According to Darwin, living organisms adapt themselves to changing environments for the continuation of their survival. The main reason which initiates the process of evolution is the demand of exploitation of a niche but inability to do so because of some structure and physical or anatomical issues. The phenomenon of adaptive radiation was first described by Darwin during his voyage to Galapagos Island. There he observed wide ranges of finches with different structure in their beaks. There, he formulated the concept of adaptive radiation.

Definition

Adaptive radiation, in the case of evolutionary biology, is a process which highlights rapid diversification of organisms from their ancestor in some new forms because of some newly introduced resources in the changed environment. This adaptation is the result of different responses by a species to the same change in environment. 

Characteristics

Four characteristics of adaptive radiation are follows:

• Adaptive radiation requires a common ancestry of different species and also a recent ancestry is more preferable. This is not the same as the monopoly which consists of participation of every single descendant of a common ancestor.
• A correlation between phenotype and environment is also important which includes an important association between the morphological and physiological characters of the organisms with the environment.
• The advantages and fitness of the traits or characters of a single organism considering the changed environment.
• Rapid evolution of more than one species when ecological and phenotypic divergence is going on.

Conditions

It is thought that adaptive radiation gets triggered by ecological opportunity. Ecological opportunity causes loss of competitors which is the key of innovation. Both predators and competitors in an environment have the potential to cause an increase in the population disrupting the stabilizing selection. Since, genetic diversity is related positively with population size, with increase of population, genetic diversity will also increase to a great extent. With reduced stabilizing selection phenotypic diversity can also increase. In addition, intraspecific competition will increase, promoting divergent selection to use a wider range of resources. These conditions in ecology promote adaptive radiation.

• In the case of Hawaii and the Galapagos, the volcano created a new ground in the middle of the ocean. This is the same for aquatic species, formation of a new water body would have done the same function. For instance, the tectonic movement which gave rise to the East African Rift and ultimately led to formation of Rift Valleys Lakes. An extinction process can provide the same result, forming niches previously occupied by different species that are extinct now. 
• The newly formed habitat is quite isolated. If a volcanic eruption occurs on land, it would destroy the mainland as well the forest areas adjacent to that causing extinction of different plant and animal species. After that, in the destroyed area rapid recolonization would occur without proper evolution. The species that would result in colonization in the destroyed parts would be rare and uncommon arrivals, if recolonization occurs. 
• The new habitat has a wide availability of niche space. The rare colonist can only adaptively radiate into as many forms as there are niches. 

Example of Adaptive Radiation

Darwin Finches

There are about 15 finches in the island of Galapagos. In the island, Darwin found there a wide range of finches lived in the single area but had different structures of beaks.  The ground finches have thick bills as their diets consist of seeds. Incase of smaller finches, the beaks are smaller for eating smaller seed and medium ground finches have medium sized beaks for consumption of intermediate sized seeds. Some robust medium ground finches have larger beaks than the smaller ground finches. The reason behind this difference is the difference in food habits to avoid intraspecific competition. This condition where the finches modified themselves when lack of resources have been observed, to avoid extinction is known as adaptive radiation.

Australian Marsupials

Australian marsupials are a great example of adaptive radiation which develops from a single ancestor in the Australian subcontinent. This highlights divergent evolution which consists of evolution of different species from a single ancestor. But when a number of adaptive radiations take place for a species, it would give rise to convergent evolution.

Conclusion

Adaptive radiation, in the case of evolutionary biology, is a process which highlights rapid diversification of organisms from their ancestors in some new forms because of some newly introduced resources in the changed environment. This adaptation is the result of different responses by a species to the same change in environment. Adaptive radiation requires a common ancestry of different species and also a recent ancestry is more preferable. This is not the same as the monopoly which consists of participation of every single descendant of a common ancestor. Conditions that enhance adaptive radiation are also discussed above.