3.4 Natural selection in Darwin's finchesAn influential study of natural selection in birds illustrates how effective, and rapid, natural selection can be. Scientists Peter and Rosemary Grant studied the medium ground finch (Geospiza fortis, Figure 16) over a long period of time, on the Galápagos island of Daphne Major. See also Video 5. Show
 Figure 16 Medium ground finch. Show description|Hide description This is a photo of a female medium ground finch; its plumage is brown with lighter streaks. (The male, in contrast, has black plumage.) This finch species has a short, stout beak adapted for eating seeds found on the ground. Figure 16 Medium ground finch. Show transcript|Hide transcript Transcript: Video 5 An introduction to Darwin's finches.DAVID ROBINSON: Although he didn't realise it at the time, the most important specimens that Charles Darwin brought back from the Galapagos were finches. Initially, he wasn't sure how they were related, but when, back in England, they were examined by the ornithologist John Gould, he reported that, in fact, Darwin had brought back 13 different species of finch, all of which were unique to the Galapagos. This realisation played a significant role in Darwin's formulation of his theory of evolution. The most important differences between the finches came in their beaks. Some were large. Some were small. Each one was suited to the availability of particular foodstuffs. Eventually, Darwin theorised that different species of finch had evolved on different islands, their distinctive beaks being an adaptation to distinct natural habitats or environmental niches. In the years since Darwin's visit, many other scientists and ornithologists have come to the Galapagos to study its finches. In this experiment, researchers are observing the woodpecker finch, using this wooden box to stand in for a tree. The woodpecker finch is one of the only birds to use tools to help it find food. A stick or small twig enables it to dig deeper into tree bark for insect larvae. This skill enables it to survive in conditions which other birds would find difficult. In the dry season, it can gather up to 50 per cent of its food in this way. Woodpecker finches are hungry birds, which in the wild need to eat every three hours. So they never turn down the chance of a free meal. End transcript: Video 5 An introduction to Darwin's finches. Video 5 An introduction to Darwin's finches. Interactive feature not available in single page view (see it in standard view). In these finches, beak size is heritable, meaning that adults with large beaks pass large beak size onto their offspring. Figure 17 illustrates this relationship. Figure 17 The beak size (measured as beak depth - the distance between the top and bottom of the beak) of offspring plotted against their parents' beak size. Based on Grant and Grant (2000). Show description|Hide description This figure is a plot of average value of beak depth in offspring against average value of beak depth in parents, measured in millimetres. The axes are labelled from 7 to 12 millimetres, with beak depth values ranging from 7.5 to 11 mm in both groups. A line of best fit is drawn through the numerous data points. The figure shows a strong positive correlation between beak depth in offspring and parents. Figure 17 The beak size (measured as beak depth - the distance between the top and bottom of the beak) of offspring plotted against their... During 1977 there was a major drought on Daphne Major and many of the plants on the island produced few or no seeds. The medium ground finch population, which depends on seeds for food, declined drastically from about 1400 individuals to a few hundred in just over two years. What the Grants' research group observed was that following the drought the population of finches had recovered; but now, the average size of the beaks was larger. The underlying reason for this was that during the drought, small seeds were exceedingly rare but large seeds with thick husks were still available. Only the large birds with large beaks were able to crack open the husks and eat the contents of the seeds. Smaller birds with smaller beaks were unable to do so and therefore starved. From this differential pattern of death, there was a rapid change in the finch population. Figure 18 illustrates how natural selection caused a rapid change in the size of the beaks in the finch population following the drought. Figure 18 (a) The pale blue bars show the total number of birds on the island with beaks in each size class, before the drought. The blue bars show the number of birds with beaks in each size class that survived the drought and subsequently reproduced. (b) The average beak size of offspring produced by adults that survived the drought. The dashed vertical lines show the average beak size from one year to the next. Based on Grant et al. (2003). Show description|Hide description This figure comprises two bar diagrams plotting frequency (numbers) of birds against beak depth. The vertical axis is marked from zero to 50 in (a) and from zero to 40 in (b). Beak depth intervals, marked along each horizontal axis, are from 7.1-7.5 mm up to 11.1-11.5 mm. Part (a) shows the data for the 1977 parents: both those that were present before the drought that occurred that year, and those that survived the drought and went on to breed. Numbers were greatly reduced following the drought; for example, the frequency for the 9.1-9.5 mm beak size interval fell from 45 down to 8. Part (b) shows the data for the 1978 offspring. Vertical dashed lines mark the average beak depth values, x̄, for each of the three groups: from part (a), x̄ of birds before the drought = 9.2 mm and x̄ of the breeding survivors = 9.9.mm; from part (b), x̄ of the offspring =9.7 mm. Figure 18 (a) The pale blue bars show the total number of birds on the island with beaks in each size class, before the drought. The blue...
What is important to note here is that the genetic variation that underlies differences in beak size was already present in the population. As a consequence of natural selection, the frequency of genetic variants that expressed the larger beak size increased in the population and those variants that expressed smaller beak size declined in frequency and may even be lost from the population. From this example, you can see how dramatic changes can occur in a population as a result of natural selection. You can also see how careful observation by the Grants over consecutive years allowed them to capture empirical evidence of natural selection. Activity 1 Finch beak size and population distributionUse the resources to answer the interactive questions on finch population distributions. Interactive feature not available in single page view (see it in standard view). During their migratory journeys, birds encounter extremes of temperature, storms, changes in altitude and shortage of food and water. Birds have evolved a suite of remarkable physiological adaptations to deal with these changes in the external environment. Before discussing some of these adaptations, how organisms in general maintain a constant internal environment is explored, a process known as homeostasis. What kind of finches are on Daphne Major?Most of the finches on Daphne Major are medium ground finches. This investigation of Daphne Major focuses on the population of medium ground finches. Young medium ground finches (fledglings), are brown in color, with streaks of lighter shades.
What do medium ground finches eat?The medium ground finches eat small insects and the fruit and seeds of plants, and the fruit and seeds of cactus. The cactus finch gets its food primarily from cactus.
What do finches eat in the Galapagos?Endemic to the Galápagos, the medium ground finch is found on ten islands: Baltra, Floreana, Isabela, Fernandina, Seymour, Pinzón, San Cristóbal, Santa Cruz, Santa Fé, and Santiago. The medium ground finch feeds primarily on seeds, although it is also known to eat flowers, buds, and young leaves, and the occasional insect.
Where do cactus finches get their food?The cactus finch gets its food primarily from cactus. The medium ground finches breed after the first big rains of the wet season. The fledglings grow to adult size within one year.
What is the primary food of the medium ground finch?The medium ground finch has a stubby beak and eats mostly seeds.
Which were the primary finches found on Daphne Major and studied by the Grants?The Grants watched nature brutalize the two main finch species on Daphne, the cactus finch (Geospiza scandens) and the medium ground finch (Geospiza fortis).
How did the medium ground finch population on Daphne Major change following the 1977 drought?During 1977 there was a major drought on Daphne Major and many of the plants on the island produced few or no seeds. The medium ground finch population, which depends on seeds for food, declined drastically from about 1400 individuals to a few hundred in just over two years.
What event occurred on Daphne Major in 1977 and reduced the medium ground finch population by 85%?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. But now their beaks were, on average, 4% deeper.
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What is the primary food of the medium ground finch on the island daphne major?
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