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Chapter 3

Chapter 3 Textbook Notes - Forces of Evolution

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University of Toronto Mississauga
Esteban Parra

Notes From Reading CHAPTER 3:T HE FORCES OFE VOLUTION PGS .61-83) Introduction - We can observe biological change either by observing organisms from generation to generation, or by detecting past change through anatomic and genetic analysis - Darwin provided the basis for understanding how natural selection worked on existing variation but had no explanation for the ultimate origin of variation - Both mutation and natural selection are evolutionary forces, but are not the only ones Population Genetics - Microevolution takes into account changes in the frequency of alleles from one generation to the next - We are interested in defining the relative frequencies of different alleles, genotypes, and phenotypes for the entire population being studied What Is a Population? - Breeding Population – A group of organisms that tend to choose mates from within the group o It is not clear what proportion of mating within a group defines a breeding population - Total population refers to everybody, whether or not they are likely to breed - Breeding population is smaller than the total population because of a number of factors o Some will be too young or too old to mate o Cultural factors and geographic distribution may act to limit an individual’s choice of mate, and as a consequence, some individuals will not breed o There may not be enough individuals of the opposite sex from which to choose a mate (often in isolated areas) Genotype Frequencies and Allele Frequencies - Genotype frequency is a measure of the relative proportions of different genotypes within a population o Obtained by dividing the number of individuals with each genotype by the total number of individuals  Ie, total of 200 people; Frequency of MM = 98/200 =0.49 Frequency of MN = 84/200 = 0.42 Frequency of NN = 18/200 = 0.09 o These frequencies are proportions and add up to 1 - Allele frequency is a measure of relative proportion of alleles within a population o Computed by counting the number of each allele and dividing that umber by the total number of alleles o This method can be used only when the number of individuals with each genotype can be determined (if one is dominant, this will not be possible)  Example of Allele Frequency Computation Genotype # of People Total # of # of M # of N Alleles alleles alleles MM 98 196 196 0 MN 84 168 84 84 NN 18 36 0 36 Total 200 400 280 (0.7) 120 (0.3) Hardy-Weinberg Equilibrium - Hardy-Weinberg Equilibrium – A mathematical statement whereby, in the absence of nonrandom mating and evolutonary forces, genotype and allele frequencies will remain the same from one generation to the next Notes From Reading CHAPTER 3:T HE FORCES OF EVOLUTION PGS .61-83) 2 2 - Note: p + 2pq +q = 1 2 2 o Frequency of the MM genotype = q = (0.7) = 0.49 o Frequency of the MN genotype = 2pq = 2(0.7)(0.8) = 0.42 o Frequency of the NN genotype = p = (0.3) = 0.09 - Hardy and Weinberg show that given certain conditions genotype and allele frequencies would stay the same from one generation to the next - HW equilibrium in population genetics is a baseline with which to compare the real world, which helps us determine why populations do change - Evolutionary Forces – Four mechanisms that can cause changes in allele frequencies from one generation to the next: mutation, natural selection, genetic drift and gene flow - Note: when allele frequencies change, so do genotype frequencies, but genotype frequencies can change without altering the underlying allele frequencies - Nonrandom Mating – Patters of mate choice that influence the distributions of genotype and phenotype frequencies - Inbreeding – Mating between biologically related individuals o More individuals will be homozygous and fewer will be heterozygous The Evolutionary Forces - In the real world, mutations, natural selection, genetic drift and gene flow can happen all at the same time Mutation - Mutation introduces new alleles into a population changing the frequency of alleles over time - Mutations are vital to evolution because they provide new variations - Polymorphisms – A discrete genetic trait in which there are at least two alleles at a locus having frequencies greater than 0.01 o If an allele has a frequency greater than 0.01, we can safely assume that this relatively high frequency is caused by factors other than mutation Natural Selection - Natural selection filters genetic variation; it does not create new genetic variation (only mutations can do that), but it can change the relative frequencies of different alleles - Fitness – An organism’s probability of survival and reproduction - Fitness refers to the proportion of individuals with a given genotype who survive and reproduce Examples of How Natural Selection Works - The assumption of no selection means that each genotype has the same chance of surviving and reproducing - Having fitness of AA = 100%, Aa = 100% and aa = 50%; individuals with the recessive homozygote (aa), have only a 50% chance of survival - Over time, the frequency of the a allele is reduced and will ultimately approach zero o Note: actual amount of selection slows down over time; an increasingly lower percentage of the population will be recessive homozygotes; consequently, fewer will be eliminated every generation - By having two copies of a recessive allele leading to genetic diseases that reduces fitness o Example: Tay-Sachs disease, an affliction caused by a metabolic disorder that results in blindness, mental retardation, and destruction of the nervous system o Children with Tay-Sachs disease generally die within the first few years of life o Heterozygotes carry the allele but do not show any major biological impairment o The fitness of individuals with Tay-Sachs diseases is zero - Natural selection can act o
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