R allele-Genetic Drifo= think "random", changes in gene frequency due to random eventsoSmall 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)oThe 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 increasedoThe 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 lossoGene flowo= alleles move In/out of populationoTransferring of alleles from one population to anotheroIncludes-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 aggressiveoResults 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 FitnessoFitness 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 SelectionoDirectional selection-Shifts character's mean value to one direction-Bell curve with original population curve shifted as evolved population-Ex. Bug beak lengthoDisruptive 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