How fast is evolution?

This question came up during a class I took during my masters studies at the ETH Zurich. What is the maximum speed of evolution? On what factors does it depend? We discussed the idea that selection curbs the speed of evolution, acting as a brake by removing some phenotypes from the population and preventing evolution from continuing down those trajectories.

In that case, evolution without brakes means evolution in the absence of selection. All individuals reproduce in a statistically equal way, and individuals carrying mutations that in the real world would be lethal have just as many offspring as anyone else.

If we think of evolution in terms of travel through sequence space, then we can ask what affects the speed of that travel. Imagine that we start with a single reproducing genotype GREEN of length K in an environment that imposes no restrictions on population size or anything else.

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Over time, the boundary representing which genotypes appear in our population moves through K-dimensional sequence space in an ever-expanding cloud. How many steps (mutations) along a given trajectory can you get in a generation? By how many steps does the overall boundary move away from the origin sequence in a generation?

Now imagine that we have a target genotype PURPL (represented by the purple dot in the figure below) that we are trying to reach from the original genotype GREEN. If we re-introduce selection or the lethality of some mutations, it makes some regions of sequence space inaccessable and impossible to travel through. In theory, this may not affect the speed with which evolutionary processes (i.e. uncontrolled and unlimited drift) can “create” sequence PURPL from sequence GREEN.

Thinking of this sequence space in terms of a fitness landscape can remind us that there may be many different evolutionary trajectories, or sequences of mutations, that can lead from sequence GREEN to sequence PURPL. We might consider the trajectory represented by the smallest number of mutations to be the “fastest.”

Given the example fitness landscape represented in the figure below, we can see how, when selection is included in the system, some sequences may be considered nonviable. This can prevent evolution from taking the shortest (and therefore fastest) path from GREEN to PURPL, therefore increasing the minimum amount of time needed before PURPL can appear in the population and slowing down the process of evolution.

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