Adaptive radiation

On the Galapagos Islands, Darwin was struck by the many kinds of small black birds, now called Darwin's finches. They looked alike but had different beaks, each suited to a different food.

He concluded that they all came from one ancestral type: seed-eating finches that reached the islands. As they spread into different habitats, their beaks changed: some became insect-eaters, others ate different plant foods. One ancestor had produced many species, each fitted to its own way of life.

This process, where many species evolve from one ancestor and spread out (radiate) into different habitats within an area, is adaptive radiation.

Another striking example is the Australian marsupials (pouched mammals). Within that island continent, many different marsupials, from kangaroos and koalas to the marsupial mole and the Tasmanian wolf, evolved from one ancestral stock.

Australia also shows convergent evolution. Its marsupials look strikingly like placental mammals elsewhere that live similar lives: the Tasmanian wolf (a marsupial) and the placental wolf, the marsupial mole and the placental mole, the flying phalanger and the flying squirrel. They are not closely related; similar habitats have simply shaped them alike.

So the two ideas run in opposite directions: in adaptive radiation, one ancestor gives many different forms; in convergence, different ancestors give similar-looking forms.