Effects of Natural Selection and Genetic Drift on Eye Color in Drosophila melanogaster Populations
Madison Cole
University of Houston – Clear Lake
October 26th, 2017
Introduction
Evolution is a naturally occurring process that, through a number of factors, will foster changes in a population that affect the genetic composition of the population. Many factors contribute to evolution, such as natural selection and genetic drift. In Drosophila melanogaster, the fruit fly, a white-eye color is a recessive trait whereas red-eye color is a dominant trait (Welsh, 2011). By using small and large populations of the fruit fly, the effects of natural selection and genetic drift can be seen in regard to frequency of eye color.
Natural selection and genetic drift are two factors that help shape a population. Natural selection promotes diversity in members of a population, whereas genetic drift contributes to loss of diversity in a population. Natural selection is the progression of members in a population that carry specific genotypes that make them better suited to their environment are able to survive longer and reproduce than others in a population, therefore, the succeeding generations carry those specific genotypes. Natural selection is a process by which members select mates based on their desirable traits, with the intention of passing on these desirable traits to progeny. This can allow for later populations to become more physically adapted to their environment and increase likelihood of survival (Urbanelli et. al, 2014). An example of natural selection is show in an experiment conducted by J. Albert C. Uy and Gerald Borgia where two populations of bowerbirds were studied. This study determined that female bowerbirds preferred nests decorated in the same fashion as a male from their population would produce over a male’s nest from different population (Uy et. al, 2000).
Genetic drift is the change of the genotypes of a population over time. This results in the change of allele frequencies over generations. While natural selection is not random, however, genetic drift is random. Where natural selection produces a directional change, genetic drift randomly affects a population. Genetic drift is shown to “have significant effects on small populations” (Hallatschek et. al 2007). Genetic drift can have either a negative or positive impact on a population. A negative impact could be that if a population were to get separated into half, one half of the population could have less desirable traits which could lead to these less desirable traits being passed on to progenies or even the population half dying out (Urbanelli et. al 2007).
Based on past studies of Drosophilia melanogaster in regard to natural selection and genetic drift, one can conclude that eye-color allele frequency will change over time. In terms of red-eye vs. white-eye color, the dominant allele will eventually be the most commonly seen phenotype in the population over the recessive allele. Therefore, at the end of the study, more red-eyed flies will be seen over white-eyed flies. Natural selection will be more easily observed in the large population whereas genetic drift will be clearly seen in the small population.
Methods
The fruit fly, Drosophila melanogaster, was an appropriate test subject as eye color is an easily identifiable physical trait in this organism. Eye color is an X-linked trait, meaning that eye color trait is determined by the female fly who has two copies of the gene as opposed to the male fly who has only one copy of the gene (Welsh, 2011). Determining gender, or “sexing”, is accomplished more definitively by using a small test subject such as the fruit fly. The life cycle of D. melanogaster is short and their culture media which they feed from is inexpensive and easily prepared. The flies used for setting up the initial populations are “true-breeding” flies or flies that are homozygous for either the white-eye or the red-eye trait (Welsh, 2011).
The following methods have been adapted from the Lab Manual by E. Welsh 2011. True-breeding Drosophila melanogaster were separated into two F0 populations. The small F0 population contained eight flies, two red and two white-eyed males and two red and two white-eyed females. The F0 large population contained forty flies, ten red and ten white-eyed males and ten red and ten white-eyed females. Both populations had a culture media that contained Formula 4-24 Instant Drosophila Medium. The populations were given a week to breed and afterwards, the parents from each population were removed in order to prevent inbreeding between progeny and their parents. The next generation, F1, were obtained from the larvae made from the F0 populations. Once the larvae hatched, the flies from each population were made unconscious by placing them in a transfer vial on ice. The flies were observed underneath a microscope and separated according to sex. The F1 populations were made by randomly selecting four males and four females from the small population and twenty males and twenty females for the large population. The flies were then placed in new culture vials containing a fresh medium. The eye color of each fly was recorded for data purposes. This process was repeated over the course of three generations of flies.
