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Mechanisms of Evolution Microevolution Population Genetics • A Population is a group of interbreeding organisms living together in space and time. (This means they are necessarily all the same species) • A Population is The Smallest Unit of Evolution • Individual organisms DO NOT evolve (in the Darwinian sense) • Natural selection acts on individuals, but populations evolve • What changes is the gene pool of the population, from generation to generation • Genetic variations in populations – Contribute to Natural Selection and are what is changed by natural selection Cuban Tree Snails Figure 23.1 Key Concepts • 23.1: Population genetics provides a foundation for studying evolution • 23.2: Mutation and sexual recombination produce the variation that makes evolution possible • 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition • 23.4: Natural selection is the primary mechanism of adaptive evolution • Concept 23.1: Population genetics provides a foundation for studying evolution • Microevolution – Is change in the genetic makeup of a population from generation to generation Figure 23.2 The Modern Synthesis • Integrates Darwinian selection and Medelian inheritance and focuses on population genetics • Population genetics (began in 1930’s) – Is the study of how populations change genetically over time – Reconciled Darwin’s and Mendel’s ideas • At the time The Origin of Species was published, little was known about inheritance. • Darwin did not know how variation appeared or how it was transmitted • His raw material for selection was variation in quantitative characters (vary along a continuum) • Mendel’s characters were discrete • Mendel’s inheritance was rediscovered in the early 1900’s, but it wasn’t until the 1930’s that scientists recognized that Darwin’s quantitative characters are genetically inherited • The Modern Synthesis was formulated in the 1940’s by many scientists. • Ernst Mayr, biogeographer and systematist emphasized: – The population as the unit of evolution – Natural selection as the primary mechanism – Gradualism as an explanation of large changes resulting from the accumulation of small changes over long periods of time Evolutionary science continues to develop • Current debate focuses on the rate of evolution and on the importance of evolutionary mechanisms other than natural selection Gene Pools and Allele Frequencies • The genetic structure of a population is defined by its allele and genotype frequencies • A population – Is a group of individuals of the same species living together in space and time • A Species – is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature. • Most species are not evenly distributed over their geographic range • They may have several localized population centers MAP AREA • The population centers may be more or less isolated • Even when centers are less isolated, individuals are still more likely to mate with others from their population center, so gene flow is reduced by the intermediate range • Fairbanks Fortymile herd range • Whitehorse Figure 23.3 Population Gene Pool • Is the total aggregate of genes in a population at any one time • Consists of all gene loci in all individuals of the population • Is made up of alleles that are combined to form the next generation – An allele is said to be “fixed” if all members of the population are homozygous for that gene – Normally there will be two or more alleles for each locus, each having a relative frequency in the gene pool Gene Pool • In diploid species, each individual will be homozygous or heterozygous for each locus because each locus is represented twice The Hardy-Weinberg Theorem • Describes a population that is not evolving • States that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work Mendelian inheritance • Preserves genetic variation in a population • Does not alter the frequency of alleles or genotypes Generation 1 CW CW CRCR genotype genotype Plants mate Generation 2 All CRCW (all pink flowers) 50% CR gametes 50% CW gametes Come together at random Generation 3 25% CRCR 50% CRCW 50% CR gametes 25% CWCW 50% CW gametes Come together at random Generation 4 25% CRCR 50% CRCW 25% CWCW Figure 23.4 Alleles segregate, and subsequent generations also have three types of flowers in the same proportions Example • 500 Wildflower plants with two alleles for flower color (CR and CW) • 320 Homozygotes CRCR are red • 160 Heterozygotes CRCW are pink • 20 Homozygotes CWCW are white • This means there are 800 CR alleles and 200 CW alleles Example 500 diploid wildflowers have 1000 alleles 320 CRCR have 640 CR alleles 160 CRCW have 160 CR alleles for a total of 800 20 CWCW have 40 CWalleles 160 CRCW have 160 CW alleles for a total of 200 The frequency of the CR allele is 0.8 and the frequency of the CW allele is 0.2 Hardy-Weinberg Equilibrium • Describes a population in which random mating occurs • Describes a population where allele frequencies do not change • Describes a population that is NOT evolving Hardy-Weinberg equilibrium • p is the frequency of one allele and q is the frequency of the other allele Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) p2 CR (80%) p2 64% CRCR CW (20%) Eggs • If there are only two alleles then p+q=1 pq 16% CRCW 16% CRCW qp 4% CW CW q2 If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: • Genotype frequencies are calculated using allele frequencies 64% CRCR, 32% CRCW, and 4% CWCW Gametes of the next generation: 16% CR from 64% CR from + CRCW heterozygotes CRCR homozygotes 4% CW from CWCW homozygotes + 16% CW from CRCW heterozygotes = 80% CR = 0.8 = p = 20% CW = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: Figure 23.5 64% CRCR, 32% CRCW and 4% CWCW plants • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p2 + 2pq + q2 = 1 – And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype • In cases of complete dominance p is usually reserved to represent the dominant allele and q represents the recessive allele • If p is 0.8 and q is 0.2 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CW (20%) pq CR (80%) Eggs p2 p2 64% CRCR CW (20%) – Then p2 is 0.64 – q2 is 0.04 – 2pq is 0.32 CR (80%) 16% CRCW 16% CRCW qp 4% CW CW q2 If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: 64% CRCR, 32% CRCW, and 4% CWCW Gametes of the next generation: 16% CR from 64% CR from + CRCW heterozygotes CRCR homozygotes 4% CW from CWCW homozygotes + 16% CW from CRCW heterozygotes = 80% CR = 0.8 = p = 20% CW = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: Figure 23.5 64% CRCR, 32% CRCW and 4% CWCW plants Conditions for HardyWeinberg Equilibrium • The Hardy-Weinberg theorem – Describes a hypothetical population • In real populations – Allele and genotype frequencies do change over time • The five conditions for non-evolving populations are rarely met in nature – Extremely large population size – No gene flow – No mutations – Random mating – No natural selection Population Genetics and Human Health • Using the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease • 1/400 African Americans have sickle-cell disease • The frequency of the homozygous recessive genotype is 0.0025 (q2) • q = 0.05 is the frequency of the recessive allele • p = 0.95 is the frequency of the dominant allele • 2pq = 0.095 is the frequency of the heterozygous carrier genotype in the African American population of the U.S.