CHAPTER 23 The Evolution of Populations Evolution and population Remember that evolution is the change in gene frequencies in the population Population is a localized group of individuals that belong to the same biological species 1 Populations 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 variations Some variation is the result of heritable changes mutation and or genetic recombination and some variations is nonheritable see fig 23 3 Variation between 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 pool The 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 2 If only one allele exists for a trait then that trait is fixed in the population o 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 homozygous heterozygous 48 120 The frequency for the p allele is 0 6 120 200 The frequencies for both alleles A and a must add up to equal 1 0 6 frequency for a 1 so the frequency for a must be 0 4 You can also get this by the same method we got the frequency for A 16 x 2 homozygous 80 200 0 4 heterozygous 48 80 Assuming 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 RR The chance of being a heterozygous individual is 2 0 6 x 0 4 0 48 This means 48 of the F1 generation is Rr The 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 is 3 0 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 evolving Remember 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 mutations 2 No gene flow Population is isolated from other populations 3 Population is very large 4 Random mating 5 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 fitness 4 Evolutionary changes aren t absolutely good but are good in the environmental context MECHANISMS 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 between populations Gene flow is genetic additions to or subtractions from a population resulting from 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 populations 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 4 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 size o the surviving population probably has a different genetic makeup The population is now more susceptible to genetic drift 2 Founder Effect a few individuals colonize a new habitat o the founding population probably has a different genetic makeup The population is now more susceptible to genetic drift Non random mating can cause evolution by shifting the frequency of genotypes There are two kinds of non random mating 1 Inbreeding 2 Assortive mating Individual in a population mate with close neighbors results in a shift in genotype frequency but not
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