Theories of Biological Evolution - Adaptive Radiation

Adaptive Radiation:

• The evolutionary process which produces new species diverged from a single ancestral form becomes adapted to newly invaded habitats is called adaptive radiation.
• Adaptive radiations are best exemplified in closely related groups that have evolved in a relatively short time.
• Darwin’s finches and Australian marsupials are the best examples of adaptive radiation.
• When more than one adaptive radiation occurs in an isolated geographical area, having the same structural and functional similarity is referred to as convergent evolution.

Examples of Adaptive Radiation:

1) Darwin’s finches:

• In Galapagos Islands, Darwin observed small black birds later called Darwin's Finches.
• He realized that there were many varieties of finches in the same island.
• During that time, Darwin's finches have evolved into 14 recognized species differing in body size, beak shape, and feeding behavior.
• The ancestor arrived on the Galapagos about 2 million years ago.
• Changes in the size and form of the beak have enabled different species to utilize different food resources such as insects, seeds, nectar from cactus flowers, and blood from iguanas, all driven by Natural selection.
• Genetic variation in the ALX1 gene in the DNA of Darwin finches is associated with variation in the beak shape.
• Mild mutation in the ALX1 gene leads to a phenotypic change in the shape of the beak of the Darwin finches.

2) Marsupials in Australia and placental mammals:

• Marsupials in Australia and placental mammals in North America are two subclasses of mammals they have adapted in similar way to a particular food resource, locomotory skill, or climate.
• They were separated from the common ancestor more than 100 million years ago and each lineage continued to evolve independently.
• Despite temporal and geographical separation, marsupials in Australia and placental mammals in North America have produced varieties of species living in similar habitats with similar ways of life.
• Their overall resemblance in shape, locomotory mode, feeding, and foraging are superimposed upon different modes of reproduction. This feature reflects their distinctive evolutionary relationships.
• Over 200 species of marsupials live in Australia along with many fewer species of placental mammals.
• The marsupials have undergone adaptive radiation to occupy the diverse habitats in Australia, just as the placental mammals have radiated across North America.
  

  Adaptive radiation of marsupials of Australia

  Picture showing convergent evolution of Australian Marsupials and placental mammals

Convergent Evolution or Adaptive Convergence:

When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution. Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be 'similar' to a corresponding marsupial. e.g., Wolf (placental) and Tasmanian wolf (marsupial). 

Parallel evolution: When adaptive convergence is found in closely related species, it is called as parallel evolution. Parallel evolution occurs when two independent but similar species evolve in the same direction and thus independently acquire similar characteristics.