Adaptive Radiations
The organisms all the time tend to establish harmony with their environment. This process, called adaptation, enables the organism to face the vagaries of the ever changing environment. Adaptation in fact sums up the whole result of evolution. You might have noticed that when organisms belonging to dissimilar groups come to occupy similar environment, they develop striking identities in structure and behaviour, giving a false impression of closer relationship. For instance, fishes and whales which belong actually to different classes are found in similar habitat that is they are both aquatic. They look so alike that for a layman both are 'fishes'. This condition where organisms belonging to different groups adapt themselves to the same environment and look and act alike, is called adaptive convergence. On the other hand, there are situations in which organisms belonging to the same or closely related groups may occupy different environments due to which they develop varied adaptations. It gives rise to diverse evolutionary lines. You know previously that house- lizard, snake, tortoise and crocodile belong to the same class, Reptilia But they look thus different from each other that one may be tempted to place them in separate classes. This circumstances in which animals belonging to same or closely related groups occupy different habitats and obtain different functional adaptations, is called adaptive divergence or adaptive radiation.