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
- 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
- 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,