Darwin first described adaptive radiation during his study on the Galapagos Islands in 1835. Darwin studied the island’s land birds and noticed that despite similar in size and appearance, after he had examined them closer, he classified them as thirteen different species finches (Greij 2008). Darwin assumed that an ancestral finch must have colonized on of the islands, eventually visiting the other islands. The finches on the different islands then adapted to the local conditions. Their most obvious differences dealt with their beak size. Overtime, the populations were separated geographically and formed new species as they adapted to ecological niches specifically food sources, in their new habitats (Greij 2008).
Research today shows that the Galapagos finches continue to go through the process of adaptive radiation. Rosemary and Peter Grant are both evolutionary biologists at Princeton University. They have been studying Darwin’s finches since 1970 (Holmes 2006). The Grants along with their graduate students focused their studies on three species of finches, the small, medium and large ground finches. These three species were adapted to eat varying sizes of seeds, specifically small, medium, and large size seeds respectively. Due to a drought in 1977, there was a large reduction in seed production and many of the small and medium ground finches died as a result (Greij 2008). This drought led to an increase in larger finches with larger beaks that were able to crack the larger, tougher seeds (Holmes 2006). The population then shifted again after the El Nino season of 1983, where downpours allowed for an increase in small and medium seeds while the larger, tough shelled seeds from drier years were limited. The larger finches were forced to eat smaller seeds, which they require much more of to sustain themselves. The larger birds eventually died off, and once again small and medium size birds became the prominent species on the islands (Greij 2008).
Greij, Eldon. (2008). Ever Adapting. Birder’s World, 22(1)
Holmes, Bob. (2006). Evolution in Action by Darwin’s Finches. New Scientist, 191(2561)
Hickman, C.P., Roberts, L.S., & Larson, A. (1998). Biology of Animals; 7th Edition. New York, NY: McGraw-Hill