A model system is a convenient way of showing what will happen in real life without going through the time and trouble of observing what happens in the wild. In this exercise, a model pond was filled with items representing variable food types to see the effect beak length has on a wading bird’s chance of survival. The model showed that a short beak meant death for a bird, and that a normal length beak (wild type) and a long beak meant survival and the production of multiple offspring for the bird in this environment.
Natural selection is a driving force created by an environment that is exerted an individual to test its ability to survive and yield offspring (Purves, 2004). The individuals that are best suited for the environment, or most fit, will most likely succeed by living and producing offspring (Purves, 2004). The individuals that are worst suited for the environment, or least fit, will most likely die and not produce any offspring (Purves, 2004). The theory of natural selection can be represented and tested by using a model system to see what might happen in the real world. When observing what happens in real life, it could take years to gather enough data to decipher what is happening. A model system is convenient because it can predict what will happen in a short amount of time. For this lab, a model system was devised to show the effects of mutation on natural selection. A model pond was filled with items representing variable food types to see the effect a bird’s beak length has on its chance of survival.
Materials and Methods
A group containing three members was formed. Each group member was to represent an individual wading bird within a population of 100 birds. Groups were supplied with a bucket filled about two thirds of the way full with tap water and items representing three different food types. The first food floated on top of the water, the second food was suspended in the middle of the water, and the last food sunk to the bottom of the water. To represent the beak of a bird, groups were supplied with two tongue depressors rubber-banded together at one end with a screw inbetween to serve as a pivot. The first beak used was denoted as the wild type and was the standard length of the tongue depressors. Each group member individually took three 15 second trials to pick up as much food as they could with the wild type beak. Food was placed back into the bucket after every trial. The counts of food “eaten” were recorded on the data sheet.
Each group member was then randomly assigned an offspring from the wild type parents with a mutation which affected beak size. One member was assigned a short beak, which was about half the size of the standard tongue depressors. A second member was assigned a long beak, which was about the size of two standard tongue depressors. The last member was assigned the wild type beak. Three more 15 seconds trials were performed, but this time the group did the trial together to represent competition. Food was placed back in the bucket after each trial. Finally, food counts for each member were again recorded on the data sheet.
In regards to the three 15 second trials with the wild type beak, the first group member averaged 11.7 total pieces (Appendix I). The second group member averaged 12 pieces of food, and the third member averaged 12.3 pieces of food (Appendix I). The group average was 12 pieces of food (Appendix I). Each member averaged 8 pieces of the food that floated at the top of the water, 3.9 pieces of the food that was suspended in the water, and 0.1 pieces of the food that sunk to the bottom (Appendix I).
In regards to the three 15 second trials when all three members competed for food, the wild type beak averaged a total of 6.3 pieces of food (Appendix I). Of those 6.3 pieces of food, the wild type beak averaged 3 pieces of the food that floated and 3.3 pieces of the food suspended in the water (Appendix I). The long beak averaged a total of 8 pieces of food (Appendix I). Of those 8 pieces of food, the long beak averaged 2 pieces of the food that floated, 4.3 pieces of the food that was suspended in the water, and 1.7 pieces of the food that sunk to the bottom. Finally, the short beak averaged 3 pieces of food (Appendix I). Of those 3 pieces, the short beak’s diet only consisted of the food that floated at the top (Appendix I). Using the guidelines for determining offspring survival provided in the lab manual, it was determined that the long beak and wild type beak both survived and produced four offspring, while the short beak died (Appendix I, Snetselaar et al., 2007).
This experiment simulated how natural selection can affect the real world. As shown through the offspring with mutations, beak size had a great affect on their chances of survival. The most dramatic demonstration of this was the short beak. It was only able to eat the food at that floated on top of the water. When all the food was gone, there was nothing left for it to eat. This was a big disadvantage for it because the other beaks could not only eat the food that floated, but they could also eat the food that was suspended in the water and the food that sunk to the bottom. The short beak was the least fit for this environment and it was determined through calculations that it would have died.
The long beak seemed to have the biggest advantage. Our long beak was the most fit out of our three variations of beaks. The long beak had a big advantage because it was able to get all three types of food. It was the only one that could reach the food that sunk to the bottom, so once the other two types of food were all eaten, it had the remainder of the food to itself. It had no competition for the food that sunk. The long beak averaged the most amount of food per trial and it was determined through calculations that it survived and produced four offspring.
The wild-type beak also fared well in the environment. It could eat both the food that floated and the food that was suspended in the water. It could not eat the food that sunk to the bottom. While it could not eat the food that sunk, its beak was well suited for eating the other two types of food. The wild type’s beak was also well suited for the environment, and it was determined through calculations that it survived and produced four offspring.
This simulation demonstrated natural selection by showing which beak types were fittest and could survive in the environment. The short beak was least fit because it obtained the least amount of food. The lack of food led to its death. The wild type and long beak were most fit because they obtained the most food. They both survived and produced four offspring. This simulation was much like the real world where there are numerous birds with very slight variations in beak size and shape. The birds of the world find niches in the environment where their beak shape and size is best suited to feed. Mutations in beak size will either make a bird more fit or less fit. The most fit birds will have the best chance of survival and passing on their genes to their offspring, which will have beak shapes and sizes comparable to them. Over time, the alleles for less fit beaks will become less common, but changes in the environment could make the once less fit beak become a fit beak. There is constant struggle for survival, so any small change in the environment can affect which alleles are favored.
One thing we did not test in our simulation is varying the amounts of the different foods. If we were to do this, there may have been different results. For example, if there was very little of the food that floated and food that was suspended in the water, but an abundance of the food that sunk to the bottom, the long beaked bird would probably be the only bird to survive. If there was an abundance of food that floated and a normal amount of the other two foods, the short beak may have had a chance of surviving. Also, the different kinds of food could have different nutritional values, which would mean eating a lot of one type of food that is low in nutrition does not necessarily mean survival. The exercise we performed did not reveal these kinds of things, but it did show generally how mutations are affected by natural selection.
Purves, William K., et al. 2004. Life: The Science of Biology (7th Ed). (Courier Companies Inc., USA).
Snetselaar, Karen M., Jonathan Fingerut, and Joseph T. Thompson. 2007. Biology III Organismic Biology: Laboratory Manual Fall 2007. (Biology Department, Saint Joseph’s University, Philadelphia, PA).