Lecture 3 Monday October 3 2011 Response to the Origin Wide acceptance of the fact of evolution The publication of the Origin led to a scientific revolution Most scientists quickly accepted Darwin s claim that evolution had occurred Disagreements about the pattern and mechanism There was and still is disagreement about the pattern and mechanism proposed by Darwin Many of Darwin s supporters did not agree with Darwin s claim that the pattern was always gradual They thought that discontinuous changes also occurred They argued that the lack of intermediate forms was also consistent with discontinuous pattern of change Part of the attraction of discontinuous change is that it is not necessary to explain what advantages intermediate forms had For some characteristics such as the wing or a bird or pterosaur it is difficult to know what advantage was conferred to an individual with a partially formed wing that is incapable of generating flight Some supporters of Darwin s theory thought that natural selection was not sufficient to cause all of evolution Everyone agreed that that natural selection could cause changes in the way described in the Origin The question was whether all changes in the history of life were caused by natural selection or whether additional mechanisms were needed Mendel Darwin Darwin did not have a convincing explanation for inheritance The theory of natural selection was criticized because Darwin could not explain why differences among individuals persisted and how those differences are inherited It appeared that natural selection would eliminate variation within species by eliminating less fit individuals In later editions of the Origin Darwin changed his discussion of inheritance but he never effectively resolved this problem Mendelian inheritance was rediscovered in 1900 Mendel in 1865 discovered what is now called Mendelian inheritance by studying characters in the common garden pea His work was ignored at the time but his achievements were later recognized when the same rules were rediscovered in 1900 and soon found to apply to a wide variety of plant and animal species including humans By the 1920s there was general agreement that all of biological inheritance was attributable to Mendelian genes The addition of Mendelian inheritance to Darwin s theory of natural selection led to what is called the modern synthesis or neo Darwinism Population Genetics Population genetics is the study of Mendialian genes in populations Population genetics was developed in part to show that Darwin s theory of natural selection acting on Mendelian genes can cause evolution to occur in the way that Darwin described in the Origin What is a population In theory a population is a group of individuals of the same species that can freely interbreed and that is partly or wholly isolated from other populations of the same species In practice a population is often the group available for study or the group for which one has some data A population is the basic unit of evolution A species is made up of at least one and usually more than one population Genotype phenotype and allele frequencies Calculating genotype frequencies and allele frequencies requires simple arithmetic In the example of flower color given in lecture the frequency of CR is 0 8 or 80 Phenotype frequencies are derived from the genotype frequencies and dominance relationships of alleles The essence of neo Darwinism is that evolution results from changes in allele frequencies caused by natural selection and other factors Species evolve because allele frequencies change Hardy and Weinberg The Hardy Weinberg H W frequencies Soon after the rediscovery of Mendelism G H Hardy and Wilhelm Weinberg independently predicted the allele and genotype frequencies in a group of offspring given allele frequencies in their parents They demonstrated that if the parents choose mates independently of genotype randomly mate then the fractions of the genotypes of the offspring are p2 2pq q2 where p and q are the frequencies of the two alleles p q 1 These are the Hardy Weinberg H W frequencies Note that the H W frequencies do not depend on the genotype frequencies in the parents only on the allele frequencies Implication of the Hardy Weinberg formula for evolution Allele frequencies do not change as a result of random mating which means that genetic variation is not lost because of random mating In later generations allele and genotype frequencies remain the same Consequently if allele and genotype frequencies do change from one generation to the next it is because some other force causes them to change Implication of the Hardy Weinberg formula for human genetics Genotypes in human populations and in most species that do not self fertilize are usually very close to their H W frequencies H W frequencies are the basis for genetic fingerprinting which is used to establish that a suspect is the source of a biological sample blood skin etc found at a crime scene As a simplified example suppose that a crime scene sample is found to be homozygous for an allele that is in frequency 0 01 in the general population and a person suspected of the crime is also homozygous for that allele The implication is that the suspect is the source of the sample because the chance that a randomly chosen individual left that sample is only 1 10 000 At present 13 genetic loci called the Combined DNA Index System CODIS loci are used to establish genetic identity CODIS loci are thought to have no effect on phenotype State and federal databases currently contain records of more than 4 6 million genotypes If an allele is in low frequency the frequency of heterozygous individuals 2pq is much larger than the frequency of homozygous individuals q2 for that allele Alleles that cause recessive genetic diseases are in low frequency Consequently there are many more heterozygous carriers of those alleles than individuals with that disease For example the frequency of an allele F508 that causes cystic fibrosis CF is about 2 in European populations That means about 4 of the European population are carriers but only 0 04 1 2500 individuals are born with CF Note that it does not matter whether the frequency of F508 is called p or q H W frequencies are relevant for ethical decisions An allele of the APOE gene is associated with elevated risk of Alzheimer s disease AD Individuals homozygous for the 4 allele called APOE 4 have a risk 15 times higher than the average risk and heterozygous
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