Genetic drift & natural selection: acting together to drive evolution

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Throughout the early 20th century there had been constant conflict between the evolutionary importance of genetic drift and natural selection. Some scientists believed that natural selection was all-important, dismissing the idea of drift. Others, such as Dobzhansky and Pavlovsky, believed that genetic drift worked in conjunction with natural selection, a theory that they tested experimentally with the aid of Drosophila pseudoobscura. Wild type flies with different chromosomal gene arrangements on their third chromosomes were collected. Race hybridisation results in an explosion of genetic variability – the possible gene combinations exceeds the number of flies. Therefore, natural selection would promote genotypes with high adaptive values, but it was up to chance which genotypes would appear in the populations. The scientists used this knowledge to design their main experiments.

The main experiment used flies from geographically different regions. Flies collected were either homozygous for the Pikes Peak or homozygous for the Arrowhead gene arrangements. Foundation stock was prepared by crossing the Pikes Peak and Arrowhead strains, creating F1 and F2 heterozygotes of 50:50 Pikes Peak and Arrowhead. A total of 20 populations, ten small populations derived from founders of 20 F2 flies, and ten large populations derived from 4000 F2 founders. Over 19 generations chromosomes in the salivary glands from larvae were analysed, showing the frequency of Pikes Peak had decreased in all founder populations, with small founder populations showing significantly greater heterogeneity. Dobzhansky and Pavlovsky concluded that greater heterogeneity is due to differences in magnitude of the founder generation of populations, and that in geographically mixed populations, the selection of the chromosomal gene arrangements is dependent on the polygenic genetic background, which is highly variable due to recombination – here, genetic drift becomes important. Finally, they observed that hybridisation led to highly variable gene pools, of which random drift determined what would be included in the gene pools of founder generations, and natural selection drove the divergent results in different populations. The smaller the population size, the more genetic drift impacted the population, resulting in greater deviations and fluctuations across generations.

Creator: Beatrice Zhang

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