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Lecture 4

ANP202 Lecture 4-2

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ANP 201
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Lecture 4-2: Evolutionary Forces 4 forces of evolution 1. Mutation *A random change in a gene or chromosome -Random change in the structure/amount -Occur at different levels- genes, chromosomes *Most mutations occur spontaneously but mutations can be induced (e.g. radiation) *Most mutations are silent (synonymous), but some mutations produce phenotypic effects (non-synonymous) -Effects of mutation may be advantageous, deleterious, or neutral *Different types of mutation -Point mutations -Frameshift mutations -Mutations due to transposable elements -Mutations due to non-disjunctions *Point mutations: replacements of a single nitrogen base with another base -Synonymous mutation: creates a triplet that codes for the same amino acid as that of the original triplet, e.g. UCU &UUC both code for seline -Non-synonymous mutation: creates a triplet that codes for a different amino acid, e.g. Chromosome 11, CTC=glutamate- normal hemoglobin, CAC=valine- sickle cell trait *Frameshift mutations: the change in a gene due to the insertion or deletion of one or more nitrogen bases -causes the triplets to be rearranged and the codons of mRNA to be read incorrectly during translation, e.g. base insertion A, TAA-Isoleucine, ATAA- Tyrosine *Transposable elements: mobile pieces of DNA that can copy themselves into entirely new areas of the chromosomes -These elements can cause deletion, insertion, and translocations -If occurs in non-coding regions, the effect may be minor or none -If occurs in coding regions significant alteration can result *Non-disjunctions: can contribute to irregular number of chromosomes in a cell -Monosomy (1) -Trisomy (3) e.g. Down syndrome, Klinefelter syndrome *Mutation is the only source of new genetic information -the ultimate source of all variation -the ultimate cause of evolution -tis is because all other evolutionary forces act on the variation originating from mutations 2. Natural selection *The process by which individuals with features that are adaptive in the environment, preferentially survive and reproduce, thereby increasing the frequency of alleles contributing to those features in the population *Produces systematic effects *Different types -Positive selection -Balanced polymorphism *Positive Selection: selection in which advantageous genetic variants quickly increase in frequency in a population, e.g. Peppered moth in industrialization *Balanced Polymorphism: type of selection that maintains polymorphism -Both advantageous and disadvantageous variants persist in a population if heterozygotes have selective advantage -E.g. hemoglobin variants (sickle cell, G6PD, thalassemia, etc.) in Malaria endemic regions -Example: natural selection that maintains polymorphism in hemoglobin, 2 phenotypes: sickle cell & normal cell in Malaria endemic regions -Other hemoglobin and enzyme abnormalities are also linked to balanced polymorphism: -Thalessemia -G6pd -Both of these show relationships to malaria, similar to those of the sickle-cell gene *Patterns of natural selection -Directional selection: selection for one extreme of the phenotypic distribution, e.g. larger brains, peppered moth -Stabilizing selection: selection against the extremes of the phenotypic distribution, e.g. birth weights
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