The allele frequency for red and white-eyed flies in both small and large populations were calculated using the Chi Square Test. The P value used was less than 0.05 with three degrees of freedom. After three generations had been completed, the eye colors and corresponding sexes of at least 100 flies from the small population and 250 flies from the large population had been assessed and recorded. From this information, the mean allele frequencies, mean heterozygosity values and Fixation index (Fst) values for both population could be calculated.
Results
In both small and large populations of Drosophila melanogaster, frequencies of the white-eye and red-eye phenotypes shifted over three generations. In the small population, the allele frequencies for the dominant allele, red-eye color, decreased slightly and then increased over the three generations while the allele frequencies for the recessive allele, white-eye color, decreased over the three generations (Figure 1). The Chi-Square value was 44.524 with three degrees of freedom (Table 1).
In the large population, the allele frequency for the dominant allele increased, decreased and then increased whereas the allele frequency for the recessive allele decreased, increased and then decreased again (Figure 2). The Chi Square value was 33.601 with three degrees of freedom (Table 2).
The heterozygosity value increased over the three generations in both small and large populations (Figure 3) and the fixation index value decreased in the both small and large populations (Figure 4).
Discussion
The theory of evolution states that progenies are different from their parental generation (Baraniuk 2015). Over the three generations observed in D. melanogaster, each generations’ mean allele frequency changed showing that each generation was evolving. In both populations, the frequencies of the dominant allele, red eye color, increased whereas the frequencies of the recessive allele decreased. In the small population, the mean allele frequency for the dominant allele, red eye color increased slowly, whereas the mean allele frequency for the recessive allele decreased. In the large population, the mean allele frequencies for the dominant and recessive alleles increased. Both populations had heterozygosity values increasing closer to 0.5, meaning that the number of heterozygotes increased, yielding more genetic diversity in the populations. The small population’s heterozygosity value dropped quickly from F0 to F1, proving that genetic drift occurred in the small population. Therefore, the hypothesis of genetic drift occurring in the small population is accepted. Both populations had fixation index values that were closer to zero, reflecting the increase of heterozygotes, also seen in the increased heterozygosity values. Both populations did not remain in Hardy-Weinberg equilibrium, showing that there was an exchange of genetic variation. Because the populations did not remain in Hardy-Weinberg equilibrium, one can conclude that evolution occurred.
Sub-population 3 for the small population maintained a consistent mean allele frequency through the three generations. Due to sub-population 3 maintaining consistent mean allele frequency, there was no production of heterozygotes. This shows that this specific sub-population somehow stayed in Hardy-Weinberg equilibrium and produced an outlier from the whole population. Sub-population 18 for the large population produced heterozygosity values that were lower than average, meaning this population contained a larger number of homozygotes, showing that no genetic drift occurred.
Through studies done on Drosophila melanogaster, it was found that white-eye flies have difficulty seeing under certain light frequencies whereas the wild-type flies that possess red eyes do not encounter this difficulty because the pigment in their eyes protects them (Cosens et. al 1972). This information would not have been available had scientists not tested these small subjects. The testing of Drosophila can be applied to other areas of science and research. Barbara Jennings states, “Genetic analysis gives essential insight into the role played by individual factors in a given biological process” (Jennings 2011). Due to many ethical concerns related to using humans as test subjects in studies, researchers are limited to using smaller subjects such as Drosophila and mice (Jennings 2011). Observing how genes are passed on through generations leads scientists to understand the role that genes play overall in production of offspring. Because eye color in D. melanogaster is an X-linked trait, observing the changing of eye color over time can help understanding how sex-linked traits and diseases are passed on through generations. Observing and analyzing phenotypic and genotypic changes in populations over a long period of time leads to greater knowledge of how evolution and natural selection occur. Understanding how these processes work will lead to a more of an understanding of humanity as a whole and how we have changed to become the humans we are now.