Natural Selection and Genetic Drift Evolutionary Forces in Drosophila Melogaster

The study aims to investigate model flies known as Drosophila Melogaster to ascertain the effects of natural selection and genetic drift. The population of the sample was categorized into two different groups; large and small. Accordingly, natural selection is gradual changes that occur in an organism resulting in rapid changes among the most reproductive of organisms. On the other hand, genetic drift are changes that take place rapidly modifying the allelomorph occurrence in a group of organisms. The experiment assessed   and recorded the allele frequency of the flies throughout four generations. The study found that both samples registered changes. Nonetheless, the changes took place differently according to the sample size.   The experiment found that the small sample indicated a fixated growth rate, with a high degree of changes in its allele frequency throughout the four generations. This shows that genetic drift force has more influence on smaller population. Conversely, the large population sample indicated high levels of disparities, concluding that natural selection impacts strongly on larger population than small ones. The final assumptions are that all the two forces have an effect on the population although at different rates.

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This experiment adds to the existing understanding of evolutionary forces and its various impacts on different sizes of the living population. The results of this experiment are important in enhancing the application of evolutionary forces in the field of natural conservation and preservation of endangered species. This study is also important in projecting future behaviors of organism, as such can aid in generating remedies to mitigate such behaviors.

The intent of the study is to examine the effects of two evolutionary forces namely, natural selection and genetic drift on small and large population sizes. The expectations of this experiment are that both the natural selection and genetic drift have an impact on the growth of small and large population of   Drosophila Melogaster species however, the rate change varies.

According to Singh and Rhomberg, the experiment on the small population is anticipated to show high changes in the allele frequencies compared to the large population due to the force of genetic drift since it consists of a lesser number of flies.   This signifies that a small population has a lesser number of genetic pools, as such in the case of minor changes or error it corrupts the allele frequency. The experiment anticipates the four generational culture samples to show varying patterns of changes. The reason for this expectation is that small the population has little gene variations hence, high rates of fixation in case of minor changes.

Geer and Green, argues that Due to natural selection forces, the white-eyed phenol type is expected to become more common than the rest, owing to increased power of natural selection force acting on allele frequency of the red-eyed phenotype, which also increases the strength of D. melogaster flies2. The same force decreases the allele frequency of weaker white-eyed characteristics.

The experiment uses the chi-square analysis to find   the difference between the observed and expected number of flies. Such a conclusion would indicate that evolution forces impact on the small population declassifying it from the Hardy-Wendy equilibrium. The outcome of the study indicates the same results as those derived in the earlier experiment. The small population showed significant variations in the allele frequency. The results also indicated high fixation rates, since half of the small sample size showed fixation of the p. allele, on the other hand, only a single culture indicated fixation of the q. allele3. Further, the small population showed variation only in the first generation, however towards the fourth generation, the variation was the same as the large population, hence the observed and expected results of the chi-square are at par with the study’s expectations. Since the observed and expected result matches the expectation of the experiment, it is proof that both genetic drift and natural selection are acting on the small sample size population. However, the genetic drift has more effect on the small population owing to its small genetic pool that can easily be corrupted by random errors, thus the more the natural selection effects on the small population, the higher the frequency of the red-eyed flies.

Due to genetic drift forces, the large population is anticipated to indicate changes in the allele frequency throughout the four generations. The reason for this anticipation is that large population has large numbers of individual flies; as such, it consists of a large genetic pool, able to resist any random errors that might occur. The outcome is expected to show less fixation rate in the large population compared to the small population, since there is more variation in the big gene pool compared to that in the small population. Further, fixation is expected to reduce in the large population because any marginal errors will not alter the allele frequency3. In the large population the expectation is that, the red-eyed phenol type will be more common in the preceding generations due to natural selection. Here, natural section increases the frequency of red-eyed phenotype, this characteristics also increases the general fitness of the flies under experiment, while decreasing the fitness of the white eyed type. Using the Chi-square test, the results are expected to indicate significant difference between the observed and expected number of flies. The outcome also indicates that evolution is taking place; as such, the population would not be classified under the Hardy-Wendy Equilibrium.

The results of the an earlier experiment were achieved showing variations in the allele frequency o f the large population, further, the large population showed low fixation index since four generation indicated fixation of the p. allele ( no generation indicated fixation of the q. allele)2. In addition, the fixation trend of p. allele towards fixation was more obvious. The results also showed decreased variation in the initial generation, however, towards the fourth generation, the variation matches that of the small pollution, a summation that matches the expectation of the experiment. The chi-square test also attests to the similarities between observed and expected results.

The increase in the number of the red-eyed flies in both the large and small population sample indicates that evolution was taking place. According to Geer & Green2, since the red-eyed flies have more red pigmentation in the eyes, it allows them to have a first choice over their mates, the white ones. Here, natural selection act by increasing the frequency of stronger allele while decreasing the strength of frequency of weaker allele4. The anticipated variations in the large population are higher throughout the experiment. This indicates that the large population has a more diverse variation to act on large population than in the small populations, which also proof that natural selection acts more on large population than in small populations.

Notable experimental errors occurred in terms of accuracy sex classification. This might have resulted in classifying one type of sex in one population more than another, leading to biased results. Secondly, some of the flies escaped during transfer and sexing, leading to an imbalance of males and females in each of the population. Thirdly, the group’s fourth generation culture got lost affecting the results of group 6.



In conclusion, perhaps future experiments should consider making procedural improvements in this experiment, for instance setting the experiments in different timeframes for phenotype testing such as 100 generations or more. Nonetheless, this experiment found that evolution process act on all organisms, altering their genotype and phenotype over given species’ generation timeframe. The experiment was instrumental in demonstrating how such changes take place. It also illustrated how scientist can observe such changes and behaviors of a given organism in its natural environment or altered one. This research is important in adding to the existing body of knowledge about the theories of evolution and their application. Such knowledge can be instrumental in generating models that can be a remedy for   predicting various populations’ phenotypes characteristics. It can also be used to predict behaviors and size of a population in given environmental conditions. In the course of scientific innovations, evolutionary models can be used in the analysis of future generational changes that remedy endangered species. Such a step will be pivotal in wildlife conservation as well as tracing the changing nature of harmful species.

Cited References

  1. Singh and Rhomberg,L.. A comprehensive Study of Genetic Variation In Natural Populations of Drosophila Melanogaster. Genetics, 1987; 117(2), 255-271.
  2. Geer B. and M. Green. Genotype, phenotype and Mating Behavior of Drosophila Melanogaster. The American Naturalist, 1962; 96 (888): 175-181
  3. Glinka, S., Ometto,L, Mousset,S., Stephen, W.,& De Lorenzo,D. Demography and Natural Seletion have Shaped genetic Variation in Drosophila Melanogastr. A multi-Locus approach. Genetics, 2003; 165 (3), 1269-1278.
  4. Luwin,I. Natural Selection in Drosophila Melanogaster under Laboratory Conditions, Evolution. 5(3), 1951; 231-242.


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