PCR, direct sequencing, and the comparative approach.

P C R has relieved much of the experimental drudgery of molecular biology and dramatically improved the sensitivity of many diagnostic procedures. These practical advantages are having a profound impact on the fields of molecular systematics and comparative biology. The reason is that PCR has made possible the analysis of previously intractable small, old, and poorly preserved samples and greatly facilitated the rapid cloning and sequencing of large numbers of samples. DNA sequences allow finer resolution of phylogenies, represent stable characters for taxonomic identification, and provide new insight into the patterns of evolutionary change in genes. This review will focus primarily on the impact of PCR on the field of molecular systematics, but I will also argue for the incorporation of a more comparative approach into molecular biology in general. Sequence data can be a common currency which can intellectually link evolutionary and molecular biology, to the advantage of both fields.