Stochastic Simulation and Evolution of Morphology-Towards a Nomothetic Paleontology

Abstract

The morphological order of evolutionary trees has been the traditional argument for the operation of directional causes in macroevolution. We show, in this work, that a similar order can be generated within stochastic systems bounded by minimal biological constraints. Our system generates an evolutionary tree by making random decisions about each lineage in each time interval given preset probabilities for branching, extinction and persistence (Raup, Gould, Schopf, and Simberloff, 1973). Morphology is determined in an independent and equally stochastic manner. Using ten hypothetical characters, the beginning lineage is given an all zero morphology. At each branching point, each character may change by one unit (in a positive or negative direction) according to preset probabilities for positive change, negative change, and no change. Our simulations display most of the ordered features generally associated with uni-directional selection: morphological coherence of monophyletic groups and incomplete filling of “morphological space” regular “unfolding” of morphology (as seen in strong correspondence between phenetic and cladistic taxonomies); marked evolutionary “trends” strong correlation among characters; large variation in rates of evolution; and specialization of derived forms. We attribute much of this order to abstract topological properties of the tree itself and urge that the data for inferences about directional causes be sought elsewhere (in functional morphology, for example). We suggest, with caution, that undirected selection may be the rule rather than the exception in nature, if a temporal unit of sufficient duration be used as the yardstick of measurement.