It is the year 1957.
A paper, titled “An Experimental Study of Interaction between Genetic Drift and Natural
Selection” has been published by the authors Dobzhansky and Pavlovsky, at Columbia University.
Dobzhansky, an established and well-known geneticist, has inadvertently intensified
the chaos present in the genetics community by seeking experimental evidence of natural
selection and genetic drift.
Throughout the 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.
In the manuscript, the authors identified two major opinions regarding drift:
1: As only a descriptor for differences in organisms with unexplained origins OR
2: That natural selection and drift are alternatives – ‘either-selection-or-drift’.
Dobzhansky did not regard this second definition to be logical, thus designed an experiment
to test for the interactions between selection and drift.
Random genetic drift encompasses all of the variations in genetic frequencies which are
indeterminate in direction.
A preliminary experiment was set up involving Drosophila pseudoobscura.
Wild type flies with different chromosomal gene arrangements on their third chromosomes
were collected.
Curiously, chromosomal heterozygotes bred from flies of different geographic regions
gave varied fitness and frequency results that were not uniform across replicate trials,
unlike those from flies of the same locality.
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, with variations on their third chromosomes, collected from
geographically different regions.
Flies collected were either homozygous for the Pikes Peak (PP) or homozygous for the
Arrowhead (AR) 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, were kept in population cages of
uniform condition for 19 generations.
All populations were allowed to fluctuate and expand within the carrying capacity of
the population cages.
Periodically, chromosomes in the salivary glands (having high visibility) were taken
from larvae and analysed.
By 19 generations, the frequency of Pikes Peak had decreased in both large and small
founder populations, with small founder populations showing significantly greater heterogeneity.
Dobzhansky and Pavlovsky concluded that: 1: Greater heterogeneity is due to differences
in magnitude of the founder generation of populations.
2: 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.
3: 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.
The results from this paper were significant for the scientific community, providing experimental
evidence for several principles.
Firstly, the experiment is largely believed to have verified Mayr’s founder principle,
as it manipulated the founder effect while controlling the variables of population size
and selection pressures.
However, the relevance of this research towards natural populations, where founder events
derived from a single locality, have been questioned.
Secondly, the manuscript is recognised for having shown experimentally the Fisher-Haldane-Wright
theory by testing the effects of genetic drift in combination with natural selection.
However, some geneticists at the time were highly skeptical about the importance of genetic
drift.
Ford, vocal against genetic drift – especially his understanding of Wright’s views, only
believed that natural selection created divergence in populations, and if drift was in play,
divergence should be less stable in smaller populations than larger ones.
This paper provided one of the earliest connections between experimental evolution and genetics,
allowing evolution to become an experimental discipline.
The experiments provided evidence for the interaction of genetic drift with natural
selection, an idea that had been widely criticised and doubted, as well as providing experimental
evidence for further theories.