Two of the key driving factors of evolution are natural selection and sexual selection. These make up two of the five mechanisms of evolution. Natural selection can be defined as the growth of a species of organism over time by the fittest surviving and the least fit dying off, and when discovered in 1835 by Charles Darwin, it marked a turning point in evolutionary thought. Before Darwin, there were only two theories behind evolution, neither of which were certain. Their basis rested mainly on assumption, and there had been little examination of their truth. False beliefs based on assumptions remained popular thought until Darwin studied finches and their beak size in the Galapagos Islands. Darwin discovered that on the island there were many of the same types of birds with different beak sizes. He later went on to discover that this was due to evolution and natural selection. The birds would develop different beak sizes due to their unique location on the islands and would adapt to their surroundings. For example, the large ground finch has the largest beak size of Darwin’s finches due to it needing to crack seeds and nuts for food. Thanks to Darwin, there is now a better understanding of evolution and natural selection today.
When selecting a mate, there are certain features or traits that make a female select a certain male over others. This is called sexual selection. Some examples of sexual selection could be colorful fins on fish. Another common example is that of Drosophila melanogaster (commonly referred to as Drosophila). Drosophila can be born as wild-type flies with wings or born as vestigial flies without wings. Drosophila males born with wings (wild type) can do a mating dance toward females that increases their chances of mating. Vestigial Drosophila, born without wings, are unable to do this mating dance, which drastically reduces their odds of mating.
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Evolution is vital because it is what allows organisms to adapt to their surroundings and become more fit to survive in their particular environment. Evolution is also important in understanding how organisms change and adapt to their environment. Knowing previous patterns in an organism’s evolution can allow scientists to predict the future and know how to respond to certain situations.
The purpose of this experiment is to determine if evolution will occur among wild-type and vestigial Drosophila melanogaster under natural and sexual selection. The goal is to see how these two mechanisms affect the allele frequency over a 10-week time period. Over this 10-week time period, it is expected that wild-type Drosophila with wings will be sexually selected over vestigial Drosophila without wings because males will able to do the mating dance and attract a female mate. We can test this by having two crates with 100 Drosophila each. 20 will be wild type (.2 frequency) and 80 will be vestigial (.8 frequency). One cage will contain a simulated predator to catch the wild-type flies, making it so that there are greater odds a female will mate with a vestigial Drosophila. For the cage without a simulated predator, I hypothesize that if Drosophila melanogaster is kept in a cage with no simulated predator, then wild-type flies with the dominant trait for wings will have a better chance to mate. This could be due to the evolutionary mechanism of sexual selection because, with wings, wild-type flies can perform a mating dance that will allow them to find a mate more easily than vestigial flies. For the cage with a simulated predator, I hypothesized that if Drosophila melanogaster is kept in a cage with a simulated predator, then wild-type flies with the dominant trait for wings will be captured (killed) and will no longer be able to mate, thus giving vestigial flies without wings a better chance to mate. This could be due to the evolutionary mechanism of natural selection. With wild-type flies being captured, vestigial flies will become the fittest and thus more likely to find a mate.
Materials and Methods
Two cages were set up during the first lab to hold flies (Drosophila melanogaster). The first cage was set up with fly paper inside of it to capture flies with wings that could fly. This acted as a simulated predator, and when a fly was captured, it could no longer reproduce so it was considered dead. The second cage did not have a simulated predator. Each cage was marked as a cage with/without a simulated predator to tell the difference when collecting data. Drosophila food was set up for each cage. In each cage, 20 wild-type flies with large wings and 80 vestigial flies with small/no wings were released. Taking turns every other lab on Wednesdays, one of the four lab groups would count flies after class, recording their data on a random sample of flies gathered. CO2 was used to anesthetize the flies and make them easier to count. A feather was used when sorting the flies while doing counts to ensure no harm was done to the flies. Each group recorded how many wild-type flies and vestigial flies were in the sample collected from each cage when it was their turn. This experiment lasted 10 weeks, but no counts were made during the fifth week when students were on spring break. Using a PopGen simulator, a graph was created in Microsoft Excel of what different expected population sizes would look like. Ten weeks later, after the completion of the experiment, more graphs and tables were made using Excel to analyze the data. A chi-square statistical analysis test was performed to find the p-values for each cage. Wild-type flies were given a .2 frequency and vestigial flies were given a .8 frequency (20 wild-type, 80 vestigial). The p-values were determined to accept/refute the two hypotheses. “If flies have the dominant allele for wings, then they will have a better chance to mate” – is the hypothesis being tested for the cage with no simulated predator. “If flies with the dominant allele for wings are caught in the fly trap, then they will no longer be able to mate, and the vestigial flies with the recessive allele will have a better chance to mate” – is the hypothesis being tested for the cage with a simulated predator.
