Evolution |
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Evolution in Action |
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Speciation |
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| Speciation | Darwin, and modern Darwinists, conceived of gradual change as taking millions and millions of years to bring about evolution of one species into another. Phyletic gradualism brings about a very slow, steady branching and slow divergence of two populations, as modification of one or both branches results in two new species. |
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| Figure legend: |
Phyletic Gradualism. Gradual changes in the gene pool slowly accumulate over long periods of time. Slowly species A changes into species B.
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However, the discovery of the founder effect, and the rapid changes that can come about in small populations suggests a different mechanism. A new model, proposed by Niles Eldridge and Stephen Jay Gould in 1972, envisions sudden sharp branches in the evolutionary tree followed by long periods of stasis and no change. They termed this model punctuated equilibrium. Eldridge and Gould suggested that small populations become isolated from large populations and new adaptations arise very rapidly in these new, smaller groups. This speciation event is rapid in geological time and produces a sudden, dramatic burst of change that quickly originates brand new species. But once a new species is formed and the population grows to a large size, the dampening effect of the huge gene pool slows down further change and makes it difficult for any subsequent modifications to take place. Such species remain virtually the same for the rest of their existence, or until they become extinct. In both the phyletic gradualism and punctuated equilibrium models, notice that: Speciation is a branching mechanism through time. |
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| Figure legend: |
Punctuated Equilibrium. After long periods with little or no change, an isolation event separates a small population of individuals. With a different gene pool or a burst of mutations, this isolated group rapidly eveolves into a separate species.
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BIOdotEDU © 2001, Professor John Blamire |
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