In the introduction to the article, the authors write "soon after the expansion of molecular biology in the 1950’s, it became evident that by comparing proteins and nucleic acids of one species with those of another, one could hope to obtain a quantitative and objective estimate of the "genetic distance" between species". what did the authors mean? how do you "compare proteins and nucleic acids" (or how did you in 1975 when this article was written)?

The author means to say that Genetic difference exist between different species.

Explanation:

Comparison between protein and nucleic acid

Similarity

1 Both nucleic acid and Proteins are Macromolecules

2 Both nucleic acid and protein are the main component of central dogma of molecular biology

Difference

1 nucleic acids are the polymer of nucleotides whereas proteins are the polymer of amino acids.

2 nucleic contain nitrogen bases whereas protein contain open reading frame composed of codons.

the answer to your question is slate