For most biologists and reasonable people, evolution is FACT. In as much as gravity could be said to be fact. However, for those who deny the existence of evolution, the difficulty of observing its occurrence in real time is proof enough that it doesn’t exist. There are a few key examples of evolution in action, however, and during these short essays I have been detailing some of them. One of the most famous examples is that of the Galapagos Finches, which inspired Darwin as he formulated his ground-breaking theory.
The Galapagos islands are an archipelago of 40 islands off the west coast of Ecuador, which vary in size and habitat. They are home to 13 species of finches, which differ noticeably in their beak size and shape. They were first discovered by Darwin during the second voyage of HMS Beagle in September 1835, but did not immediately grab his attention. He collected specimens of a variety of species of bird and mammal, and during his visit to the islands was particularly interested in the mockingbirds and giant tortoises. Shortly after his return to England, however, the respected orniothologist, John Gould identified the specimens and published a report, describing 12 distinct species of finch “so peculiar [as to form] an entirely new group”. It was only now that Darwin realised that each island harboured its own separate species of finch, adapted to the particular habitat and resources provided by that island. This realisation would later be integral to the development of his theory of natural selection.
The case of the Galapagos finches is not unique, in fact the same phenomenon has been found for island species world-wide. The mechanism is simple: a few members of a mainland species colonise an island, and, over time, begin to adapt to the peculiarities of that island; food sources, predators, habitat, climate. Evolution proceeds, as it does everywhere else on Earth where life persists. After a time, the creatures living on the island are distinct from the mainland species they descended from. They have evolved to exploit a niche. In groups of islands, particularly where the islands form a chain, some individuals of that species may find their way to another island, where, again, they must adapt to the particular conditions found there. This happens over and over until, thousands of years later, when biologists come to study the island species, they find a series of clearly related, but slightly different species, all decended from an original mainland population, but adapted to the specifics of the island where they reside.
In the case of Darwin’s finches, the main adaptations occurred in beak morphology. Each of the Galapagos islands has a slightly different selection of fruit and seeds available as food, and on each island, the finch species present have beaks which allow them to take advantage of the food available. The finches, all decended from a single south American ancestor, range in size from 8 to 38 grams, and their beaks range from short to long, narrow to wide, depending on the size and hardness of the seeds upon which they feed.
“Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.” Darwin (1845)
DNA analyses on the Galapagos finches confirms that they are decended from single a species of seed-eating tanager finch found in South and Central America, which arrived on the Galapagos between 2 and 3 million years ago. Biologists have also now located a number of genes involved in beak morphology. The genes are extremely similar across finch species, but the timing of their expression during development determines the final shape and size of the beak. Changes in gene expression may lead to more rapid evolution than changes in gene function, and this may in part explain the rapid diversification of the Galapagos finches.
Long-term study of the Galapagos finch populations has revealed rapid evolutionary change in response to changing climatic conditions. For 30 years, Peter and Rosemary Grant monitored beak shape and size in two species of Finch found on the island Daphne Major, and they found dramatic changes over time. In 1977 a severe drought had a dramatic impact on the flora and fauna of Daphne Major. Plants with small seeds suffered particularly badly during the drought, and this in turn killed many of the finches. Only those with larger beaks, able to eat the larger seeds, were able to survive. The effect was so strong that each generation beak size in the finches increased by 4%. Extreme rains caused by the 1983 En Niño event caused a reversal of this process, with an increase in availability of small seeds, and corresponding decreases in finch beak size. The rapid evolutionary change exhibited by these populations in response to frequent climatic changes offers us a rare opportunity observe natural selection in action, and offers insight into the processes which shaped these populations across millennia.
Articles in this Series:
- Intro: Reasons Why Evolution is True
- Part One: The Panda’s Thumb
- Part Two: Parasitoid Wasps
- Part Three: Ring Species
- Part Four: Galapagos Finches
- Part Five: The Quirky Human Eye
- Part Six: Homology
- Part Seven: Coevolution
- Part Eight: PreCambrian Rabbits
- Part Nine: DIY Evolution
- Part Ten: Convergent Evolution
Want to Know More?
- Darwin’s Finches
- Beagle Investigations Return with Darwinian Data
- Twin Beaks Darwin (1859)
- On the Origin of Species by means of natural selection. J. Murray, London.
- Grant and Grant (1996) High Survival of Darwin’s Finch Hybrids: Effects of Beak Morphology and Diets Ecology77,500-509
- Abzhanov et al (2004) “Bmp4 and morphological variation of beaks in Darwin’s finches” Science 305,1462-1465