Chapter 23 The Evolution of Populations 2021 Pearson Education Inc Lecture Presentations by Nicole Tunbridge and Kathleen Fitzpatrick Key Concepts and Questions Vocabulary Discuss how natural selection and evolution are related What three mechanisms cause allele frequencies and how do they work What causes Genetic Variation Understand how Nucleotide Variability works What is phenotypic variation How do mutation rates vary Calculate genotype frequencies Calculate allele frequencies What are the conditions if the hardy Weinberg equilibrium Define describe and apply natural selection genetic drift and gene flow Define describe and apply the founder effect and the 2021 Pearson Education Inc Figure 23 1b 2021 Pearson Education Inc What mechanisms can cause the evolution of populations Natural selection acts on individuals but only populations not individuals evolve Microevolution the change in allele frequencies in a population over generations is evolution at its smallest scale Three mechanisms cause allele frequency change Natural selection adaptation to the environment Genetic drift chance events alter allele frequencies Gene flow transfer of alleles between populations 2021 Pearson Education Inc CONCEPT 23 1 Genetic variation makes evolution possible Genetic variation is a prerequisite for evolution by natural selection Recall Gregor Mendel Genetic variation refers to the differences in genes or other DNA sequences among individuals Natural selection can only act on variation with a genetic component At the gene level genetic variation is quantified by the percentage of heterozygous loci in a population At the molecular level genetic variation is quantified by comparing the nucleotide sequences of two or more individuals 2021 Pearson Education Inc Nucleotide variability Nucleotide variability rarely results in phenotypic variation Most nucleotide differences occur within noncoding DNA segments introns Variations in coding regions exons rarely change the amino acid sequence of the encoded protein 2021 Pearson Education Inc What causes phenotypic variation Phenotypic variation can result from environmental influences as well as genetic differences Only the genetically determined part of phenotypic variation can have evolutionary consequences 2021 Pearson Education Inc Sources of Genetic Variation Genetic variation originates when new genes and alleles arise by mutation gene duplication or other processes Sexual reproduction can produce genetic variation by recombining existing alleles which can happen in three methods Crossing over exchange of genetic material between homologous chromosomes during meiosis Independent assortment random distribution of chromosomes into gametes during meiosis Fertilization random combination of gametes New allele formation stemming from mutation Some are harmful most are neutral variation few are beneficial Altering gene number or location Most are harmful but duplications over generations add up and could result in a new function for the gene 2021 Pearson Education Inc Rapid Reproduction Mutation rates are low in animals and plants averaging about one in every 100 000 genes per generation Prokaryotes have lower rates but shorter generation times allow mutations to accumulate rapidly Mutations can accumulate rapidly in viruses due to short generation times and rapid mutation rates 2021 Pearson Education Inc CONCEPT 23 2 The Hardy Weinberg equation can be used to test whether a population is evolving Genetic variation is required for a population to evolve but does not guarantee that it will One or more factors that cause evolution must be at work for a population to evolve 2021 Pearson Education Inc Gene Pools and Allele Frequencies A population is a group of individuals of the same species that live in the same area and interbreed Geographically isolated populations rarely exchange genetic material If populations are not isolated individuals still usually only breed with members of their own population 2021 Pearson Education Inc The gene pool consists of all copies of every allele at every locus in all members of the population A locus is fixed if all individuals in a population are homozygous for the same allele If there are two or more alleles for a locus individuals may be homozygous or heterozygous Each genotype and each allele has a frequency in the population that can be calculated 2021 Pearson Education Inc Practice Problem Consider a population of 500 wildflowers with 320 red flowers CRCR 160 pink flowers CRCW and 20 white flowers CWCW To calculate genotype frequencies divide the number of individuals of each genotype by the total number of individuals in the population CRCR is 0 64 320 500 CRCW is 0 32 160 500 CWCW is 0 04 20 500 2021 Pearson Education Inc Allele frequencies can also be calculated for a population For diploid organisms the total number of alleles at a locus is the total number of individuals times two Count two dominant alleles for each homozygous dominant individual and one for each heterozygote The same logic applies for recessive alleles 2021 Pearson Education Inc Consider the population of 500 wildflowers with 320 red flowers CRCR 160 pink flowers CRCW and 20 white flowers CWCW Calculate the number of copies of each allele CR 320 2 160 800 CW 20 2 160 200 2021 Pearson Education Inc By convention if there are two alleles at a locus p and q are used to represent their frequencies The frequency of all alleles in a population will add up to 1 That is p q 1 2021 Pearson Education Inc To calculate the frequency of each allele divide the number of copies of each allele by the total number of alleles in the population p frequency of CR 800 800 200 0 8 80 q 1 p 0 2 20 The sum of alleles is always 1 100 0 8 0 2 1 2021 Pearson Education Inc The Hardy Weinberg Equation The Hardy Weinberg equation describes the expected genetic makeup for a population that is not evolving at a particular locus Such a population is in Hardy Weinberg equilibrium If the observed genetic makeup of the population differs from expectations under Hardy Weinberg then the population may be evolving 2021 Pearson Education Inc More Practice Imagine all alleles for a given locus for all individuals in a population are placed in a bin Reproduction occurs by randomly selecting and combining alleles from the bin Apply this analogy to the population of 500 wildflowers with 1 000 alleles where p frequency of CR 0 8 q frequency of CW 0 2 2021 Pearson Education Inc
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