This Week's Citation Classic

This week's citation classics are a pair of papers published by John Hubby and Richard Lewontin in 1966.

J. L. Hubby and R. C. Lewontin, "A Molecular Approach to the Study of Genic Heterozygosity in Natural Populations. I. The Number of Alleles at Different Loci in Drosophila pseudoobscura," Genetics 54 (1966): 546-595.

R. C. Lewontin and J. L. Hubby, "A Molecular Approach to the Study of Genic Heterozygosity in Natural Populations. II. Amount of Variation and Degree of Heterozygosity in Natural Populations of Drosophila pseudoobscura," Genetics 54 (1966): 595-609.

A central tenet of Darwin's theory of natural selection is that genetic variation exists in natural populations. Selection on genetic variation is the fundamental basis for evolutionary change. However, at the time of Hubby and Lewontin's work, whether or not this genetic variation, in fact, existed was largely unknown.

Lewontin, a population geneticist, paired up with Hubby, a biochemist, to use a revolutionary new technique to examine genetic variation at dozens of loci for the fruit fly, Drosophila pseudoobscura. This new technique was acrylamide gel electrophoresis.

When a mutation occurs in a gene, it can change the amino acid sequence of that gene following translation. Different amino acid sequences can have different net electrical charges. Hubby and Lewontin isolated proteins from D. pseudoobscura, placed them in wells of a slab of acrylamide gel and ran a slight current across the gel. The proteins then migrated thru the gel at a speed proportional to their net electrical charge. Hubby and Lewontin observed that the same protein isolated from different members of the population frequently migrated across the gel at different speeds (represented by the "bands" in the photo above), a result they correctly attributed to genetic variation. This supposition was supported by the fact that the variation segregated in a Mendelian fashion.

Hubby and Lewontin's results were stunning. While some genetic variation was expected, no one was quite prepared for the enormous amounts of variation their experiments revealed. Hubby and Lewontin concluded that there was genetic variation at 39% of loci in the D. pseudoobscura genome. In fact, this method leads to an underestimate of the true amount of genetic variation because it does not account for mutations that do not lead to amino acid substitutions (i.e., silent mutations) or changes in the net electrical charge of the protein.

"This was a breakthrough, a revolutionary finding," recalled Brian Charlesworth, "It led to an explosion of work. Everyone in the field rushed out to duplicate these studies in other organisms, including humans, and they found more and more examples of genetic diversity."

The data was distinctly at odds with the Classic and Balance theories of molecular evolution (a series of posts on this from hpb are available here: controversy, redux and final) championed by R. A. Fisher and Sewall Wright respectively. In fact, data such as that obtained by Hubby and Lewontin led Motoo Kimura to propose a neutral theory of molecular evolution.

Curiously, paper II (Lewontin and Hubby) is cited far more often than paper I (Hubby and Lewontin), despite Dick Lewontin's assertions that the papers form an indivisible pair. Lewontin attributes this to the Matthew Effect.

Update: Larry Moran provides some background.