A Series of Fortunate Events by Sean B. Carroll

Author:Sean B. Carroll
Language: eng
Format: epub, pdf
Publisher: Princeton University Press
Published: 2020-08-03T00:00:00+00:00

CLIMBING STEPS

In the woolly mammoth, researchers found that at least two of the three changes in the mammoth hemoglobin independently affect oxygen delivery. It is reasonable to infer that each of these changes provided some incremental benefit to mammoths in improving their performance in the cold and expanding their range. But given the improbability of two mutations happening at once, how could the two mutations (or three) wind up in all woolly mammoths?

The process is stepwise, not instantaneous.

If one mutation provides a significant performance advantage, then over time it can increase in frequency in a population by increasing the reproductive output and survival of those that inherit it versus those that don’t. This is the competitive process of natural selection. For example, if a mutation confers just a 3 percent advantage, meaning that those with it produce 103 viable offspring for every 100 produced by those lacking it, the mutation can become ubiquitous in a large population in fewer than 1,000 generations.

If and when a second beneficial mutation occurs, then it will arise in an animal that already possesses the first mutation and can subsequently spread as well. This is the process of cumulative natural selection. It works like climbing a set of stairs, with new mutations providing each new step up (the rise in the stair), and natural selection providing each step forward by spreading the mutation in a population (the run of the stair) before the next step up occurs (Figure 5.7). The process takes a long time in slowly-reproducing creatures, but we can see this happen in real time in rapidly multiplying creatures like microbes and viruses, and cells in our own bodies. Take a good look at Figure 5.7; you are going to see this set of stairs again.

FIGURE 5.7. The staircase of evolution

Multiple changes can occur in a gene through a cumulative process. New mutations provide each stepwise change in a gene (the rise in the stair) while natural selection spreads each mutation in a population (the run of each stair). In this manner, multiple changes can accumulate in a gene. Illustration by Kate Baldwin.

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