Our example here says the graph below shows beak depth of medium ground finches on Daphne Major in the Galapagos before and after a major drought. The graph displays both beak depth for the population before the drought in white and for the survivors that reproduced after the drought in black. Use the graph to answer the following questions. Alright. Let's analyze this graph.
We see the white bars, and those black bars overlaid and on the y-axis, which represents a count, and on the x-axis, which represents beak depth — the height of the beak where it attaches to the face of the bird. Note that this data comes from Rosemary and Peter Grant, two scientists who spent years on Daphne Major cataloging every single bird and measuring all kinds of traits to track evolution in this population over time. In the legend, it shows the average beak depth for both groups.
Our first question here is, by how much did the average beak size change after the 1977 drought? The average for the breeding population before the drought in 1976 was 9.2 millimeters, and for those who bred after the drought, the average beak depth was 9.9 millimeters. So the calculation would be: 9.9−9.2=0.7 mm. Thus, the beak size change is 0.7 millimeters, showing a small yet documented change in this population.
Next, which birds had higher fitness during the drought? Observing the graph, those birds with deeper beaks appear to have had a better chance of surviving and reproducing. Even though each size class had survivors, the percentage was higher in classes with deeper beaks. Thus, birds with deeper beaks had a higher fitness.
Do you think your answer to the previous question is always true on Daphne Major? From the given information, it's hard to generalize, but the answer is likely no. If deeper beaks were always advantageous, it would be reasonable to expect all birds to have developed deeper beaks over time, yet the beak size before the drought was smaller. Historical populations had smaller beaks.
Finally, what do you think the beak depth of the offspring produced in 1977 will most closely resemble? Considering those that survived and reproduced had deeper beaks and assuming this trait is heritable, their offspring likely had larger beaks. The average beak depth for the offspring was approximately 9.7 millimeters, indicative of a slight evolutionary change towards deeper beaks, driven by natural selection. Even though the change was about half a millimeter, it still represents an evolutionary adaptation.