Results
Average allele frequencies between both cages throughout the ten-week period were compared. Both cages started with the same frequencies, but as time passed, the cage without a simulated predator began to show higher dominant allele frequencies, and these allele frequencies would continue to rise until the end of the ten-week experiment. In the cage with a simulated predator, dominant allele frequencies rose from their original frequencies, but began to settle near the fourth week at around a .4 value. T-tests were performed to find p-values representing how close our actual results came to our expected results. If a p-value gives a value of < .05, then there is a significant difference. If a p-value gives a value of > .05, then there is no significant difference.
Discussion
The expected model to compare with the cage with no simulated predator was model 1, and the expected model to compare with the cage with a simulated predator was model 3. The cage without a simulated predator had a p-value of 3.58351E-06. This p-value is less than .05, indicating a significant difference and implying that in the cage with no simulated predator, there was indeed evolution. The cage with a simulated predator had a p-value of .0564, which is greater than .05. This implies that there was no significant difference and evolution did not occur. As stated earlier, the predicted model for the cage with a simulated predator was model 3. Model 3 suggests that there is no change in the allele frequencies from start to finish. Perhaps a better model to use to represent this cage could have been model 2. Model 2 suggests that the final WAA is 1 and Waa is .8. This is more accurate to our results as the final dominant allele frequency for a simulated predator population was about .37. The p-value to determine if the cages differed was .000847, which is less than .05, suggesting that there was a significant difference. This was expected because with one cage having a predator and one not, it would not be expected for the two populations to evolve in the same way. The p-value found between the non-predator cage and the predicted model (model 1) was .6718, which is well above .05. This suggests that there was no significant difference between our actual results and expected results, meaning that model 1 was a good representative of what would actually happen in the experiment. The p-value found between the predator cage and the predicted model (model 3) was .345, which is greater than .05, suggesting that model 3 was also a good representative of what the actual results would look like in this experiment.
The hypothesis for the non-predator cage was accepted. The hypothesis for the simulated predator cage was also accepted. The two hypotheses were accepted because both p-values were greater than .05, meaning that, according to the data, the chosen model for each cage was an accurate representation of what happened. This makes sense for the non-predator cage because if flies have the dominant allele for the wing trait, then they will have wings and be able to perform the mating dance which helps them attract a mate. This also makes sense for the simulated predator cage because the simulated predator was placed in the air so that it would only catch wild-type Drosophila that could fly. If a significant portion of wild-type Drosophila had been captured, then female Drosophila would have less of a choice in their mating partner. In this case, the ‘more fit’ flies are those without wings or vestigial. They cannot be captured by predators and survive by natural selection.
Final Thoughts
In the future, more experiments could be conducted that test more mechanisms of evolution, such as gene flow, genetic drift, etc. The cages testing natural selection and sexual selection could be kept, and more cages could be added to test these other mechanisms. Also, a way to possibly make results more accurate is to do counts every week instead of every other week. This way there would be more data to make a more definitive conclusion. The purpose of this experiment was to examine the relative fitness of Drosophila melanogaster and to understand how its fitness can impact its evolution. The research is globally important because it tests the evolutionary mechanisms in species with a fast reproduction rate so that extensive amounts of data can be compiled in quick periods of time. The information found can then be compared to other species to make predictions regarding other species and factors that may affect their evolution. The big picture is in understanding the different evolutionary mechanisms, specifically natural selection and sexual selection, and being able to predict future outcomes based on these criteria. In this experiment, it was important to use a model organism such as Drosophila melanogaster because of its quick generation time, thus allowing us to compile data for an entire generation in just a short amount of time.
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