The idea that mutation rates can evolve shatters the predisposition that mutations are entirely random. However, environmental factors can certainly act on the evolution of mutations and their frequency throughout a population. According to Lynch et al., the evolution of mutation rates depends on three main factors: first, the production rate of a deleterious mutation in a population, second, the reduction in fitness of each mutation, and third, the heritablity or persistence of that mutation through generations (2016). Consider a scenario where a natural disaster randomly wipes out a significant amount of a species' population. Through this event of genetic drift, we can imagine that the variation and mutation frequency in the species would be fairly high; thus, this has become a perfect population for selection to act on. As selection acts against the force of genetic drift, the mutation rate will decrease until an equilibrium is reached over time. This is displayed by the red and black lines in Figure 1 from the study by Lynch et al. (2016). The steep slope of the black line points towards a quickly changing mutation rate in the population while the green line depicts a relatively unchanging mutation rate.
A study on the Hsp90 protein in Drosophila was done by Rutherford and Lindquist (1998) to demonstrate other mechanisms for increasing variation under stressful conditions. Hsp90 is a heat shock protein that acts as a chaperone and regulates developmental processes under normal conditions. However, the study found that it masks its second function. Under heat stress conditions, the Hsp90 chaparone dissociates from it's normal protein complex and moves to stabilize other proteins that would have been denatured by the stressed environment. In conclusion, this study showed that changes in the environment, such as genetic drift, can cause mutators operate more quickly, or when change is necessary. My understanding is that overall, higher mutation rates allow for selection to act upon a population; thus, allowing for greater adaptive fitness in the population over time.
Lynch, Michael, et al. “Genetic Drift, Selection and the Evolution of the Mutation Rate.” Nature Reviews Genetics, vol. 17, no. 11, 14 Oct. 2016, pp. 704–714., doi:10.1038/nrg.2016.104.
Metzgar, David, and Christopher Wills. “Evidence for the Adaptive Evolution of Mutation Rates.” Cell, vol. 101, no. 6, 9 June 2000, pp. 581–584., doi:10.1016/s0092-8674(00)80869-7.
Rutherford, S., Lindquist, S. Hsp90 as a capacitor for morphological evolution. Nature 396, 336–342 (1998). https://doi.org/10.1038/24550
That's a great graph! It's a solid example for mutation rates!!
ReplyDeleteI like that you went and found a scientific study to help back up what you were saying and to help show other students an example of mutation rates evolving. It's always helpful to be able to see something, such as a graph, instead of just trying to picture what you're reading in your head.
ReplyDeleteHm, a correct conclusion but an interesting way of getting there! You're right; higher mutation rates frequently lead to strong selection, as selection acts on the suddenly-more-common mutations. But your opening sentence is confusing; is what way does the evolution of mutation rates "shatter" the idea that mutation is random?
ReplyDeleteRemember, random mutation just means that we cannot predict what will mutate, or when. It doesn't mean we can't pinpoint the *cause* of a mutation. Frequently, high rates of mutation are caused by alterations in the DNA transcription mechanisms. It just makes more random mistakes! So don't be confused by correlation and causation. Higher mutation rates are correlated with areas of frequent disturbance because higher mutation rates promote population survival - not because such places *cause* higher mutation rates.
If you're still confused, I encourage you to reach out to Michi! He's probably got a more succinct way of explaining this.
Thanks so much for your comment, Madison. Your explanation makes things a lot more clear. Also, after completing the recent R-exercises, I realized that I didn't quite have the right conceptual ideas about mutation rates. Your note really helps though!
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