Jacob Alfaro BIOL 251 Whittier College
Jacob Alfaro BIOL 251 Whittier College

Effect of Ebony and Wild Type Phenotypes on the
Natural Selection and Fitness of Drosophila
November 2015
Effect of Ebony and Wild Type Phenotypes on the
Natural Selection and Fitness of Drosophila
November 2015

INTRODUCTION: There are many different Drosophila phenotypes. This experiment focuses on the ebony and wild type phenotypes. These phenotypes affect color. The ebony flies more dark and the wild type flies are much lighter. The ebony is recessive and therefore carries two copies of the recessive allele, therefore the Drosophila are homozygotes. But wild type is dominant, meaning that heterozygotes and homozygotes represent the wild type phenotype. A phenotype is a physical characteristic as a result of a genotype affected by its environment (Brooker et al, 2011). A genotype is the set of genes of an individual (Brooker et al, 2011). Drosophila are ideal for this experiment because there is an abundance of them, they reproduce quickly, and they have short life cycle (Jennings, 2011).
Natural selection and fitness are related to evolution in a population. Natural selection is when certain traits survive because they are selected for by the environment. These traits enhance or increase survival and reproduction have adapted and learned to function well in their environment with those traits, making them advantageous (Ayala, 1969). This would result in those specific traits expanding in the population in future generations. Fitness is related to the chances of a genotype passing on to the next generation (Brooker et al, 2011). So a certain genotype may be in many individuals in a future population. The higher fitness individuals are the ones with phenotypes that are best compatible with its environment. Their survival is heightened because the individuals have the traits that enable them to survive and reproduce. Therefore, they leave more offspring for the next generation.
We predicted that if the two phenotypes we are testing have equal fitness, we expect that both alleles will continue to thrive in the Drosophila population. This idea falls into the Hardy Weinberg equilibrium. The Hardy Weinberg equilibrium involves the following equation: p2 + 2pq + q2 = 1. This equation measures the allele frequencies in a stable population (Brooker et al, 20110. In order for a population to be in equilibrium, assumptions have to be made. The population cannot have any mutations, no migrations, random mating must take place, the population must be large to avoid sampling errors, and all genotypes have to survive equally (Brooker et al, 2011). Once equilibrium is obtained, all future generations will maintain that equilibrium (Brooker et al, 2011).
We predict that there will be more mating of the heterozygotes with the homozygotes which therefore will result in more Drosophila with the wild type phenotype. The Drosophila that is heterozygous has higher fitness than the Drosophila that is homozygous for wild type and for ebony (Kyriacou et al, 1978). Higher fitness will affect the natural selection of the heterozygotes. One big factor that effects the mating of the Drosophila is partial blindness (Kyriacou et al, 1978). Homozygous males that are partially blind are at a disadvantage because it affects the fitness. It is difficult for the partially blind Drosophila to identify mates, giving the Drosophila with normal sight a better success rate at finding one (Kyriacou et al, 1978). Heterozygotes, which have the wild type phenotype, have normal sight which gives them the advantage of finding a mate and have a better success with attracting a mate with their courtship song (Kyriacou et al, 1978). The male heterozygous Drosophila had a better advantage at mating than the homozygotes, which were the ebony, and therefore we predict there will be more wild type by the end of the experiment.
METHODS: In this experiment, the instructor provided us with 30 ebony individuals and 20 wild type individuals. In order to get an exact amount of each type, we anesthetized the flies and counted them off by gently using a fine point paint brush. Then all 50 Drosophila were put into a population cage which had a lid that had six holes for the centrifuge tubes. Two food tubes and four clean, empty tubes were added on the first day. Each food tube consisted of half a cup full of food mixed with 6-7 milliliters of water. This was the fly medium. The food should turn blue once the water is added. Each tube was labeled with a number and with the date. Every two to three days we added one more food tube until all 6 tubes contained the fly medium. After all 6 tubes were filled, the following days after we exchanged the first food tube with a new food tube. At the end of the experiment, we fed the flies with a total of 8 food tubes. Then the flies were anesthetized, again. At the end of this four week lab, the number of living ebony and wild type Drosophila were recorded. Then the data from all the groups were combined. A spreadsheet was set up by the instructor that showed the phenotype frequencies for ebony and wild type for each date and then the data we calculated was averaged to represent each week. The q and p values were calculated for each date.
Results:
Week | Ebony | Wildtype | 2 | 0.498878 | 0.85 | 3 | 0.482612857 | 0.917826509 | 4 | 0.39243684 | 0.944444478 | 5 | 0.865241348 | 0.53156935 |
Table 1. shows the change in relative fitness of the ebony and wild type alleles for each week.
The wild type have the higher relative fitness until the end of the experiment at week 5. The fitness levels are not balanced between the two phenotypes and therefore it supports that the population is not in Hardy Weinberg equilibrium.
Allele Frequency Over Time
Figure 1. shows the average allele frequency.

Week | Observed | Expected | Observed | expected | 1 | 30 | 30 | 20 | 20 | 2 | 1 | 3 | 4 | 3 | 3 | 5 | 6 | 5 | 6 | 4 | 5 | 8 | 15 | 12 | 5 | 8 | 8 | 12 | 11 | 6 | 129 | 77 | 63 | 115 |
Table 2. shows the average phenotype frequency.
Discussion: The observed population for both the ebony and wild type phenotypes did not meet our expected results, therefore this Drosophila population was not in Hardy Weinberg equilibrium. All five assumptions for a population to be in equilibrium were not all fulfilled. These assumptions were that the population cannot have any mutations, no migrations, random mating must take place, the population must be large to avoid sampling errors, and all genotypes have to survive equally (Brooker et al, 2011). The population size was large when combined with the entire class. But parts of the assumptions were maintained. The short time span of the experiment did not allow for any new mutations to occur and there were also no migrations because the flies were separated in population cages.
In this experiment, we didn’t expect both the ebony and wild type phenotypes to have the same frequency because we assumed they had equal fitness. But the fitness was not equal. The homozygous ebony had a better reproduction rate than the heterozygous wild type Drosophila, which affected the natural selection after the first week.
Our prediction that there would result in more wild type Drosophila than ebony Drosophila was not true. Our data showed that we had more ebony than wild type Drosophila. We predicted otherwise because the heterozygous wild type had the advantage because they had a better chance at finding a mate during the day and night because they had normal vision (Kyriacou et al, 1978). But the homozygous ebony only had a better chance during the day due to the mutation of partial blindness (Kyriacou et al, 1978). But the ebony were able to reproduce more than the wild type.
LITERATURE CITED
Ayala, F.J. 1969. Evolution of fitness. v. rate of evolution of irradiated populations of Drosophila. Proceedings of the National Academy of Sciences of the United States of America, 63: 790-793.
Brooker, R.J., Widmaier, E.P., Graham, L.E., and Stiling, P.D. 2011. Biology (3rd ed). McGraw Hill, New York, NY. pp. 325, 463, 480 – 481, 482 – 483.
Jennings, B.H. 2011. Drosophila – a versatile model in biology and medicine. Materials Today, 14, 190-195. Kyriacou, C.P., Burnet, B. and Connolly, K., 1978. The behavioural basis of overdominance in competitive mating success at the ebony locus of Drosophila melanogaster. Journal of Animal Behavior, 26: 1195-1206.