The experiment addresses the interaction between natural selection and genetic drift for several generations in a short period of time. D. melanogaster, the fruit fly, is a commonly used model organism for studying the role of genetic and environmental factors in action. Two small and two large independent populations of D. melanogaster were each split into a light and dark treatment, crossed, and monitored until three generations have passed. After each generation, the frequency of the white-eyed and red-eyed phenotype were recorded to estimate how the allele frequencies varied over time. An allele frequency that reached 0 for both light treatments and a higher proportion of extinction in the N=12 population supports the hypothesis that selection was strongest in the large light population and signs of genetic drift influence are shown in the dark population. The study of insects in biology has always played an important role. It is relatively difficult to illustrate concepts for evolution in a short period, which is why D. melanogaster is commonly used in laboratories. Characteristics such as: a short and easily manipulated life cycle, inexpensive to rear thousands of individuals in laboratory, and simplified genome is what makes D. melanogaster an ideal candidate to study evolution (Nguyen 2018). There are four distinct stages in the life cycle of the D. melanogaster: egg, larvae, pupa, and adult (Nguyen 2018). During the pupa stage, metamorphosis occurs and can last from 4-5 days. This stage ends when a mature adult emerges from the pupal case, and becomes sexually mature in 6-8 hours. Therefore, the flies are collected within 6 hours after emerging from the pupal stage, so that we are certain the flies are virgins and can be used in our own genetic crossing scheme (Nguyen 2018). The original stock of flies will all be homozygous, either red-eye or white-eye phenotype, and true-breeding. The red-eye phenotype, wt, is the wild type and dominant to the white-eye, w-, phenotype which is the mutant type. The conditions in the experiment are separated between two populations, a small population of 12 and large population of 40, and two treatments, a light and dark environment. The purpose of this study is to perform and evaluate a long-term experiment to illustrate evolution using D. melanogaster under different treatments of selection strength and population size. There is a smaller and larger population of flies to observe the effect of genetic drift. Genetic drift is a mechanism that can lead to evolution, where stochastic factors that have no influence on fitness cause the allele frequency in one generation to not exactly equal the allele frequency in the previous generation (Nguyen 2018). It’s the result of the random sampling of alleles at each generation, and its effects are most prevalent in smaller populations. Therefore, genetic drift does not favor one allele over the other. Tribolium Confusum, the flour beetle, is another organism used to study the effects of genetic drift in laboratories. The beetles have a body color of either black or brown, governed by two alleles at a single gene locus (Rich 1979). The experiment set up populations of 10, 20, 50 and 100 with equal ratio of black and brown beetles. Subsequently, a change in allele frequencies occurred in all population sizes, but the smallest population of 10 displayed the most extreme fluctuations (Rich 1979). The two different environments, a light and dark cycle, is needed to observe the effects of natural selection. Natural selection is another mechanism that can lead to evolution, where differences in individual fitness cause some organisms to outcompete others. Natural selection acts to increase the frequency alleles that give organisms higher fitness. For natural selection to act there must be a variation in fitness within a population that is heritable (Nguyen 2018). Unlike genetic drift, an organism within a population will have higher fitness than others; allowing their offspring to inherit one allele that consistently experiences greater success and become more prevalent. Both mechanisms will eventually cause one allele to reach fixation the other to extinction (Gillespie 1985). If genetic drift has a greater effect in small populations, then I predict in the dark treatments the wt and w- allele will experience a random distribution of fixation. If selection acts against the white-eyed phenotype, then I predict that the light treatment selection will be the strongest in the large population, and the wt allele will reach fixation while the w- allele to goes extinct. When looking at figure 1, the w- allele frequency is decreasing over time in various rates in each of the treatments. The N=40 LIGHT treatment decreased the fastest, the N=12 LIGHT treatment also decreased but not as fast compared to the N=40 LIGHT treatment. Both dark treatments decreased at a slower rate, but the N=40 DARK decreased the slowest. Figure 1 also shows the standard deviations of all four treatments in all generations can be seen. The standard deviation in G3 is 0.27 in N=12 LIGHT, 0.32 in N=12 DARK, 0.17 in N=40 LIGHT, and 0.38 in N=40 DARK. In figure 2, the w- allele frequency declined to 0 from G1 to G2 in the N=12 LIGHT and N=40 LIGHT treatment. The N=12 DARK treatment spiked, went extinct, and then increased again. While the N=40 DARK treatment spiked in G1 and slowly declined in the next two generations, but did not reach 0. In figure 1 it can be interpreted that the allele frequency of the w- allele decreases over time in all four treatments. The light treatment for both the small and large population decrease at a faster rate, while the dark treatment for both the small and large population decrease at a slower rate. The decrease is because white mutant flies are optomotor blind but are also blinded by the light in daylight (Krstic 2013). The decrease in the w- allele can also be due to the w- mutant allele on courtship behavior. The white-eyed male flies display bisexual behavior, leading to the reproduction of little or no offspring in the next generation is suggested to be a direct result of the mutant allele (An 2000).
While looking at figure 2, it illustrates that natural selection has a strong impact in the light populations, and little impact in the dark population. Both the N=12 LIGHT and N=40 LIGHT treatments experienced a substantial drop after siring generation 1. Therefore, selection acted the strongest in both light treatments, because according to table 1, the allele frequency of the w- allele reached 0 meaning both the N=12 LIGHT and N=40 LIGHT went extinct. Meaning the wt allele frequency increased, allowing the red-eyed flies to have higher fitness. Another reason the w- allele went extinct can be caused by the over-flow of light and over-excitation of photoreceptors, suppressing their sexual arousal (Krstic 2013). However, selection acted the weakest in the N=40 DARK treatment, because both wt and w- allele survived in the last generation. Though the N=12 DARK and N=40 DARK treatment did not go extinct, the N=12 DARK treatment increased tremendously in generation 1, went extinct in generation 2, and then increased again in generation 3. While the N=40 DARK treatment increased in generation 1, and slowly declined in the next two generations. Due to the sudden increase and decrease in allele frequency of the w- allele in the N=12 DARK, this is the result of random sampling from generation to generation; demonstrating how genetic drift is most prevalent in smaller populations.
The allele frequencies of the w- allele from each generation was calculated for both population sizes and both treatments, were meant to prove whether the hypotheses would be rejected or accepted. In this case, I predicted genetic drift would be strongest in the both dark treatments, and selection is strongest in the N=40 LIGHT treatment where the w- allele will go extinct and the wt allele will reach fixation. The w- allele did not reach fixation and went extinct selection was higher for the wt allele. The allele frequency of 0 in both light treatments, and the random change in frequencies in the N=12 DARK and N=40 DARK treatment support my hypothesis.
An experimental error occurred when siring from generation 1to generation 2, because the males in the N=40 LIGHT treatment experienced a genetic bottleneck. There weren’t enough males when twenty flies were needed. And a difficulty that existed throughout the experiment, was anesthetizing the flies with just the right amount of FlyNap so that they able to wake up and mate for the next generation. An improvement to this difficulty would be to present FlyNap in another form, CO2, where it is simple and efficient (An 2000). An improvement to this experiment can be observing the flies’ flight performance with a change in their wing shape. No FlyNap would be needed, and natural selection and genetic drift could still vary depending on wing shape.
The overall experiment shows how natural selection and genetic drift interact with one another. Natural selection and genetic drift do not act on its own. Instead they work against each other to slow the fixation or extinction of an allele.