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Chapter 8: Activity Case Studies: Natural Selection and Darwin's Finches

Natural Selection and Darwin's Finches
by Martin Wikelski, University of Washington

Introduction:
One of the classic studies in the evolution of natural populations was conducted by Rosemary and Peter Grant and coworkers on Darwin's finches. It is among the first–and is certainly the most elegant–study to document evolution in a wild population of vertebrates. Rosemary and Peter investigated Darwin's finches for almost 3 decades and conducted most of their field work on two small islands, Daphne Major and Genovesa, in the Galápagos archipelago, Ecuador.

About 30 years ago, Peter Grant started from the question of how interspecific competition influenced the formation of animal communities. He decided to study the 'classic' case of character release. David Lack suggested in 1947 that the bill sizes of Darwin's ground finches differ between islands, depending on the presence or absence of other (competing) species of Darwin's finches. However, there was hardly any knowledge of why this should be so. Nevertheless, the data are very clear-cut–the beak sizes of medium ground finches (Geospiza fortis) from Daphne Major are smaller than the ones from the same species on the nearby island of Santa Cruz Island, although the islands are only about 10 kilometers apart.

The articles by Boag and Grant (1984) summarize the enormous body of their work.

P. T. Boag and P. R. Grant. 1984. The classical case of character release: Darwin's finches (Geospiza) on Isla Daphne Major, Galápagos. Biological Journal of the Linnean Society 22: 243-287.

The prediction of evolutionary change is the real goal in evolutionary analysis. To make clear how the measurements from previous studies can be used to predict microevolutionary change, read:

P. R. Grant and B. R . Grant. 1995. Predicting microevolutionary responses to directional selection on heritable variation. Evolution 49: 241-251.

To truly appreciate the scope and detail of their research, consider these points:

• The study of evolution is necessarily a long-term goal, especially in natural vertebrate populations. Observing the finch population of Daphne Island for three decades revealed that natural selection can push traits of animals into different directions, depending on the environmental conditions.
• Rosemary and Peter followed individual birds and determined both their fate, as well as, the fate of their offspring. It cannot be emphasized enough how important data on individuals are to an understanding of evolutionary change. We can only observe evolution if we know how the offspring of individually known parents succeed. Furthermore, nearly all finches of the entire island of Daphne Major were individually known. This is unique for a natural population of this size. It shows that not only a subset of a population has been studied, but the entire population.
• There are very strong episodes of selection in the Galápagos islands, caused by both unusually wet (El Niño) and unusually dry (La Niña) weather conditions. These selection episodes were witnessed by Peter and Rosemary, who analyzed the quantitative responses of birds to those events. Thus, although the Grant et al.'s studies are not experimental studies in a strict sense, the environmental perturbations are the equivalent of experimental manipulation.
• The best test to see whether we really understand evolution is if we can predict evolutionary change under a set of environmental conditions. To achieve a prediction, we need to understand how natural selection works. This implies that one has to measure all the ecological details like the force it takes to crack seeds, the availability of seeds in the environment and the like.

Hypotheses:
Boag and Grant examined four hypotheses to explain the evolution of the intermediate beak sizes of G. fortis on Daphne Major Island.

1. Genetic drift and/or founder effect contributed to the intermediate beak sizes.
   
2. Hybridization with the small ground finch (Geospiza fuliginosa), which is also present on the island, contributed to the intermediate beak sizes.
   
3. Local adaptation to food supply was influential in contributing to intermediate beak size.
   
4. Character release in allopatry (i.e., in a situation where only one of several species exists).

Boag and Grant captured (with mistnets) and measured the beaks of hundreds of finches on Daphne and Santa Cruz islands to find out whether the formerly observed character release was still present. Captured birds were banded with individually numbered aluminum and colored plastic rings. Nestlings were banded each year during the breeding season. Ecological parameters like plant cover, seed abundance, seed diversity, feeding behavior, rainfall etc. were measured during the same periods.

Most importantly, the data confirmed that indeed the medium ground finches from Daphne were smaller than the conspecifics from nearby Santa Cruz. With regard to the four hypotheses, Boag and Grant could show that:

1. There was no strict isolation of the Daphne population from the Santa Cruz population, because birds fly between the large (Santa Cruz) and the small island (Daphne). Furthermore, immigrating birds mated and reproduced on Daphne. Although immigrants occur at a very low frequency, immigration is enough to counteract the effects of drift. Thus, genetic drift is unlikely to account for the differences in beak sizes between the two populations of the same species.
   
