When an ancestral species stock develops rapidly into many different species filling different environmental niches, it's referred to as adaptive radiation. Genetic mutation, natural selection for the most advantageous traits, and the isolation of small groups of a population all play a key role in adaptive radiation.
The most commonly-used example of adaptive radiation and natural selection is Darwin's finches on the Galapagos Islands. The thirteen or fourteen different species of finch all rose from common ancestral stock from the South American mainland, 600 miles away. Without the population-reducing pressure of predators or disease, mutations in finches were more likely than usual to be advantageous, and finch adaptation into several different genii and species proceeded rapidly.
The three types of adaptive radiation are:
- General adaptation, in which a species change is advantageous for accessing previously-unreachable resources - a giraffe's long neck or bird's wings.
- Environmental change, in which species adapt in response to an environmental difference
- Archipelagoes, where colonization of isolated environments, like islands, allows rapid variation in species due to a decrease in environmental stresses.
Speciation in adaptive radiation tends to happen rapidly, followed by long periods of species stability.
Web Resources On Adaptive Radiation
Fact Monster: Adaptive Radiation
Book Resources On Adaptive Radiation
The Ecology of Adaptive Radiation by Dolph Schluter
Molecular Evolution and Adaptive Radiation by Givnish & Sytsma (Editors)