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Biology 301
12/3/2012
The Effects of Relative Fitness of Drosophila Melanogaster on Evolution

Abstract
This experiment was conducted to study the relative fitness of two phenotypes of the Drosophila melanogaster and how fitness can affect evolution in the population. The phenotypes were placed in two different environments, one in which contained a predator and another with no predator. Results of the experiment would show how the fitness of each phenotype is affected by providing a mechanism, and if evolution was occurring in the population. Two hypotheses were inferred, one for each environment. For the cage uninfluenced by a predator, we hypothesized that evolution would occur due to sexual selection, and that sexual selection would be in favor of the wild-type drosophila. For the cage containing the predator, we hypothesized that the vestigial flies would have a higher relative fitness due to natural selection. A ratio of wild-type to vestigial flies was determined, and was set up in each environment. 10 wild-type to 40 vestigial flies was chosen, giving a total of 50 flies for each environment. Each week the flies were fed, and every two weeks they were counted to represent a new generation. At the end of the 13 week experiment, the last generation of flies were counted and recorded in a data table. The results of the experiment show that evolution was occurring in both cages, and that wild type flies were dominant regardless of the environment. Introduction

The species studied was the Drosophila melanogaster, or more commonly known as the fruit fly. This experiment was conducted to see how the relative fitness of two different phenotypes of the Drosophila melanogaster can affect evolution in the population of two different environments. The relative fitness of an organism is the ability of that organism to both survive and reproduce in its environment. A value of 1 is assigned to an individual is who is best suited for its environment, and all other individuals are assigned a number less than 1 (Kundapur 2008). If a species reaches a relative fitness level of 1, then the population is undergoing fixation, and all other species would be extinct. If no evolution was occurring in the population, then it would be undergoing Hardy- Weinberg equilibrium. The Hardy- Weinberg principle states that both the allele and genotype frequencies remain constant, for each generation, unless disturbances occur in the population. For this transpire, evolution, the change in the inherited characteristics of a population over time, cannot be present (Ko 2007). If evolution is not occurring, then after each generation, the allele frequencies and genotypes of the population would be the same. This means that the ratio of the beginning generation of species must be consistent with the ratio of species to infinite generations. The two phenotypes of Drosophila used in the experiment were wild-type and vestigial. The wild-type flies, possess the dominant allele, have fully grown wings, and aren’t inhibited to flight. Vestigial flies, which are homozygous recessive, have a mutation to their wings in which they are unable to fly. Males are relatively smaller than female flies and have a small black patch at the end of their abdomen (Santos et al 1997). This is important to know when separating flies, to ensure not all one gender is present in a population, and reproduction is able to occur. Fruit flies have a unique way in which they mate. The male fly, orients its body towards the female and sings for the female fly by spreading its wings and vibrating them rapidly (Kundapur 2008). This would mean if a wild-type fly mates with a female, the offspring will be wild-type, as well as for vestigial. The experiment would consist of 50 flies in a ratio of 10 wild-type to 40 vestigial flies. Half of each type would be male and the other half female. Two environments would be considered in the experiment, one containing a predator and the...
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