形态比率

Speciation does not always happen at the same speed. Scientists describe two patterns of how evolutionary change happens during speciation: gradualism and punctuated equilibrium.

Imagine a bird species living on an island with varied food sources.

As natural selection favors birds with beak shapes better suited to available food sources, beak shapes change slightly over time.

After many thousands, or even millions, of years, these tiny changes add up, and a new species forms.

This process describes gradualism, in which populations change in small steps over many generations.

Now consider a sudden event, such as a storm, that carries a small group of these birds to a new island.

The available food is limited and different from before, which makes survival more difficult.

Birds with beaks that are better suited to the new food survive and reproduce more, so the population’s beak shape changes.

A new species forms over a relatively short period and then remains stable for a long time, sometimes for thousands or even millions of years, until another rapid change is triggered.

This pattern describes punctuated equilibrium, with brief bursts of speciation followed by long periods of stability.