- H. F. Osborn (1902) coined the term adaptive radiation. According to him adaptive radiation is with great variety adaptations to different niches”.
- John Relethford – The formation of many new species following the availability of new environments or the development of new adaptation” is called adaptive radiation.
- Lawrence S. Dillon – Evolutionary divergence of the members of simple major stock into a number of distinctive types” is adaptive radiation
- Simpson (1953) – adaptive radiation is the rapid proliferation of new species from a single ancestral group
Adaptive radiation shows how organisms of the same group or closely related groups exhibit great divergence in their morphology when they are found indifferent habitats.
Prof. Osborn has formulated the law of adaptive radiation which holds that “Each isolated region if large and sufficiently varied in its topography , soil , climate and vegetation will give rise to diverse fauna.
The larger the region and more diverse the conditions, the greater will be the varieties of animals found . Therefore adaptive radiation is evolution in several specialized directions standing from a common and generlised ancestral type, or the entry of the organisms of the original stock to new adaptive zones.
Adaptive radiation illustrates a simple but important principle of evolution. It may be stated thus: When a new form arises , it will diverge into as many variations as follows depending on
• its adaptive potential and
• the adaptive opportunities of the available zones.
The structural and functional ability of the body of the organisms reveals the adaptive potential of the organism. An organism may have great adaptive potential or less adaptive potential depending upon its biological feature. The ecological niches or the available zones may have their own potentials to provide adaptive opportunities in the form of resources such as food, shelter, and space. All this reveals how adaptive radiation and evolution are inter-related in an important way.
Adaptive radiation and evolution are inter-related in another way. Adaptive radiation involves the transition from generalized to specialized characteristics. A particular trait which is adapted for many functions is termed generalized whereas a trait that is limited to a narrow set of ecological functions is called specialized.
Adaptive Radiation Among Primates
The order Primates comprises more than one hundred and fifty living species. These tall within two sub-orders: Prosimii and Anthropoidea . Though in many ways the primate species show remarkable similarities, they have adapted in distinct ways both anatomically and otherwise . The spread of arboreal primates, the old world monkeys, into trees of the tropical forests is an example. Thus, all the chief groups of primates today include species with contrasting dietary habits. Insect eating, seed eating, leaf eating, and more or less omnivorous genera recurred in different branches of the primates as these branches departed more or less from the ancestral line.
Adaptive radiation is well exemplified by the history of the mammals. The mammals evolved in many distinct lines. The rodents specialised for gnawing, the carnivores for hunting, the hoofed animals for grazing, the primates and sloths took to the trees, the whales, seals, and sea cows adapted for life in the oceans, and the bat took to the air. Furthermore, each of these mammalian orders in turn gave rise to sub lines that colonised new environments by acquiring new modes of life. Many of today’s mammals are far different from their primitive common ancestors to the Paleocene epoch.
This adaptive radiation within the branches is thus accompanied by parallelism between the branches and by convergence of adaptations towards those of some non primate lines.