Rates Of Speciation

Speciation sometimes requires millions of years. But apparently some species can form more rapidly. For example, Polynesians introduced banana trees to the Hawaiian Islands about a thousand years ago. Today, there are several species of moths that are unique to the Hawaiian Islands and that feed only on bananas. These species likely descended from ancestral moths during the past thousand years, since bananas were introduced to Hawaii.

The idea that speciation occurs at a regular, gradual rate is called gradualism. However, some scientists think that speciation happens in "bursts" relative to the geologic time scale. The fossil record holds evidence that many species existed without change for long periods of time, whereas in some cases a great diversity of new forms seems to have evolved rapidly. That is, change occurred in a few thousand, rather than a few million, years. Scientists call this pattern of species formation punctuated equilibrium. The term punctuated refers to sudden, rapid change, and equilibrium refers to periods of little change. Figure 16-14 illustrates these two contrasting models as they might apply to the evolution of snakes.

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