Which finches were most likely to survive the drought?
Video transcript- This here is a picture of the ground finch of the Galapagos Islands. And one of its primary sources of food is seeds that it finds on the ground. And if we go back to 1976, we can look at the distribution of beak depths. And these beak depths, I would assume these are given in millimeters. Finches are quite small birds. And you can see in 1976, you have a large number of finches that had a beak depth of 8.8 millimeters, but it was a distribution around that. Now, after this data was collected, it turns out there was a drought and there were fewer seeds. So the smaller seeds, which were easy to eat for all of these finches, would have been consumed quickly. And all that would have been left were the larger seeds, the ones that you need a larger beak in order to crack and get at the good stuff. So what do you think would have happened to the distribution of beak depths over the course of the next two years? Well, you might guess that the birds, the finches that have larger beak depths, are more likely to survive because they're more likely to be able to crack the larger seeds. And the finches that are more likely to survive are also more likely to reproduce, and pass their large beak trait to their offspring. And that is indeed what scientists observed. As we go from 1976 to 1978, the distribution has shifted a pretty good bit to the right. Now the most common beak depth is 9.8. So this is an example of an environmental change, a drought, changing the food supply, because now there's fewer small seeds available, that changed a distribution in beak depths over just two years. Now, environmental changes don't always cause adaptation. If it's too severe, if the drought was so strong that there were no seeds, you could have extinction. So the species disappears altogether. But here we have an example that in just two years, a species was actually able to adapt a pretty good bit. Now, no individual member of the species knew to somehow grew their beak. But as we saw, you always have a variation of beak depths. And year after year, the birds with the bigger beak depth, were more likely to survive and reproduce. So the species as a whole adapted to the new environmental conditions. Show
The differences in shape and size of beaks in Darwin’s finches illustrate ongoing evolutionary change. Key Points
Key Terms
Visible Evidence of Ongoing Evolution: Darwin’s FinchesFrom 1831 to 1836, Darwin traveled around the world, observing animals on different continents and islands. On the Galapagos Islands, Darwin observed several species of finches with unique beak shapes. He observed these finches closely resembled another finch species on the mainland of South America and that the group of species in the Galápagos formed a graded series of beak sizes and shapes, with very small differences between the most similar. Darwin imagined that the island species might be all species modified from one original mainland species. In 1860, 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.” Figure \(\PageIndex{1}\): Darwin’s Finches: Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources. This illustration shows the beak shapes for four species of ground finch: 1. Geospiza magnirostris (the large ground finch), 2. G. fortis (the medium ground finch), 3. G. parvula (the small tree finch), and 4. Certhidea olivacea (the green-warbler finch).Natural SelectionDarwin called this mechanism of change natural selection. Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, the characteristics of organisms are inherited, or passed from parent to offspring. Second, more offspring are produced than are able to survive; in other words, resources for survival and reproduction are limited. The capacity for reproduction in all organisms exceeds the availability of resources to support their numbers. Thus, there is a competition for those resources in each generation. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Out of these three principles, Darwin reasoned that offspring with inherited characteristics that allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called “descent with modification,” or evolution. Studies of Natural Selection After DarwinDemonstrations of evolution by natural selection can be time consuming. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important demonstrations of the operation of natural selection. The Grants found changes from one generation to the next in the beak shapes of the medium ground finches on the Galápagos island of Daphne Major. The medium ground finch feeds on seeds. The birds have inherited variation in the bill shape with some individuals having wide, deep bills and others having thinner bills. Large-billed birds feed more efficiently on large, hard seeds, whereas smaller billed birds feed more efficiently on small, soft seeds. During 1977, a drought period altered vegetation on the island. After this period, the number of seeds declined dramatically; the decline in small, soft seeds was greater than the decline in large, hard seeds. The large-billed birds were able to survive better than the small-billed birds the following year. The year following the drought when the Grants measured beak sizes in the much-reduced population, they found that the average bill size was larger. This was clear evidence for natural selection of bill size caused by the availability of seeds. The Grants had studied the inheritance of bill sizes and knew that the surviving large-billed birds would tend to produce offspring with larger bills, so the selection would lead to evolution of bill size. Subsequent studies by the Grants have demonstrated selection on and evolution of bill size in this species in response to other changing conditions on the island. The evolution has occurred both to larger bills, as in this case, and to smaller bills when large seeds became rare. Figure \(\PageIndex{1}\): Finches of Daphne Major: A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the finches’ average beak size between 1976 and 1978.What finches survived the drought?Medium ground finches with smaller beaks, however, survived a severe two-year drought better than medium ground finches with larger beaks, the researchers report in the journal Science.
Which type of birds survived the drought?Because the drought reduced the number of seeds and finches with bigger beaks were able to eat the larger and harder seeds so more of them survived.
Which Galapagos finches survive and reproduce better during droughts and why?The adult survivors of the drought were the ones with the largest beaks because they could still crack large seeds. These birds then mated and because beak size is heritable and is passed on to offspring, the chicks from these birds inherited large beak size.
How did the 1977 drought affect the finches?A major drought hit the island in 1977, and 85% of the birds died. Having big beak raised the odds of a bird surviving, because it meant the animal could crack the hard spiked seeds. The Grants discovered that within a few years the population of finches the recovered.
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