Darwin’s theory of evolution is based on the fact that all the different types of species of plants and animals found today are the descendants of common ancestors. Gradual variation among them occured due to changes in organism-environment relationship in both time and space. The organism- environment relationship is different for different populations at different places and time. If a homogeneous population living in a particular environment migrates to a new habitat the organism- environment relationship may change. Depending upon the organism-environmental relationship the nature may select out those combination of genes which are better adapted under the given circumstances. In long course of time a few members of the population may show distinct adaptation which is marked by variation of characters. 

The environment selects out the better gene combination rom the existing gene pool of the population. The newly adapted individual if become reproductively isolated from its parent population, it establishes itself as a new species and breeds among the newly adapted individual. Such a population which is different, even marginally, from its parent population may migrate to new habitat. If the migrated species overcome competition and adapt to a new geographic area and evolve through further specification, the evolutionary line radiating from the initial population to new geographic area is said to have undergone adaptive radiation. 

The adaptive radiation from initial stock first gave rise to races or varieties which had attended the level of species through further adaptation and reproductive isolation. The following two examples will illustrate the concept. 

Darwin’s finches are examples of adaptive radiation from a common ancestor. Galapagos islands are situated on the equator some 900 km. west of Ecuador. These islands were originated form volcanic eruption which were later occupied by plants and animals. A few plants and birds originally entered into the islands and later being separated from the parent stock evolved through selection by nature. Darwin had noticed that there were 13 different species of finches Fish were similar to the species in the mainland. The original species migrated to the new lands which provided greatly relaxed selection pressure because of the absence of predators and competitors. 

The relative isolation of the islànds permitted the establishment of unique hind races or subspecies. From these had arisen the 13 different species. These species had given rise to more subspecies. But the fact that in the mainland there was only one species. So the 13 species had a common ancestor on the mainland. The marine species of giant iguana wizard similarly arose from terrestrial species to avoid competition on land. The finches also to avoid competition developed different food habit and therefore they occupied different ecological places. Their beaks had evolved to suit different kinds of food. 

The homology of forelimbs of mammals illustrates common ancestry. The forelimbs of bat, hale, monkey, pig, horse, mole, ant eater conform to the basic pentadactyl pattern. They were probably originated from common ancestor and evolved in different lines depending upon the need in different ecological niches.