Principles of Biology

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From questions 1 and 2 above, the data shows that the mean beak depth for non-survivor is 9.11 mm while that of the survivor medium ground finch is 9.67mm.  This means that on average beak



Lab 10 Analysis of evolution

Purpose of analysis: Practice data analysis and using the information to make a conclusion about the results.

Learning outcomes:

  • Name the genus and species of medium ground finch that the Grants analyzed for more than 40 years
  • Be able to read and interpret data from a graph and draw a conclusion
  • Be able to explain the specific causes of the evolution changes observed in the medium ground finches by the Grant’s research
  • Be able to explain natural selection
  • Be able to explain phenotype and genotype of an organism

Instructions: Read background information below about the medium ground finch research completed by the Grant research team over more than forty year period. Analyze the graphed data provided of the phenotypic traits measured in the medium ground finches before and after a long period of drought (no rain) in the Galápagos Island of Daphne Major. Answer questions that follow. Work as individual student. Submit the completed analysis by November 17, 11PM in Assignment Lab 10. Grading will be based on accuracy of answers.

Background information:

In 1973, scientists Peter and Rosemary Grant began observing and studying finches on several islands in the Galápagos archipelago. They wanted to understand how species change over time and, in particular, how changes in the environment can influence a species’ physical characteristics. As part of their work, the Grants intensively studied the population of medium ground finches (Geospiza fortis) on the island of Daphne Major.

Every year for forty years, the Grants measured the wing length, body mass, and beak depth of hundreds of individual medium ground finches. They focused on these characteristics because they vary widely among individual birds within the same species; for example, some birds in a population will be larger than other birds or have bigger beaks, even though they all belong to the same species. It is normal for heritable traits to vary among individuals in a population because no two individuals, except for twins, are genetically identical. In some cases, individuals with one form of a trait, such as a larger beak, will have a survival advantage over individuals with a different form of the trait, such as a smaller beak. Those advantageous traits may make it more likely for some individuals to survive and produce more offspring, and therefore those traits are more likely to be passed on to the next generation. This process is what Charles Darwin called 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 18-month long drought resulted in the death of more than 80% of the medium ground finches on the island.

Figure 3 shows two graphs of beak depth measurements for the 50 medium ground finches that died in 1977 and did not survive the drought (identified as non-survivors – graph 3A) and the 50 medium ground finches that lived beyond 1977 and survived the drought (identified as survivors – graph 3B). Each graph includes average (mean) beak depth and standard deviation (s) for that group of birds in units of millimeters (mm).

Standard deviation quantifies the amount of variation in a set of measurements. Standard deviation is a measure of how spread out the numbers are. The larger the standard deviation, the more the data points are spread out for a measured characteristic, such as beak depth, in a population.

In the two graphs in Figure 3, the standard deviations are 0.88 mm and 0.84 mm respectively, meaning that most birds in the non-survivor data sample have beak depths that are plus or minus 0.88 mm of the mean and most birds in the survivor data sample have beak depths that are plus or minus 0.84 mm of the mean.

Graph questions

  1. Analyze Figure 3A non-survivor graph for medium ground finches then answer the following questions based the data.
  2. (5 pts) What is the non-survivor mean beak depth (mm)?
  3. (5 pts) What is the non-survivor standard deviation (mm)?
  4. (10 pts) What are the beak depths that occur most frequently? (Hint: Refer to the y-axis for the number of birds for each data bar presented in graph).


  1. Analyze Figure 3B survivor graph for medium ground finches and answer the following questions based on the graph.
  2. (5 pts) What is the survivor mean beak depth (mm)?
  3. (5 pts) What is the survivor standard deviation (mm)?
  4. (10 pts) What are the beak depths that occur most frequently?
  5. (10 pts) Review your answers to questions 1 and 2. What does this data show for the survivors compared to the non-survivors of the drought of the mean beak depth in the medium ground finch?
  6. (10 pts) What happened that appears to have caused the change in beak depth? Be specific with cause and its effect on the food supply.
  7. (10 pts) Does the evidence shown in the graphs of Figure 3 support or not support the mechanism of evolution by natural selection in this species? If needed refer to Module 11 to review the concept of natural selection. Explain your thoughts about this evidence.
  8. (15 pts) Name the genus and species of the medium ground finch that live on this island. Correct spelling and capitalization are counted.


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