Biology 1B Evolution Lecture 3 March 1 2010 Neodarwinian Synthesis After On the Origin When Darwin and Wallace jointly published their paper on natural selection they began a new era of evolutionary study While their work was important they were still wrong about inheritance a process which they believed was the result of blending of traits Mendel s Principles Alternative forms of genes known as alleles account for variation Offspring individuals inherit two copies of parents DNA in most cases these are known as diploid organisms If the possible alleles of a gene differ one may be dominant meaning that it would mask the phenotypic expression of the other allele Two alleles for a heritable trait segregate during meiosis usually independently of other traits except for case of linked genes which are close together on the chromosome Dominant alleles mask all other phenotypes known as recessive but there is also the case of co dominance think of one red and one white flower making a pink flower or look at the examples below Moritz Lecture 3 Notes page 1 th Figure 14 10 pg 272 8th edition Figure 14 5 pg 266 8 edition Alleles A Genotypes Possible Phenotypes AA Dominant AB Codominant Dominant B BB Recessive In this case AA and BB are known as homozygous same allele type and AB is known as heterozygous Examples of recessive traits in humans Albinism cystic fibrosis Examples of dominant traits in humans achondroplasia one form of Dwarfism Huntington s chorea Example of codominant trait in humans Sick cell anemia Population Genetics A population in genetic terms is a randomly breeding group of individuals that is largely isolated from others Key evolutionary processes mutation the only source of variation sampling processes also known as genetic drift the various forms of natural selection exchange of genes through migration and non random mating Moritz Lecture 3 Notes page 2 Mathematical Models Hardy Weinberg Equilibrium Example of a natural population of flowers Phenotype Frequency Red Flowers 320 Pink Flowers 160 White Flowers 20 Genotype Frequency CRCR 0 64 CRCW 0 32 W W C C 0 04 Allele Frequency p f CR 0 8 q f CW 0 2 Figure 23 7 pg 474 8th edition Now the general case Expected proportions p2 2pq q2 1 Also p q 1 because there are only two possible alleles f A1 p2 1 2 2pq p p q p meaning that the next generation will in theory have the same gene frequency as that of the parents Moritz Lecture 3 Notes page 3 Conclusions from Hardy Weinberg math Inheritance alone does not cause the frequency of alleles to change between generations better known as evolution This is because Hardy Weinberg acts on these assumptions Random mating only for this gene trait No mutation or selection on population in question This is an isolated population with no gene flow from outside i e no migration This is only true for a large population with no sampling error Based on these assumptions we can call Hardy Weinberg a null hypothesis for evolution That means that if a population does not conform to Hardy Weinberg Equilibrium for a certain trait then evolution has occurred Example of HWE as a Null Hypothesis Wild Oats Genotype Frequency Note that there are far fewer heterozygotes than HWE would 0 548 A1A1 predict Some explanations for this would be that the wild oats 0 071 A1A2 do not practice non random mating or that heterozygotes are 0 381 A2A2 selected against in the environment the oats inhabit Moritz Lecture 3 Notes page 4
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