BIOL 152 1st Edition Lecture 3 Outline of Last Lecture Intro to Evolution Outline of Current Lecture Evolution of Populations Microevolution change in genetic makeup of a population from generation to generation In the past the blending hypothesis was still widely accepted Mendel s hypothesis not published so the blending hypothesis was still popular Review some definitions Species a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature Population localized group of individuals belonging to the same species Natural selection acts on individuals but populations evolve Describing variation Polymorphism More than one readily noticeable morph in a population Average heterozygosity is a fraction of individuals in a population that are heterozygous for a particular locus Geographic clines is a gradual change of a trait across a geographic area between populations o The song sparrow Melospiza melodia 31 subspecies or races of these song sparrows Geographic cline affects body size plumage and song characteristics of birds o Genetic versus Environmental Factors Not all variation is due to genetic differences environment also has an effect on an individual s phenotypes Selection upon Variation Fitness the genetic contribution of an individual to the next generation relative to the contribution of others To be successful an individual must survive to reproductive age and successfully reproduce These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Survival of fittest is misleading Why Natural Selection cannot Fashion perfect Organisms 1 Can only act on existing variation 2 Limited by historical constraints 3 Adaptations are compromises 4 Chance natural selection and environment interact How does natural selection change populations Certain phenotypes selected for or selected against Selection alters the frequency of alleles o When only one extreme of alleles survive Directional o Centered around a mean where only the intermediates survive Stabilizing o When the intermediates survive and the extreme alleles can t successfully reproduce Disruptive Sexual Selection Individuals in a population typically males differ in their ability to attract mates Non random mating Sexual dimorphism Intrasexual direct competition of males for female Intersexual the female chooses the mate How is variation maintained The diploid status of chromosomes allow recessive alleles to persist in population Heterozygote advantage individuals who are heterozygous at a gene sometimes have higher fitness Sources of Variation Mutations only mechanism by which new alleles originate if mutations occur in the gametes it will be passed to the offspring Mutation rate varies between organisms Sexual Reproduction offspring with different combinations of alleles than parents shuffling of existing alleles Recombination Independent Assortment Fertilization Other Effects on Variation Genetic Drift when allele frequencies fluctuate randomly between generations Over time it typically diminishes in variation Especially true for small populations Genetic Drift a biological form of Sampling Error Sampling Error refers to chance events random departures from expectations based on underlying probabilities Chance events are more likely in small samples than in large samples Thought experiment One student flips a coin 6 times expectation is 3 heads 3 tails 5 H 5T One student flips a coin 60 000 times same expectation of 5H 5T If you had to bet on which student had results closer to expected probability which one would you bet on 2 types of Genetic Drift Bottleneck Effect Founder Effect Consequence the new population is not represented of the original Gene flow is the allele movement between populations over time gene flow reduces differences between populations Why is genetic variation important Populations with heritable variation i e genetic diversity have the capacity to adapt to the changing environment Populations with little or no genetic diversity have little or no capacity to adapt What are 4 processes that change the allele frequencies in a population 1 Mutation 2 Gene Flow brings new alleles or changes existing alleles 3 Genetic Drift affects small populations frequencies change randomly 4 Selection Analyzing change in allele frequencies Common powerful approach to studying how natural selection affects populations Develop mathematical models Collect data from real populations to test the predictions of the model Model developed by Hardy and Weinberg 1908 Predict future allele frequencies at a single locus when none of the four evolutionary processes are operating it is a null model Hardy Weinberg Principle Frequencies of alleles and genotypes in a population remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work Mathematically track the change in allele frequencies between generations We now focus on outcome of all mating events in a population rather than Mendel s mating between 2 parents All gametes are considered as one gene pool o Two gametes can be chosen at random from gene pool to represent an individual o We can calculate the genotypes in the offspring by considering what would happen with repeated sampling from the gene pool Defining the allele frequencies p frequency of allele CR q frequency of allele CW The frequency of both alleles 1 p q 1 If p 0 8 then q 0 2 What is frequency of CRCR CRCW CWCW The frequency of CRCR genotype is p2 The frequency of CRCW genotype is 2pq The frequency of CWCW genotype is q2 p2 2pq q2 1 Used to predict the allele frequencies in subsequent populations and assess whether allele frequencies are changing i e evolution is occuring p2 0 8 0 8 0 64 2pq 2 0 8 0 2 2 0 16 0 32 q2 0 2 0 2 0 04 0 64 0 16 0 80 CR 0 4 0 16 0 20 CW A change in allele frequencies evolution The Hardy Weinberg Principle p2 2pq q2 1 If frequencies of A1 and A2 are given as p and q then the frequencies of each genotype will be p 2 2pq and q2 for every successive generation ONLY WHEN certain conditions apply Hardy Weinberg Assumptions 1 No natural selection 2 No gene flow 3 Random mating 4 No mutations 5 Large populations Therefore if alleles in a population do NOT follow the Hardy Weinberg principle one of these assumptions must be false i e Sickle Cell heterozygote advantage Phenylketonuria Current