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Lecture 15

lecture 15 notes from prof.Barrett

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Spencer Barrett

Lecture 15: NeoDarwinism & the evolutionary significance of genetic variation Recall: Requirement for Darwin’s theory Ÿ Variation: variation among individuals in a population Ÿ Heredity: progeny resemble their parents more than related individuals Ÿ Selection: some forms between at surviving and breeding in a given environment Basic terms used in genetics Ÿ Genotypes: genetic constitution of an organism- used in relation to a particular gene or gene combination (ex. AA. aA) Ÿ Phenotype: the organism as observed- used when discussing a trait of a feature of an organism that varies Ÿ Genome: the entire organism’s DNA including both genes and non-coding regions What is a gene? Ÿ The functional unit of inheritance Ÿ A unit of hereditary information located on the chromosomes consisting of DNA Ÿ A DNA sequence composed of codons essential for a specific biological function Ÿ DNAàRNAàprotein synthesis Where does genetic variation come from? Ÿ Mutation Ÿ Recombination Ÿ Gene flowànew members coming into the population Ÿ Hybridizationàmixing between species Independent assortment & Recombination Ÿ Independent assortment & recombination during meiosis generated enormous diversity (in 23 humans with n=23 chromosomes, 2 possible gamete combination) Ÿ Most genetic variability in a population results from sexual reproduction; in any given generation input from mutation very small. Mutation in fruit fly Single recessiveàwhite eye color Sickle-cell anemia “Jumping genes” or mobile (transposable) genetic elements in corn. Arise by mutation and can move around the genome. Petal color mutant in daisy (3 scenarios) Ÿ Pollinators not visit it, this mutation will have no fitness Ÿ Maybe there is another mutant, replace white by yellow. Ÿ The two forms coexisting, both the yellow and white. Mutation-ultimate source of genetic variation Ÿ Stable change in DNA sequence resulting in a change of genotype Ÿ Occurs at a very low but variable rate in all organisms Ÿ Effects: neutral (no effect), deleterious (although it does kills you, but will cause 0 fitness), lethal (YOU ARE DEAD), beneficial; in many cases their fitness effects depend on environment. Ÿ To be important for evolution, must occur in germ cells (sperm/egg)-somatic mutations NOT inherited Characteristics of mutation Ÿ Mutation is an unstoppable phenomenon ○ Despite cellular mechanisms to correct errors during DNA replication Ÿ Mutation is not directed by the organism or the environment ○ Random with respect to effects on fitness genes Ÿ Rates depends on the type of mutation ○ Also varies among genes Ÿ Environmental insults can affect mutation rate ○ Mutagens, high temperature Types of mutation Ÿ Point mutations ○ ATGCAGTàATCCAGT Ÿ Insertions/deletion (including “jumping genes”) ○ ATGCAGTà ATGGCAGT Ÿ Changes in repeat number ○ ATGATGATGATGà ATGATGATGATGATG Ÿ Chromosomal rearrangements (mostly lethal) The distribution of fitness effects o
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