Natural Selection

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© Sewall Wright, "The Role of Mutation, Inbreeding, Crossbreeding, and Selection in Evolution," Sixth International Congress of Genetics, Brooklyn, NY: Brooklyn Botanical Garden, 1932.

Sewall Wright, in this 1932 drawing, considered the path different populations might take across the fitness landscape through random mutations and selection pressure. For effective evolution to take place, the population must be able to move from a lower peak to a higher one by moving across a saddle where the level of fitness is lower. In each case, the dashed line represents the initial population sitting on a peak (not necessarily the highest) in the fitness landscape. The shaded area is where the population ends up through natural selection. The top row shows the fate of populations so large that individual mutations have little effect on the whole. In the upper left, the mutation rate is high and the selection pressure is weak, so the population meanders around the fitness hilltop. In the top center, the population concentrates at the fitness peak after living under unchanging conditions for a long time with either a low mutation rate or high selection pressure. In the top right, the environment suddenly changes and the population slowly works its way to a new fitness peak. The lower row shows the fate of smaller populations. In the lower left, a population small enough to be affected by virtually every mutation meanders across the landscape in random steps. In the bottom center, the small population is affected nearly equally by mutations and selection pressure, and easily escapes small peaks while moving very slowly up higher ones. The lower right shows the conditions Wright considered optimal: many small, isolated populations that occasionally interact. These groups can change quickly, but are spared the uncontrolled random wandering of a single small group by virtue of their interaction with one another. (Unit: 9)