Ch 23 Evolution of a PopulationPopulation genetics answer questions Darwin couldn’t answer. How do we see evolution? Why is there so much diversity?Darwin and Modern GeneticsDarwin vs. Mendel- Darwino Thought there was continuous variation in specieso Accumulation of differences in offspring- Mendelo Chromosome theory published in 1900o Discrete genetic factor in individualso No blending of traits, no accumulationA 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 groupsNatural selection acts on phenotype at an individual level, but evolution is at a population levelMicroevolution is evolution with in a single populationGenetic 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 twoor 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 genomesDiploidy 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 EquilibriumGenetic 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=1p= frequency of the dominant alleleq= frequency of the recessive allelep+q=1Hardy-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 occurringo 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 factorsMechanisms of MicroevolutionNatural 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 environmentGenetic 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 populationsNatural SelectionFitness 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 SelectionDirectional Selection occurs when one extreme phenotype is favored. This results in the movement of the graph left or right. It occurs when habitats changeDisruptive 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 VariationVariation is maintained through- Diploidyo Low frequency of recessive alleles are hidden in heterozygotes- Disruptive selectiono Extremes are favored over intermediates- Heterozygote advantageo The heterozygote is fitter than either homozygote (malaria and sickle cell)- Frequency-Dependent Selectiono 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 individualo Adaptively neutral features “tag along”- And adaptation may be a compromise in form due to competing needs (deer standing stilland running)- It can only act on existing variationo Extinction happens when adaptation is impossible- Constrained be genetic ancestry- Chance, environment, and natural selection interacto History