Adaptive Radiation

Adaptive radiation is the evolutionary process by which new species, all diverging from a single ancestral form, become adapted to a range of newly available habitats or resources, typically over a comparatively short evolutionary time span. Darwin's finches on the Galapagos Islands are the textbook example: a single ancestral finch species arrived on the islands roughly two million years ago, and its descendants have since diversified into 14 recognised species differing in body size, beak shape and feeding behaviour (Fig. 6.5), exploiting food resources as varied as insects, seeds, cactus nectar and even blood from iguanas. This diversification is linked to genetic variation in the ALX1 gene — even a mild mutation in ALX1 produces a measurable change in beak shape, giving natural selection raw material to act on as different beak shapes proved advantageous for different food sources. A second, independent example is the parallel radiation of marsupial mammals in Australia and placental mammals in North America: separated from a common ancestor for over 100 million years and evolving on different continents, the two lineages nonetheless produced strikingly similar-looking species occupying similar ecological roles (similar locomotion, feeding and foraging strategies), while retaining very different modes of reproduction. Over 200 marsupial species live in Australia today, having radiated to fill the continent's …