2. Hybridization does occur between the medium and the small ground finch--between 1.1 and 3.5% of breeding pairs were hybrid pairs. Because the variation in beak size has a genetic component, i.e. beak size is heritable, they expected the hybrid offspring between the medium and the small ground finch to have smaller beaks than pure medium ground finches. However, compared to the large selection pressures exerted by changes in environmental conditions, the influence of hybridization on beak sizes is small. Please also note that hybridization between the medium ground and the cactus finch pulls beak size upwards.
   
3. Natural selection acts through the size of food items that are available. This was the most important factor determining beak size for each population. If there are only large food items available, birds with large beaks can crack them better and thus survive better. To the contrary, if the seed size is very small, then small birds survive better. During the current study, large birds survived better.
   
4. Boag and Grant also found evidence for character release–medium ground finches expanded their diet during periods of food shortages. Boag and Grant could also show that there was indirect competition between the medium ground finch and the cactus finch by determining the time course of diet similarities between different finch species.

In summary, the last three hypotheses probably contribute to the difference in morphology between the Daphne and the St. Cruz island populations.

The main conclusion from this study was that the medium ground finch phenotype "... represents a balance between introgression with G. fuliginosa, selection for larger ... size in dry years and selection for smaller ... size in wet years." The major importance of this study was that the evidence for all hypotheses could be quantified in a natural situation. From these (and many other) results, Rosemary and Peter Grant could go ahead and predict evolutionary change. The importance of a prediction cannot be emphasized enough. A quote from the famous evolutionist J. B. S. Haldane might make this clear (after Grant and Grant 1995): "No scientific theory is worth anything unless it enables us to predict something which is actually going on. Until that is done, theories are a mere game of words, and not such a good game as poetry."

A sample data set is available for this case study. The file is an Excel spreadsheet file.

Exercise:
This exercise will allow you to verify the conclusions arrived at by Boag and Grant. The data file contains representative data from the Boag and Grant study. The following procedures provide a general guide to help you analyze the data. (Bold letters refer to the variables in the data file.)

1. Classify the beak depths (Daphne Island and Santa Cruz Island) into classes that range 0.2 mm (thus, one class would contain all values from 8.01 to 8.2 mm beak depth, the next class all values from 8.21 to 8.4 mm beak depth).
2. Determine the number of animals in each class. Relate this number to the total number of animals (i.e., determine the percentage of animals in each class).
3. Use these values to plot histograms of the beak depths of medium ground finches from Santa Cruz, Daphne birds in 1976, and Daphne birds in 1978 (those individuals that survived, i.e. Survived = 1).
4. Determine the mean beak depth of the parents (Mother Beak and Father Beak) for each offspring in 1976.
5. Plot this mean value (the "midparent" beak depth) on the x-axis against the offspring beak depth on the y-axis.
6. Fit a regression line to those data and determine whether there is a significant relationship. If this is the case, determine the slope of the regression. The slope is a measure of the heritability of the trait 'beak depth' between parents and offspring, reflecting the degree to which resemblance is due to shared genes.

What does your analysis of this data suggest? Does it support the conclusions drawn by Boag and Grant?

Additional exercises:

1. Look at the extremely high values for beak depths for medium ground finches from Daphne. These values exceed the values for the medium ground finches from St. Cruz. Why is this so? Discuss what this could mean for the species concept. Do you think these birds are hybrids between medium and large ground finches? Also consider the fact that different numbers of birds were measured on both islands (see also point 2).
   
2. Determine the coefficient of variation (calculate the standard deviation, multiply by 100 then divided by the mean) for each group of birds. What does it tell you with regard to the force of natural selection between 1976 and 1978?
   
3. Plot the histograms both as percentages and as absolute values. Does this demonstrate the strength of natural selection?
   
4. Plot the means and standard errors of beak depths of birds from St. Cruz, Daphne 1976 and Daphne 1978. If you use two standard errors in each direction (which then is the 95% confidence interval), you are able to estimate whether two means are different at the 0.05% level. As an approximation, when the 95% confidence interval does not overlap with the mean of another value, the two values differ on the 0.05% level. Also compare the absolute difference in beak depth before and after the episode of selection. By how many millimeters does the mean beak depth differ?
   
5. Determine the residuals of the regression of midparent beak depth against offspring beak depth. How large are the residuals and what does the amount of residual variation tell you?