Ch 23 Evolution of a Population Population genetics answer questions Darwin couldn t answer How do we see evolution Why is there so much diversity Darwin and Modern Genetics Darwin vs Mendel Darwin o Thought there was continuous variation in species o Accumulation of differences in offspring Mendel o Chromosome theory published in 1900 o Discrete genetic factor in individuals o No blending of traits no accumulation A population is a group of individuals living in the same area and with the potential to interbreed and produce fertile offspring It is somewhat isolated from other groups Natural selection acts on phenotype at an individual level but evolution is at a population level Microevolution is evolution with in a single population Genetic variation Discrete variation consists of genes that exist at only one gene locus with two or more alleles There is an either or choice in the genetic code Quantitative variation occurs when a phenotype is produced by the additive effects of two or more genes The average heterozygosity measured within populations is the average percentage of loci genes that are heterozygous in and individual population Variation may also exist between populations known as geographic variation A cline is a graded variation along a geographic axis p 1196 The ultimate source of variation is mutation in the gamete forming cell line Point mutations replacement of a single base pair may or may not create a new allele Chromosome alterations deletions duplications translocations expand genomes Diploidy increases allelic diversity because new and possibly harmful recessive mutations hide in heterozygotes Sexual reproduction creates new phenotypes through new and infinite combinations of alleles Hardy Weinberg Equilibrium Genetic make up of a population measured through The Gene Pool which includes all of the alleles of all the genes in a population The Genotypic Frequency is the percentage of each genotype in a population The Allelic Frequency is the percentage of each allele in the population The Hardy Weinberg Principle If a LARGE population is performing completely RANDOM mating sexual and no other factors are acting THEN the genetic make up will not change in the next generation p2 2pq q2 1 p frequency of the dominant allele q frequency of the recessive allele p q 1 Hardy Weinberg Equation MUST haves No mutation Random mating No natural selection Large population No gene flow emigration immigration The Hardy Weinberg Equation lets us define evolution at a population level Hardy Weinberg Equation is a null hypothesis o We can compare the expected Hardy Weinberg with the actual genotypic ratios to see if evolution is occurring o If the actual ratios do not equal the expected Hardy Weinberg ratios then the population is evolving microevolution Microevolution is any change in the genetic make up of a population over generations The change may be ADAPTIVE due to natural selection The change may be ADAPTIVELY NEUTRAL or MALADAPTIVE due to other factors Mechanisms of Microevolution Natural selection acts non randomly on the phenotypes of individuals The change in allelic frequency in a population is non random It leads to adaptation of a population to an environment Genetic drift is the change in a gene pool die to random events It is very likely in small populations It is much like sampling errors in statistics The allele frequencies may vary wildly and randomly Decreases the genetic diversity It may fix alleles which means the entire population is homozygous for an allele even harmful ones The founder effect occurs when a few founders start a new population that is isolated from the original population The bottleneck effect occurs when an event drastically cut a population s size o The survivors survived only by sheer dumb luck they did not have any phenotypic advantage over those that died Gene Flow occurs when alleles move in and out of a population causing a change in allele frequencies Sources include gametes adults moving seeds larvae Results in microevolution which may but not always be adaptive More gene flow results in a decreased difference between populations Natural Selection Fitness is the best phenotype to survive and lead alleles to the next generation best reproductive success Fitness is relative to other individuals in a population Relative Fitness Forms of Natural Selection Directional Selection occurs when one extreme phenotype is favored This results in the movement of the graph left or right It occurs when habitats change Disruptive selection occurs when both extremes are favored over intermediates Often occurs in patch habitat It helps maintain diversity African seed cracker bimodal population Stabilizing selection occurs when extremes are less successful and intermediates are favored It reduces variation infant birth weight Sexual selection is based on differential mating success based on traits not directly related to reproduction or survival An individual s traits may not help with survival but does help with sexual reproduction birds This leads to sexual dimorphism Intraselection occurs when males fight to win females Interselection occurs when females chose males based on coloration or behavior Maintaining Variation Variation is maintained through Diploidy o Low frequency of recessive alleles are hidden in heterozygotes Disruptive selection o Extremes are favored over intermediates Heterozygote advantage o The heterozygote is fitter than either homozygote malaria and sickle cell Frequency Dependent Selection o Fitness of a phenotype is dependent on how common it is in the population The more common the less successful Limits of Natural Selection It acts only on the phenotype of an entire individual o Adaptively neutral features tag along And adaptation may be a compromise in form due to competing needs deer standing still and running It can only act on existing variation o Extinction happens when adaptation is impossible Constrained be genetic ancestry Chance environment and natural selection interact o History matters