When an environment changes in a cyclic pattern (e.g., dry summers, then wet summers, for instance), the “more fit” organisms will shift back and forth. This can result in no net change in the organisms, or “frozen” evolution.
For instance, suppose that a few consecutive summers are very hot. Rabbits with shorter fur may survive better since they will stay cooler in these summers, and so suppose natural selection sifts out many of the long-haired rabbits and preserves the short-haired ones. Then suppose that a few consecutive winters then become very cold with more mild summers. Now, the rabbits with longer fur will likely be favored and preserved by natural selection. If this goes back and forth, the rabbits’ fur will go from longer to shorter, but the end result will be nothing. The rabbits as a whole will not change over the years, and many selection resources will be wasted.
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