Natural Selection Natural Selection

Natural Selection Natural Selection By definition, genetic variations constantly occur within populations. This makes some members of the populations to be better suited to the prevailing environmental conditions. As a result, such better suited populations adapt faster to the environment, while the members with lower adaptability are selected against through the process of natural selection. In Drosophila pseudoobscura, heterozygosity and homozygosity is a common occurrence on the third chromosome. In natural settings, variations in the constitutions of the genetic makeup of the organisms vary from one locality to another. Evidently, the process of natural selection has been shown to play a role in seasonal variations in homozygotes and heterozygotes. To this end, it has been ascertained that carriers for different chromosomal variations occur at different frequencies at different seasons. Such genetic constitutions can be passed on to offsprings. This leads to some offsprings that are better suited to environmental disparities, while others are least suited to prevailing environmental conditions.

In this article, the variations in seasonal adaptability among natural populations of Drosophila pseudoobscura are analyzed. Basically, the article unveils a study targeted towards determination of the impact of the gene combinations on the third chromosome of Drosophila pseudoobscura in their adaptation to varied environmental setup. It aims to show that natural selection plays an integral role in determining the survival of organisms with specific gene combinations. The study was prompted by the observations that seasonal variations occurred within the populations of the organisms. More specifically, it was observed that while certain seasons favored heterozygotes, other seasons were more favorable to homozygotes.

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To test the hypothesis to ascertain the heritability of seasonal adaptation, the research uses experimental designs which involved parent-offspring comparisons under artificial environment, in a population cage. Through this comparison, the study was able to show differential death rates with inversion heterozygotes more favored than homozygotes. Further, this study involved use of specifically selected eggs representative of heterozygote or homozygote states exclusively. Such eggs were produced through well-programmed breeding. Despite the conduct of the experiment in artificial setting, the study confidently outlines that the determination of the artificial experiment is an accurate account of the developments in natural habitat. It affirms that frequencies of specific gene arrangements are under direct selective pressure. The determination of the influence of natural selection on the trait also involved calculations of heritability. To produce pure heterozygotes and homozygotes, the study employed inbreeding, with siblings being mate immediately after the emergence from the pupae. However, assertive mating and random population differentiation was also carried out. Nevertheless, the study does not specify the number of generations used in the analysis.

In summary, the article is able to point out that the survival of populations of Drosophila pseudoobscura in their wild setting is subject to selection pressure. However, the nature of the variation has been shown to vary with sex of the fly. For instance, the study is able to experimentally show that the adaptability of adult male flies to the environment was deviant from the normal binomial square rule. In this respect, the article puts it across that the homozygotes are less compared to heterozygous variants. Above all, the study concludes that chromosomal variation observed in the third chromosome is due to natural selection, forcing better suited species to adapt and reproduce while least suited species perish.


Dobzhansky, T., & Levene, H. (1948). Genetics of natural populations. XVII. Proof of operation of natural selection in wild populations of Drosophila pseudoobscura. Columbia University, New York.

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