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Lecture 17

Biology 2581B Lecture 17: Mechanisms of Genome Evolution

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Biology 2581B
L.Graham Smith

Lecture 17: Mechanisms of Genome Evolution - What do we talk about when we talk about genome evolution? o Differences in genome architecture: size, structure, content, modifications ▪ There’s huge diversity in genome architecture: within cells, within lineages, and between different lineages – how do we explain this diversity? o Forces responsible for differences - Think about genome evolution at a molecular level: the level at which mutations occur in genes - The only way to get different genome architecture is through mutation o Different levels of mutations: type, context, frequency, bias 1) Type - Includes point mutations: T-A site changes to C-G site - Deletion, insertion, large insertions/deletions, duplications, rearrangements, fragmentation, fusion, conversion (one sequence basically copies itself onto another sequence) o Large insertions can occur through HGT, mobile elements, endosymbiotic gene transfer 2) Context: where is it occurring? Is it occurring in non-coding DNA? If so, is it regulatory? - Is it happening to a region, or to the whole genome? - Is it impacting the whole cell? 3) Frequency: does it happen often or rarely? - Often you get a system where you have many point mutations, and very few fragmentation events, or vice versa 4) Bias - One type of point mutation could be more frequent than another type – mutational spectrum Mutations alter genomes - Mutations are a reflection of the environment the cell lives in - The actual cellular machinery that we have could alter genomes as well - Some organisms have really good DNA maintenance machinery – this is pretty rare o This means you hardly get any mutations (e.g. plant mitochondrial genomes) - Sometimes you have a really crappy DNA maintenance machinery – more common o Doesn’t work well, always inserting mutations (e.g. animal mitochondrial genomes) Thinking about evolution - Evolution is a population-level process - The little white things in the image are algae - Let’s add a mutation into the population, so we’ve changed the genome of one of these organisms - When this happens, we could get one of two outcomes - Over time, this change will either get fixed in the population, or lost - What determines whether a mutation will get fixed or lost? - First question: is the mutation beneficial, deleterious, or neutral? o If beneficial, you’d want to have it fixed - Next question: is this population effectively large or small? o “effectively” because you can have huge populations that behave like small populations o what determines this is the probability that a member can pass on its genes to its offspring o If population is selectively large, natural selection is efficient - If you have many competitors, a little mutation that’s beneficial can give you an advantage - In tiny populations, natural selection sucks o E.g. remember the woolly mammoth example,
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