Few people have the tenacity of ecologists Peter and Rosemary Grant, willing to spend part of each year since 1973 in a tent on a tiny, barren volcanic island in the Galapagos. Even fewer would have the patience to catch, weigh, measure, and identify hundreds of small birds and record their diets of seeds. But for the Grants, the rewards have been great: They have done nothing less than witness Darwin's theory of evolution unfold before their eyes. That would have stunned Darwin, who thought natural selection operated over vast periods of time and couldn't be observed. In their natural laboratory, the 100-acre island called Daphne Major, the Grants and their assistants watched the struggle for survival among individuals in two species of small birds called Darwin's finches. The struggle is mainly about food -- different types of seeds -- and the availability of that food is dramatically influenced by year-to-year weather changes. The Grants wanted to find out whether they could see the force of natural selection at work, judging by which birds survived the changing environment. For the finches, body size and the size and shape of their beaks are traits that vary in adapting to environmental niches or changes in those niches. Body and beak variation occurs randomly. The birds with the best-suited bodies and beaks for the particular environment survive and pass along the successful adaptation from one generation to another through natural selection. Natural selection at its most powerful winnowed certain finches harshly during a severe drought in 1977. That year, the vegetation withered. Seeds of all kinds were scarce. The small, soft ones were quickly exhausted by the birds, leaving mainly large, tough seeds that the finches normally ignore. Under these drastically changing conditions, the struggle to survive favored the larger birds with deep, strong beaks for opening the hard seeds. Smaller finches with less-powerful beaks perished. So the birds that were the winners in the game of natural selection lived to reproduce. The big-beaked finches just happened to be the ones favored by the particular set of conditions Nature imposed that year. Now the next step: evolution. The Grants found that the offspring of the birds that survived the 1977 drought tended to be larger, with bigger beaks. So the adaptation to a changed environment led to a larger-beaked finch population in the following generation. Adaptation can go either way, of course. As the Grants later found, unusually rainy weather in 1984-85 resulted in more small, soft seeds on the menu and fewer of the large, tough ones. Sure enough, the birds best adapted to eat those seeds because of their smaller beaks were the ones that survived and produced the most offspring. It occurs when members of a single species occupy a variety of distinct niches with different environmental conditionsConsequently, members evolve different morphological features (adaptations) in response to the different selection pressures
An example of adaptive radiation can be seen in the variety of beak types seen in the finches of the Galapagos Islands
- These finches have specialised beak shapes depending on their primary source of nutrition (e.g. seeds, insects, nuts, nectar)
Adaptive Radiation
Daphne Major Daphne Major is a volcanic island that forms part of the archipelago that is collectively referred to as the Galapagos Islands - It is the native habitat of a variety of bird species known as Darwin’s finches (subfamily: Geospizinae)
Darwin’s finches demonstrate adaptive radiation and show marked variation in beak size and shape according to diet
- Finches that feed on seeds possess compact, powerful beaks – with larger beaks better equipped to crack larger seed cases
In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection
- Dry conditions result in plants producing larger seeds with tougher seed casings
- Between 1976 and 1978 there was a change in average beak depth within the finch population
- Finches with larger beaks were better equipped to feed on the seeds and thus produced more offspring with larger beaks
The finches in the above video were collected from the Galápagos Islands in 1835 by Charles Darwin and his colleagues during the second voyage of HMS Beagle (1831-1836).The different finch species on the islands are closely related to each other, but show wide variations in beak and body size and feeding behaviour. In particular, changes to the size and shape of the beaks have enabled the different species to specialise in different types of food: seeds, insects, cactus flowers and fruits or even bird blood. The Galápagos finches are a classic example of adaptive radiation. Their common ancestor arrived on the islands a few million years ago. Since then, a single species has evolved into different species that are adapted to fill different lifestyles. Darwin didn't recognise the significance of these birds when he first encountered them as he didn't realise how closely related they were to each other. Later, he wrote: '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.' (Journals of Researches, 2nd Edition, 1845) Suggestions for use in classThe video above shows six different species of finch found by Darwin and his colleagues on the Galápagos Islands. Overall, there are about 15 closely related species of Darwin's finches. The video could be used as starter on a lesson on evolution, adaptation and natural selection, or as a part of a discussion on the evidence gathered by Darwin for his theory of evolution. The video asks: 'Can you spot the differences between these finches?' The birds differ in plumage and body size but the most obvious differences between the birds are the size and shape of their beaks, which are dependent on their food preferences and specialisations. The thinnest beak belongs to the green warbler finch which uses it to probe for insects. The massive, stout beak of the large ground finch enables it to crush big hard seeds. The long pointed beak of the common cactus finch helps it extract seeds from cactus fruit.
Which is the explanation for the different types of beaks Darwin observed on the finches in the Galapagos Islands?
On the Galapagos Islands, Darwin also saw several different types of finch, a different species on each island. He noticed that each finch species had a different type of beak, depending on the food available on its island. The finches that ate large nuts had strong beaks for breaking the nuts open.
What was Darwin's explanation for the different beaks?
Long, pointed beaks made some of them more fit for picking seeds out of cactus fruits. Shorter, stouter beaks served best for eating seeds found on the ground. Eventually, the immigrants evolved into 14 separate species, each with its own song, food preferences, and beak shapes.
Why were the beaks on the finches different depending on what island they were on?
Explanation: Each island has a different environment. The differences in environment selected different variates from the possibilities of the DNA in the finches. Also within a given island there are different niches.
Which of the following is the best explanation for why finches on one of the islands?
d. Finches with large beaks are always better adapted to their environment. Finches with small beaks only survive on islands where there were never finches with larger beaks.
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