Adaptive Radiation: Organisms that are present around our surroundings; all of them share a common ancestor. The ancestors diverged into different organisms with little unique properties that we call different species. But they are no longer precisely the same as the common ancestors. Adaptive radiation is where all the organisms that we see are produced from one common ancestor with little differences in their molecular, morphological, and anatomical properties which help them to survive and fit well into their surrounding environment.

What is Adaptive Radiation?

The evolutionary diversification of different species starting from a point in a geographical area and finally radiating to other areas of geography (habitats) is called adaptive radiation. The concept of adaptive radiation was developed by HF Osborn (1902). Adaptive radiations are best observed in closely related groups that have evolved in a relatively short time. This plays an important role in macroevolution.

Adaptive Radiation Characteristics

Adaptive radiation has the following four characteristics (Schluter 2000).

1. Common Ancestry: All the species of adaptive radiation show a single common ancestor.

2. Phenotype-environment Correlation: There is a relation between the diverse phenotypes of the descendant species and the divergent environments in which they live.

For example, the body and limb size of Anolis lizards matches the twig diameter in their habitat.

3. Trait Utility: The morphological or physiological traits that differ between the descendant species of adaptive radiation are helpful to exploit the respective ecological niche or often happens when an organism enters a new area with lots of ecological opportunities—for example, Darwin’s finches.

4. Rapid Speciation: In adaptive radiation, speciation is typically rapid. Speciation during adaptive radiation can be allopatric.

Adaptive Radiations Causes

The major causes which can trigger adaptive radiation are:

1. Ecological Opportunities– The ecological opportunities that act as the trigger of adaptive radiation have been demonstrated in numerous examples using living and extinct organisms. Ecological opportunity permits a group to experience rapid diversification in species number and morphological attributes.

2. Mass Extinction- When extinction of a more significant number of species occurs in a short period of geological time, mass extinction is considered. These can be caused due to climate change, asteroid impacts, massive volcanic eruptions, or a combination of these causes. These can change faster than the rate of evolution. The extinct species are soon replaced by new species which are more adapted to the new conditions.

3. Acquisition of Novel Adaptive Traits- Evolutionary novelties can cause variation in the basic pattern that produces something unique which adapts to a different role. For example, feathered wings on birds that evolved from reptilian scales represent preadaptation of flight. A slight alteration in regulatory genes or mutation in genes can result in major structural changes in the organism.

Adaptive Radiation Examples

1. Adaptive Radiation in Darwin’s Finches

I. The Galapagos Island has located approximately 1000 km from the coast of Ecuador, South America
II. Darwin observed a tremendous amount of diversity in organisms in Galapagos Island.
III. These are considered Darwin’s finches.
IV. Darwin differentiated 14 species of finches.
V. There were many variations in the beak structure of the bird’s varieties that might have evolved in the island itself.
VI. It must have arisen from an essential seed-eating habit.
VII. Evolution might have allowed insectivorous and vegetarian food habits.
VIII. He conjectured that these varieties evolved from the original seed-eating ancestor bird present in South America.
IX. Some of them flew to these islands, and according to the needs of different environments, they developed different shapes of beaks and feeding habits. Some became insect-eating (tree finches), some fruit-eating, some cactus eating finches have thicker, decurved, flower-probing beaks, some crushed seeds (ground finches), woodpecker finch possess a stout, straight beak, small beaked finch feed on small grass seed.

Fig: Adaptive Radiation of Darwin’s Finches of Galapagos Islands

2. The Australian Marsupials

I. The Australian marsupials show adaptive radiation.
II. The number of marsupials, each different from the other, like koalas, kangaroos, wombats, etc., originated from an ancestral marsupial, but all within Australia.
III. The placental mammals also underwent adaptive radiation and developed into different varieties.
IV. It was seen that many placental mammals resembled the marsupial mammals in structure and in leading similar ways of life.
V. These similarities in these two different types of mammals (marsupials and placentals) are due to living in similar ecological niches.
VI. The convergent evolution resulted in similar-looking members in both the groups, e.g., Placental wolf and Tasmanian wolf-marsupial.

Fig: Adaptive Radiation of Marsupials of Australia

3. Adaptive Radiation in Mammals

I. Placental mammals illustrate an excellent example of adaptive radiation.
II. About 180 million years ago, from the cynodont ancestors of mammals, adaptive radiation occurred.
III. Multiple diverse forms of mammals have developed from the common lineage of a primitive, short-legged, insectivorous, rat-like creature that coexisted with dinosaurs.
IV. The extinction of dinosaurs triggered a global adaptive radiation event that resulted in the rich mammal diversity that exists today.

4. Locomotion in Mammals

I. The locomotion in mammals is an excellent example of adaptive radiation.
II. Humans are the most evolved species on the planet and have also undergone many changes over time from our ancestors.
III. Humans are bipedal, meaning they can walk on two limbs. The other limbs, the arms, and subsequent hands have adapted to do precision tasks such as typing or tying shoelaces, everyday tasks that were performed in our environment.
IV. The amount of melanin in our skin represents the environment we live in, i.e. dark-skinned people occupy hotter climates.
V. The structure of our body indicates the temperature in our climate also. Humans who have produced offspring that successfully live in a cold environment tend to be broader and smaller in stature, while thinner, taller humans occupy hotter environments.

5. Adaptive Radiations in the Mouth Part of Insects

I. All insects have similar mouthparts- one labrum (upper lip), one labium (lower lip), one pair of the mandible, one pair of the maxilla and one hypopharynx.
II. These mouthparts are modified in the different niches for the collection of different types of food.
III. Some parts are well developed, and some parts are reduced.
IV. This leads to the formation of different species among insects.

Impacts of Adaptive Radiation

The impacts of adaptive radiation are as follows:

1. Adaptive radiation allows organisms to adapt in the favourable environment leaving the unfavourable one and thus increasing their rate of survival and hence lowering the possibility of their extinction.
2. Adaptive radiation gives rise to species diversity in a geographical area.
3. This allows the emergence of new species, which exhibit different morphological and physiological traits.
4. The evolutionary process is rapid because of adaptive radiation.
5. This allows organisms to utilise different ecological niches.

Summary

Adaptive radiations are best observed in closely related groups that have evolved in a relatively short time. It often happens when an organism enters a new area with lots of ecological opportunities. Example: Darwin’s finches of Galapagos islands. It can also be caused due to mass extinctions or when new traits evolve. Example: Mammals after extinction after dinosaurs. This allows organisms to adapt in the favourable environment leaving the unfavourable one. This also allows organisms to utilise different ecological niches.

Frequently Asked Questions (FAQs) on Adaptive Radiations

Q.1. What is adaptive radiation?
Ans: The evolutionary diversification of different species starting from a point in a geographical area and finally radiating to other areas of geography (habitats) is called adaptive radiation.

Q.2. Give one example of adaptive radiation.
Ans: Darwin’s finches of the Galapagos are one example of adaptive radiation.

Q.3. Why is adaptive radiation necessary?
Ans: Adaptive radiation allows organisms to adapt in a favourable environment leaving the unfavourable one.

Q.4. What factors contribute to adaptive radiation?
Ans: The phenomena of adaptive radiation result from natural selection, artificial selection, sexual selection, mutation pressure, genetic drift or migration. It indicates evolutionary variations that are quite adaptive to a specific environment.

Q.5. Who first coined the term “adaptive radiation” in evolution?
Ans: The term “adaptive radiation” was first coined by HF Osborn (1902).

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