Adaptive Radiation

Adaptive radiation can occur due to a number of factors:

• Mass extinction wipes out competitors and predators, which allows the surviving species to explore new habitats and resources. For example, the extinction of dinosaurs around 65 million years ago opened up ecological spaces for mammals, which underwent significant adaptive radiation for their survival. 

• Ecological opportunities arise when new habitats or resources become available.

Species that colonize these new environments often experience less competition and can evolve to exploit unoccupied ecological roles. For example, the diversification of Darwin’s finches in the Galápagos Islands occurred because the birds encountered diverse environments with varying food sources on these islands. With fewer competitors, the finches evolved into multiple species, each with a different beak shape to suit specific needs.  

• Accumulation of novel adaptive traits can also cause adaptive radiation. These new traits provide a competitive edge, which allows a species to thrive in niches in which they previously could not exist. An example of this is the evolution of winged flight in birds. Once birds acquired wings, they could fly and get access to food sources and habitats, such as the sky and treetops, that were unavailable to ground-dwelling species. This trait led to the diversification of bird species.

Darwin hypothesized that natural selection was the mechanism behind this diversification. He reasoned that, over time, the ancestral finches had spread across the different islands, each of which experienced distinct environmental conditions and food sources. Due to these varied environments, the birds needed to develop the required traits to survive. He proposed that finches with beaks best suited to the available food in their environment were more likely to survive and reproduce, passing their advantageous traits to the next generation. Over many generations, this led to the formation of different species, each with a specialized beak for its specific feeding strategy.

Apart from the evolution of Darwin’s Finches, there are many other examples of adaptive radiation in nature that have led to macro and micro-evolution.

Cichlid Fish in the African Great Lakes

The cichlid fish in the African Great Lakes (Lakes Victoria, Malawi, and Tanganyika) experienced adaptive radiation. Cichlids exhibit a wide range of feeding strategies: some species feed on algae growing on rocks, others prey on smaller fish, and some filter plankton from the water or specialize in crushing snails. These diverse feeding strategies are complemented by distinct morphological variations in jaw structure, tooth shape, and body size, all of which are adapted to their specific diets and habitats.

With time, these new species developed distinct mating behaviors and color patterns, which prevented them from interbreeding with their parent species population. Thus, sexual selection, diversity in the ecological niches, and geographical isolation have resulted in the adaptive radiation of cichlid fish.

Hawaiian Honeycreepers

Hawaiian honeycreepers have evolved from a single ancestral species. These birds diversified into numerous species with varying beak shapes and feeding habits. Some honeycreepers have long, curved beaks for sipping nectar from flowers; a few others have strong, thick beaks for cracking seeds, while the rest develop slender beaks for catching insects. 

Hawaiian Silverswords

The current Hawaiian silversword species that are found in a wide variety of habitats ranging from dry, barren volcanic deserts to wet, high-altitude alpine regions have all diversified from a common ancestral species. 

The new species show a wide range of variation in leaf shape, growth form, and flowering structure. For example, some species develop tall, spiky plants with sword-like leaves, while others grow low-growing rosettes. The diversity in the landscapes, together with reproductive isolation and genetic bottlenecks, has resulted in the speciation of Hawaiian silverswords.

Mammals

The present-day placental (eutherian) mammals are the result of diversification of once short-legged animals that fed on insects and coexisted with dinosaurs. The mass extinction of the dinosaurs triggered a series of adaptive radiations that have led to the diversification of mammals to occupy a wide range of ecological niches across Australia. They evolved to occupy the roles of placental mammals in other parts of the world.  

For example, the kangaroo evolved into a large herbivore, similar to grazing animals like deer, while the koala adapted to a tree-dwelling herbivorous lifestyle that feeds exclusively on eucalyptus leaves, much like how sloths are tree-dwellers in other ecosystems. The Tasmanian devil developed into a carnivorous scavenger, playing a role similar to hyenas, and the marsupial mole evolved to become a burrowing insectivore, similar to placental moles.