Biochemical Systematics and Evolution

Abstract

The major aims of systematics are (a) to determine the number of species in a group, (6) to classify these species in an ordered hierarchy that reflects their evolutionary relationships and/or phenetic similarities, and (y) to evaluate variation within species and the processes of speciation. Molecular biology has much to contribute to all of these levels of taxonomic work and the field is currently undergoing a major transformation because of the use of the techniques of molecular biology to provide solutions to many of the unsolved problems of systematics. Since the time of the introduction of the concept of evolution into taxonomy, taxonomists have understood that the underlying basis of their classifications is the genetic relationships of organisms, even though these could only be estimated from the phenotype. With the advent of molecular biology, it became possible to sequence DNA and gene products and thereby actually compare portions of the genotypes of organisms. Since distinct species differ from each other genetically, it is usually possible to easily determine the number of species at a locality by an electrophoretic analysis of their proteins. Hybridization between species may easily be detected since the hybrids are heterozygous for the loci that distinguish the parental species. A most unexpected and especially exciting finding for the taxonomist, was the discovery that the rate of substitution in many structural genes over time is relatively constant regardless of the differential rates of morphological evolution. Thus a molecular clock became available for use in estimating the approximate times of cladistic events in the phylogenetic trees leading to all living organisms. Taxonomists in their wildest dreams could not have wished for a more useful finding: methods to determine the real evolutionary relationships of living taxa. Heretofore we have been plagued by the well-known problems of unequal rates of evolution and the possibility of extremely rapid changes in the phenotype when a line shifts to a new adaptive zone, adaptive correlations between morphological characters, convergence, unequal developmental changes in different stages of the life cycle, character displacement, and many other problems that have resulted in innumer-