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Hannah Kyer
Caitlin Rauchle

                                                                Natural Selection and the Giraffe


The purpose of this experiment is to determine the type of selection pressure involved in the giraffe evolution and to determine whether there is evidence of coevolution taking place.  Fossil records suggest that there were once two giraffe genotypes.  Some giraffes had long necks while others had short necks.  Through natural progression and evolution, natural selection selects for the genotype that is most fit for the environment.  This natural selection often occurs during periods of stress such as droughts and floods.  Only those individuals with characteristics that allow them to survive the period of stress can reproduce and therefore produce offspring with those same genetic characteristics.  Conversely, those individuals that do not have characteristics that allow them to survive in their environment die off and do not have the chance to pass on their genetic characteristics. 

The type of selection pressure taking place in the giraffe evolution model will be determined by creating a feedback loop that shows how two different genotypes of giraffe (long neck and short neck) respond to changes in their environment.  A model is a mathematical tool used to simulate a concept that is not testable in a given time or space.  In the giraffe evolution model, drought conditions lead to a feedback loop for each genotype.  Due to the drought, the acacia trees do not regenerate and so there is less food for the giraffe population.  Because of this, the death rate is higher than the birth rate.  Only one giraffe genotype will have the characteristics that allow them to survive the drought and pass on their traits.  In this lab investigation, the independent variable is the drought.  The dependent variable is the frequency of genetic characteristics in the giraffe population.   

If the long and short-neck genotypes for the giraffe species are tested in a model of their evolution during a time of drought, then directional selection will occur because the long-necked giraffes with be favored and the short-necked giraffes will be eliminated due to the fact that the long-necked giraffes will have sufficient amounts of food because they can reach the leaves on the tall acacia trees while the short-necked giraffes will die off because they can only reach the leaves on the short acacia trees.


To simulate the evolution of the giraffe population, a feedback loop model was created on Stella (see Fig. 1).  Once the feedback loop was created, the number of individuals in each of the four species (long-necked giraffes, short-necked giraffes, tall acacia trees and short acacia trees) was inserted into the Stella model.  Additionally, the birth and death rates for the two giraffe genotypes and the regeneration and consumption rates for each of the acacia tree genotypes were inserted into the model.  The model was then run and the results were put into the form of graphs and data tables.  One graph shows the birth rate of the long and short-neck giraffes over a period of twelve years (see Graph 2).  A second graph shows the populations of the two giraffe genotypes and two acacia genotypes over the same twelve year period (see Graph 1).  Lastly, a data table shows the number of individuals in the two genotypes of the giraffes and the acacia trees each year for the twelve years.  The data table also shows the number of births for the short and long-necked giraffes each year for the twelve years (see Data Table 1).

Discussion and Analysis:

            The results from the giraffe evolution model show that the giraffes with the long necks and the tall acacia trees were naturally selected for and the short-necked giraffes and short acacia trees died off because they were not fit for their environment. More tall acacia trees survived the drought than short acacia trees.  Therefore, the long-necked giraffes were selected for because they had more food to eat because they could reach the leaves on the tall acacia trees.  The short-necked giraffes could not reach the leaves on the tall acacia trees and there were very few short acacia trees due to the drought. 

            Because the long-necked giraffe genotype was favored and the short-necked giraffe genotype was eliminated, the conclusion can be made that the type of selective pressure involved in the giraffe evolution is directional selection.  Due to natural selection, the long-neck trait survived and was able to pass on the long-neck allele because there was enough food for these giraffes to survive.  Alternatively, the short-necked giraffes were eliminated because the short-neck trait prevented them from obtaining a sufficient amount of food in order to survive.  Because the short-necked giraffes died due to lack of food, they were not able to pass on the short-neck allele.  As a result, the long-necked giraffes were naturally selected for.

            It is important to remember that in this evolution model, the giraffe population is very small.  As a result, dramatic steps of evolution occur.  If the population was larger, evolution may occur much more slowly.

Most likely, there are no sources of error because the experiment was conducted using a computer program.  Because of this, all constants were kept unchanged and nothing could have altered the experiment.  The one thing that could have been a source of error is if the data such as birth, death, regeneration and consumption rates were not entered into the flow chart correctly. 

The reliability of this experiment is very high because it was conducted using a computer program. Therefore, as stated in the above paragraph, there are very few to no sources of error. 


The findings from the experiment do support the hypothesis that directional selection will occur in favor of the long-necked giraffes.  The long-necked giraffes survived the drought and were able to pass on the long-neck allele while the short-necked giraffes were eliminated and therefore the short-neck allele was eliminated for the giraffe population.  This conclusion is reliable because there were very few or no sources of error when conducting the experiment.

This lab report that I wrote for AP Environmental Science meets the expectation for improvement in skills and knowledge because this is one of the first lab reports that I wrote using the professional, college level style.  The way this lab report is written is very different from how I had been taught to write lab reports prior to this class. This report also meets the expectation for development and support of conclusions because I had to gather information and data, create a conclusion, and then use evidence to support the conclusion.