Pattern and process in paleobiology: the role of cladistic analysis in systematic paleontology

Abstract

Systematics and paleontology have had a long conceptual relationship, united by the common goal of reconstructing the history of life. Yet, with few exceptions, paleontologists have had little input into formulating systematic theory and methodology. The reasons for this apparently relate to two conceptual-philosophical traditions of post-Darwinian paleontology: (1) the widespread adoption of a species concept in which taxa are viewed as nondiscrete, arbitrarily designated segments of evolutionary continua, and (2) the belief that phylogenetic reconstruction is primarily an empirical matter of tracing evolutionary change through the stratigraphic record.Available systematic evidence supports the hypothesis that species are real, discrete units in space and time and that, unless they are postulated to be directly ancestral to another species, they can be defined by the possession of one or more evolutionary novelties (derived characters). Species beginnings are delineated by speciation (vicariance) events and their terminations by subsequent speciation events or by extinctions.Natural groups are composed of taxa that have shared a common genealogical history. Cladistic analysis is a method to construct and test hypotheses of monophyly and thereby define natural groups. Cladistic hypotheses are necessary to investigate many of the major questions within contemporary paleobiology. Virtually no studies of evolutionary rates, patterns of taxonomic diversity, modes of taxic evolution, and patterns of morphological diversification can be undertaken without reference to cladistic hypotheses about the composition of natural groups.Because paleobiology is historical in its content, paleontologists are greatly limited in their ability to use paleontological data to investigate questions about the evolutionary process. According to current evolutionary theory, the concepts of adaptation and natural selection relate to genetic and ecological processes that take place within local populations (microevolution). If so, then data relevant to examining these phenomena are likely to be lacking in paleontological samples. Consequently, explanations of paleontological pattern that include process-related concepts such as adaptation and natural selection are axiomatic in their logical structure and thus cannot be falsified or critically evaluated by that paleontological pattern.