CHAPTER 23: The Evolution of Populations >>>Evolution and populationRemember that evolution is the change in gene frequencies in the population.---Population is a localized group of individuals that belong to the same biological speciesPopulations are the smallest relevant evolutionary unit-individuals don't evolve, populations do-evolution measured by change in population-evolution acts only on heritable traits>>>>>>Populations show variationsSome variation is theresult of heritablechanges (mutationand/or geneticrecombination) andsome variations isnonheritable (see fig23.3)>>>>>>Variationbetween populations- Population of the same organism can show variations. This population variation can often be geographic. The variation between populations can be discrete or continual.- Frequently continual variation in a population will exist as a cline (see fig 23.5)--- Cline is a graded variation in a trait that parallels a gradient in the environment>>>>>>The integration of Darwinism (evolution) and Mendelism (genetics)When the field of evolution (and the study of populations) merged with genetics, the science of population genetics was formed.- population is the unit of evolution- natural selection is the primary mechanism for evolutionary change---Population genetics is the study of how populations change genetically over time>>>>>>A population has a genetic structure defined by its gene poolThe genes in a population make up the gene pool for that population---Gene pool is the total aggregate of genes in a population at any one time- All the alleles at all the loci for all individuals in a population- most alleles in the gene pool will be combined to produce the next generation. If you add up all the alleles for a particular trait, the allele frequency for an allele can be determined.1- If only one allele exists for a trait, then that trait is "fixed" in the populationo The allele frequency is 1.0 (100%)- Most traits have multiple alleles and each allele has a frequency of occurrence.>>>>>>The Hardy-Weinberg Principle In the absence of other factors, the segregation and recombination of alleles during meiosis and fertilization will not alter the overall genetic makeup of a population.- The gene frequencies will remain constant unless nature forces a change.A population of 100 peas The peas are diploid for the pea shape gene and are either round or wrinkled. The round allele (R) is dominant over the wrinkled allele (r). In the population 84 plants have round seeds, and 16 have wrinkled seeds. Of the 84 plants with round seeds 36 are homozygous (RR) and 48 are heterozygous (Rr).Since each plant has 2 genes for shape there are 200 pea shape genes in the population.120 of those shape genes are dominant (R) (36 x 2) + 48 = 120homozygous heterozygousThe frequency for the p allele is 0.6120/200 The frequencies for both alleles (A and a) must add up to equal 10.6 + frequency for a = 1so the frequency for a must be 0.4You can also get this by the same method we got the frequency for A(16 x 2) + 48 = 80homozygous heterozygous80/200 = 0.4Assuming that mating in the population is completely random, then the frequencies of R and r will remain constant.The chance of getting two R genes in the F1 generation is 0.6 x 0.6 = 0.36 This means 36% of the F1 generation is RRThe chance of being a heterozygous individual is2 (0.6 x 0.4) = 0.48 This means 48% of the F1 generation is RrThe reason you multiply by 2 is there are 2 ways to get a heterozygous individual. You can get R first followed by r or you can get r first followed by R.The chance of getting two r genes in the F1 generation is20.4 x 0.4 = 0.16 This means 16% of the F1 generation is rr.NOTICE These percentages are the same as the P generation If reproduction continued with segregation, recombination and random mating the frequencies would remain the same (i.e., no evolution)>>>HARDY-WEINBERG EQUILIBRIUM- Genotype frequency stays the same from generation to generation because the frequency of alleles (gamete frequency) stays the same.- The population is not evolvingRemember the Hardy-Weinberg Principle stated:In the absence of other factors, the segregation and recombination of alleles during meiosis and fertilization will not alter the overall genetic makeup of a population.The conditions needed to maintain the equilibrium:1. No mutations2. No gene flow - Population is isolated from other populations3. Population is very large4. Random mating5. No natural selection These criteria are essentially never met and the result is evolution.Four important points about evolution:1. Natural selection doesn’t cause genetic change in individuals 2. Outcome of natural selection is evolution, which affects the population 3. Evolution is spread because of differential reproduction or fitness4. Evolutionary changes aren’t absolutely good, but are good in the environmental contextMECHANISMS OF EVOLUTION>>>>>>Mutations: adding a new allele to the gene pool.- Mutations are the only source of new alleles - Mutations occur spontaneously- The creation of new allele forms- Some are meaningful; some are not, some you can’t tell.>>>>>>Gene flow can cause evolution by transferring alleles betweenpopulations---Gene flow is genetic additions to or subtractions from a population resultingfrom the movement of fertile individuals or gametes- The gene pool of one population mixes with the gene pool of another- If the mixing is extensive then the two populations become one.- Migration high = little genetic variation between populations3- Migration low = more genetic variation between populations>>>>>>Genetic drift: evolution by chance. (See fig 23.9)---Genetic drift is the unpredictable fluctuations in allele frequencies from one generation in allele frequencies from one generation to the next because of a population’s finite size- In a small population chance events have a greater impact. - The larger the population the less important genetic drift becomes.Two situations which result in small populations:1. Bottleneck Effect (see fig. 23.10)- drastic reduction in the population sizeo the surviving population probably has adifferent genetic makeup- The population is now more susceptible to geneticdrift.2. Founder Effecto a few individuals colonize a new habitat- the founding population probably has a differentgenetic makeup- The population is now more susceptible to genetic drift.>>>>>>Non-random mating can cause evolution by
View Full Document
Unlocking...