Why has evolution not ceased

Regardless of the shape and arrangement of adaptive peaks, natural selection will operate to move a population up the slope of an adaptive peak, from lower degrees of adaptation to higher degrees of adaptation. Sooner or later, every evolving animal or plant (if evolving via natural selection) should wind up on top of a local adaptive peak. Once a species reaches the top of an adaptive peak, stabilizing selection should operate to keep that species in that position in the adaptive landscape. Evolution should cease.

Evolution has clearly not ceased (we, the species Homo sapiens, are a mere 200,000 years old). Yet we know that life has existed on Earth for at least three and one-half thousand million years, and that surely should have been enough time for all life to have reached all possible adaptive peaks, or not? Why does life continue to evolve new forms?

If we examine this question using the concept of the adaptive landscape we can quickly see that there are two possible causes of continued evolution: the first is the possibility that life might be able to overcome the effect of stabilizing selection by jumping directly from one adaptive peak to another without going downslope into the adaptive valley between them. Evolution via jumping from one peak to another is an interesting concept, the consequences of which the adaptive landscape concept allows us to quickly visualize. In Figure 2.14 we see a variety of adaptive peaks, some low, some high. The higher the peak, the deeper the valley produced by their longer adaptive slopes. The distance within the landscape between the higher peaks is thus greater than the distance between the lower peaks, and the depth of the valley between the higher peaks is deeper than the depth between the lower peaks. A jump from

Morphological Trait

Figure 2.14. Modelling evolution via peak jumping in an adaptive landscape. The higher the peak, the deeper the valley between the peaks. A jump off a high peak is much more likely to result in a large drop in adaptive value of the new mutant morphology than jumps off low peaks surrounded by shallow adaptive valleys. In such a landscape, random short jumps off low peaks are much more likely to be successful than random long jumps off high peaks. Thus the adaptive landscape concept predicts that evolution by peak jumping should occur in organisms that are not highly adapted, in organisms that are generalists in their environments rather than highly adapted specialists.

Morphological Trait

Figure 2.14. Modelling evolution via peak jumping in an adaptive landscape. The higher the peak, the deeper the valley between the peaks. A jump off a high peak is much more likely to result in a large drop in adaptive value of the new mutant morphology than jumps off low peaks surrounded by shallow adaptive valleys. In such a landscape, random short jumps off low peaks are much more likely to be successful than random long jumps off high peaks. Thus the adaptive landscape concept predicts that evolution by peak jumping should occur in organisms that are not highly adapted, in organisms that are generalists in their environments rather than highly adapted specialists.

one high peak to another high peak thus requires a long distance to be covered, whereas the closer proximity of the smaller peaks requires only a short jump to go from one to another.

In such a landscape (Fig. 2.14) we can predict that random short jumps off low peaks are more likely to reach an adjacent peak successfully than random long jumps off high peaks. Such a conclusion matches the empirical observation of geneticists that larger random mutations are more likely to be lethal than smaller mutations. That is, if you make a long jump off a tall peak in the landscape, the most probable consequence is a drop in the degree of adaptation, a long fall into the valley. Thus the adaptive landscape concept would predict that, if evolution by peak jumping occurs, then it should occur in organisms that are not highly adapted, in organisms that are generalists in their environments rather than highly adapted specialists.

The second possibility is that the adaptive peaks themselves are not stable in time. The morphology that has a high degree of adaptation today may not have that same high degree of adaptation tomorrow;

that is, the position of the adaptive peak in the landscape has moved, or the peak itself has vanished entirely, leaving behind only a flat nonadaptive plain.

The position of an adaptive peak in the landscape is a function of environmental and ecological factors, of abiotic and biotic conditions. Clearly, those conditions may change with time. In fact, the longer the period of time that elapses, the less likely that the environmental and ecological conditions that were present at the beginning of that period of time will still be present at the end of that period of time. What is now fertile farmland and forest in parts of northern Europe and North America was frozen tundra just eleven to twelve thousand years ago, and the varied habitats of most of present day Canada, Norway and Sweden did not exist at all, as those regions were covered by immense ice caps. Times change.

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