Rates of single-copy DNA evolution in herons

DNA-DNA hybridization was used to discover the extent of single-copy DNA similarity among 13 species of herons and one ibis. Genetic distances among taxa were summarized as Tm values in a folded matrix. From this matrix, trees with the same branching pattern were constructed by least squares under one of two assumptions: (1) that sister branches are equal in length and (2) that sister branches are not necessarily equal in length. The residual sums of squares of these trees were compared by F-test to see whether the branches of the tree built under assumption (2) fit the matrix data significantly better than those of the tree built under assumption (1). By this method the existence of different rates of DNA evolution in different heron lineages was established. Bittern single-copy DNA has evolved at a rate approximately 25% faster, and boat-billed heron (Cochearius) and rufescent tiger heron (Tigrisoma lineatum) DNA has evolved approximately 19% slower, than that of day and night herons. It appears that the differences in rates of DNA evolution may increase proportionally with genealogical distance.