Adaptive Radiation

The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation. 

• Adaptive means suited to a particular environment or way of life.​
• Radiation means spreading out from one point into many forms.​
• Together, the term describes branching evolution from one ancestor into multiple related species adapted to different ecological conditions.

1. A common ancestral species is present in a region.​
2. Members spread into different habitats or ecological niches.​​
3. Different environmental pressures act on different groups.​​
4. Natural selection favours different useful variations in each group.​
5. Over time, these populations diverge and form distinct species.

• The finches of the Galapagos Islands are the most widely cited example of adaptive radiation.
• They are believed to have evolved from an original seed-eating ancestral finch from the American mainland.
• As the ancestral population spread across the islands, different groups adapted to different food sources and ecological conditions.
• This resulted in several distinct finch species with varying beak structures - suited for seed-eating, insect-eating, cactus-feeding, and other modes.
• Darwin observed these birds during his voyage and recognised their adaptive significance.

A variety of beaks of finches that Darwin found in the Galapagos Islands

• Australia provides another classic example of adaptive radiation.
• A large number of marsupial species are believed to have evolved from a common ancestral marsupial.
• These marsupials diversified into many forms adapted to different ecological roles within the Australian continent.

Adaptive radiation of marsupials of Australia

• Placental mammals in Australia show a comparable pattern of diversification.
• Certain placental mammal forms closely resemble corresponding marsupial forms in appearance and ecological role despite being phylogenetically distinct.
• This resemblance between unrelated groups that evolved independently under similar environmental pressures is termed convergent evolution.
• A commonly cited paired example is the placental wolf and the Tasmanian wolf (a marsupial), which share similar body forms despite separate lineages.

Picture showing the convergent evolution of Australian Marsupials and placental mammals

• Adaptive radiation is the evolution of different species from a common ancestor, radiating into different habitats and ecological niches in a given geographical area.
• Mechanism: common ancestor → spreads into new habitats → different environmental pressures → natural selection favours useful variations → distinct species form.
• Darwin's finches (Galapagos): an ancestral seed-eating finch evolved into many species with different beak shapes for different food sources.
• Australian marsupials: many marsupial species evolved from a common ancestor and adapted to different ecological roles.
• Convergent evolution: unrelated placental mammals and marsupials (e.g., the placental wolf and the Tasmanian wolf) evolved similar forms under similar environments.