Simpson's Tempo and Mode it saw the publication of a monograph by Theodosius Dobzhansky and Carl Epling (Contributions to the Genetics, Taxonomy, and Ecology of Drosophila pseudoobscura and Its Relatives ) that would usher in an interest in reconstructing phylogenetic history on the basis of genetic information. The method relies on the sequential composition of chromosomes, a premonition of the molecular methods that rely on the sequential composition of the DNA. The third chromosome of Drosophila pseudoobscura exhibits a rich polymorphism with more than 40 alternatives. One vexing problem is rooting the topological relationships, that is, identifying the ancestral element. Aleksandar Popadic and Wyatt W. Anderson, in Chapter 15, examine the nucleotide sequence of a DNA fragment included within the chromosomal elements and conclude that only two of the elements are possible ancestors, one of them ("Santa Cruz") with higher probability.
The last chapter is a display of molecular biology virtuosity. Daniel L. Hartl and colleagues have produced a physical map of the chromosomes of the fruit fly Drosophila melanogaster, by ordering sequentially 2461 different DNA fragments, each about 80,000 nucleotides long. Eighty-five percent of all genes are included in these fragments. The methods are the same in use for mapping human chromosomes, and they can readily be extended to other Drosophila species, a possibility of genetic and evolutionary consequence.