Lecture 20 Population Genetics OBJECTIVES Explain Hardy Weinberg Principle Explain assumptions of Hardy Weinberg Equilibrium Apply HWE model to make predictions about allele and genotype frequencies in a population Describe and explain the evolutionary processes that can cause allele genotype frequencies to deviate from HWE Population Genetics study of the genetic variation within and among natural populations and the evolutionary processes that generate and maintain this variation FOCUS genotype and allele frequencies in population and how they change thru time Population a group of individuals of the same species living in the same area that are capable of interbreeding and producing fertile offspring Gene pool all the alleles for all genes in a certain population Genetic Variation within Natural populations levels of genetic variation are related to historical population sizes Humans vary by 0 1 in genomic sequence Genetic variation in natural populations of Eukaryotes stems from the interaction of two very Fruit flies in field differ by 1 in their genomic sequence different processes MUTATION and SEXUAL REPRODUCTION Mutation rare and random process resulting from errors in DNA replication and or cell division original source of new alleles new mutations are often slightly deleterious or neutral rarely advantageous deleterious alleles are typically rapidly purged individuals experience low fitness Sexual Reproduction most genetic variation arises from this Meiosis crossing over and independent assortment and random fertilization constantly reshuffle allelic combinations BUT CANNOT CHANGE the frequency of alleles in a population ONLY evolutionary processes change allele frequencies Measures of Population s Genetic Make up 1 Genotype Frequencies a Proportion of one particular genotype relative to total of individuals in the population 2 Allele Frequencies a Proportion of one particular allele relative to total of alleles NOTE IMPORTANT the shuffling of alleles due to meiosis and random fertilization of gametes has no effect on the overall gene pool of a population HINT meiosis and random fertilization are like shuffling a deck of cards proportion of hearts spades clubs and diamonds in deck does not change Population s gene pool will remain constant UNLESS evolutionary processes are acting on the population Hardy Weinberg Principle After 1 generation of random mating genotype frequencies for two alleles at a locus will be p2 2pq q2 p dominant q recessive p q 1 PREDICTION Genotype and allele frequencies will remain constant in succeeding generations as long as certain assumptions are met Assumptions of HW Principle 1 Random Mating no inbreeding 2 Infinitely large population 3 Closed population no immigration or emigration 4 Alleles are static no selection 5 Equal probabilities of reproduction among individuals 6 7 No mutation Locus is not sex linked Advantage of HW Principle for disorders like Phenylketonuria homozygous recessive if we know frequency of phenotype we can use equation to calculate frequency of heterozygous carriers Inbreeding mating between relatives reduces frequency of heterozygotes and increases frequency of homozygotes in each generation Self fertilization is most EXTREME form heterzygosity falls 50 each generation Inbreeding does change the allele frequencies in a population Inbreeding depression is decreased fitness that occurs as more deleterious recessive traits are expressed in the phenotypes of homozygous recessive individuals Evolutionary Processes Natural selection Sexual selection Genetic drift o Process by which individuals with certain heritable traits survive and reproduce at a higher rate compared to individuals lacking those traits o Process by which individuals that possess certain heritable traits are more successful at obtaining mates and thus reproduce at a higher rate than individuals that lack those traits o Process by which chance events lead to changes in allele frequencies in a population Particularly important in endangered species Very small populations are prone to experience significant loss of allelic diversity due to chance variation in reproductive success of individuals o Founder Effect occurs when a group starts a new population If founding group is small allele frequencies probably differ from source o Bottleneck Effect change in genetic diversity caused by catastrophic decrease in population size by chance some alleles may be lost completely Ex northern elephant seal population reduced to 20 individuals because of hunting for blubber Protected rebound population to 100 000 However LOW genetic diversity potentially more vulnerable to new pathogens Gene flow o Process by which alleles are transferred from one population to another o Due to migration of individuals gametes o Lots of gene flow between 2 populations makes them genetically more similar Mutation o Process by which new alleles arise in a population o Due to errors in DNA replication or cell division o Ultimate source of variation in populations Lecture 22 Evolutionary Perspective on Life Learning Objectives Explain differences between science and religion in terms of how they seek to understand the nature of reality Explain how the contributions of earlier and contemporary scientists impact Darwin s thinking about evolution Explain Darwin Wallace s two big ideas Describe how most contemporary biologists define biological evolution Explain the requirements for evolution by natural selection Evolution why the controversy Byfar the most controversial topic in US science education Why in US and not many other countries Study by Jon Miller found 3 factors most strongly linked to individuals who rejected evolution o A poor understanding of genetics o A literal interpretation of the bible o A conservative political ideaology A large majority of americans believe that humans are products of divine creation o Directly As envisaged by michaelangelo Indirectly intelligent design o 64 open to idea of teaching creationism together w evolution o 38 favor replacing evolution with creationism o NO substantive debate between scientists vast majority no desire to see aspects of religious thought incorporated into science curricula Science religion are DIFFERENT disciplines o very different approaches to seeking understanding Science is a Profoundly Material Discipline science is a formal process of generating new knowledge that uses evidence to construct testable explainations and