Evolution, 4th Edition

178 CHAPTER 7

vast majority of alleles fall into one of two categories. Some alleles are strongly

selected (s >> 1/Ne), and drift has little effect on their evolution. Other alleles are

nearly neutral (s << 1/Ne), and drift dominates selection in their evolution. These

rules explain the differences we see among the graphs of Figure 7.13. A critical

point is that it is the relative strengths of drift and selection that matter, not their

absolute strengths.

There is an interesting implication of these results: a mutation that is strongly

selected in one species can evolve in another as if it were selectively neutral (FIG-

URE 7.14). Some organisms have very large effective population sizes. Drosophila

melanogaster has an effective population size of about 1 million individuals, and the

populations of some bacteria (like the E. coli that live in our guts) are bigger still (see

Figure 7.11). For these species, selection is much more powerful than drift for alleles

that have fitness effect as small as s = 10–5. These species have features that reflect

exceedingly precise adaptation. One is codon bias. Earlier, we said that synonymous

mutations are selectively neutral. In fact, different codons that code for the same

amino acid can have minute differences in fitness because they affect how accu-

rately and efficiently a gene is transcribed and translated [12]. The genomes of spe-

cies with very large Ne tend to be biased toward the codons that are most efficient.

Adaptation is less precise in species with smaller population sizes. Gray whales

have an effective population size in the tens of thousands (see Figure 7.11). Mutations

with fitness effects of s = 10–5 evolve in that species as if they were selectively neutral.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_07.13.ai Date 11-14-2016 01-18-17

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FIGURE 7.13 Simulations of the spread of

beneficial mutations in populations with

different effective sizes. The mutations start

at a frequency of p = 0.25 and have a se-

lective advantage of s = 0.01. Each graph

shows five replicate populations. With

Ne = 500,000, allele frequencies follow

trajectories like those seen for an infinite

population in Figure 5.7. As Ne becomes

smaller, the effects of drift become stron-

ger. When the population size is so small

that 1/Ne is much less than s, there is a high

probability that the beneficial allele will be

lost by drift, as seen in the graphs with

Ne = 50 and Ne = 5.

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