Molecular Systematics and Evolution

Abstract

Studies that rely on genetic and molecular information to address evolutionary questions fall roughly into two different categories: the reconstruction of the evolutionary relationships of organisms (including the times of divergence of groups or lineages), and the formation and emergence of morphological novelties that distinguish or characterize different organisms. The former endeavor is sometimes referred to as molecular systematics, and when it is applied to primates and especially the relatedness of humans and apes, “molecular anthropology.” The debate over the regularity of molecular change, resulting in a molecular clock, lies within the realm of molecular systematics, and especially that of molecular anthropology. In the past decade, in particular, the primary focus of molecular systematists has been DNA sequences, both nuclear and mitochondrial. Throughout, the assumption has been that the degree of similarity reflects the degree of evolutionary relatedness, because differences accrue once lineages have diverged from a common ancestor. The popularity of molecular systematics in recent years is also predicated on the notion of “a law of large numbers,” that is, the thousands of bases that produce DNA sequences. The question is whether these assumptions are sustainable.

The rather newly defined discipline of evolution and development, or “evo-devo” as it has been nicknamed, is less involved in the interpretation of molecular data for purposes of reconstructing evolutionary relationships than it is in trying to identify molecular elements—whether they be transcription factors or other kinds or classes of proteins, as well as “genes” themselves—that are relevant to, or participate in, the processes of development of structure and form. Here, what is important is not a particular RNA, DNA, or amino acid sequence, but the signal transduction pathways or sequences of communication between different molecules in the regulation of development and the origin of structure. Sometimes, hypotheses of when and at what level within groups of organisms specific features emerged are generated as a result of overlaying this information on a presumed theory of relationships of the organisms under consideration. It appears that insights from developmental genetics will prove to be more useful than sequence data alone for systematic and phylogenetic inquiry.

This entry will attempt to summarize the main aspects of these fields of inquiry, including their underlying assumptions, and suggest possible avenues for future research that might bring these disciplines together in light of new ways of thinking about evolution.