Which of the following best explains the directional shift and big length in these birds?

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3.4 Natural selection in Darwin's finches

An 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.

Figure 16 Medium ground finch.

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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.

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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.

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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).

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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 parents' beak size. Based on Grant and Grant (2000).

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).

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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 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).

• What was the difference in average beak size of all adults before the drought in 1977 and the average beak size of adults that survived the drought?

• Before the drought, the average size of adult beaks was 9.2 mm. The average beak size of adults that survived was 9.9 mm.

• Explain how the average beak size of birds changed from before the drought in 1977 to after the drought in 1978, once the population had recovered and started to breed again.

• Before the drought, the average size was around 9.2 mm. After the drought, it shifted to around 9.7 mm.

• Taking the information from Figures 17 and 18 into account, explain how natural selection could have caused an increase in beak size in this population of ground finches.

• 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. Birds with large beaks therefore produced a large fraction of the next generation and because of this, the average size of beaks of medium ground finches increased quickly within the population.

• What type of selection is operating in this example?

• Directional selection.

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 distribution

Use the resources to answer the interactive questions on finch population distributions.

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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.

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