Does Common Equal Primitive?

Abstract

One criterion that has been suggested as a basis for estimating the direction of evolutionary trends among the states of a character is to choose for the primitive state the one that occurs most frequently. A concept is presented of what it means for a character to sttpport an estimate of evolutionary relationships. It is argtued that if the common state is chosen as primitive, then more characters can support the same estimate of evolutionary relationships. Estimating evolutionary relationships among plant species (or higher taxa) continues to be one of the fascinations of modern plant systematics. Recent estimates based on traditional considerations have been published by Takhtajan (1969) and Cronquist (1968). Estimates based on molecular data have been made for selected species by Boulter et al. (1972). Numerous estimates of the evolutionary relationships within various lower taxa have been constructed, many using various objective methods recently suggested. Many workers approach constructing an estimate of evolutionary relationships among the species of a genus or family by recognizing several bases for comparison (characters) and by recording the description of those bases (character states) for the various species under study. In practice this procedure need not always be carried out at the species level. For example, genera could be compared and described instead of species. Thus, I shall use the term evolutionary unit (EU) to refer to the entity that is described by a character state, and I shall refer to the collection of EU's whose historical relationships are to be estimated as the study collection. If the approach of structuring data as characters is taken, the problem of estimating evolutionary relationships is one of determining an evolutionary tree for the EU's in the study based on the descriptions of the EU's given by the characters. There are many facets to the problem of estimating evolutionary relationships. What is an evolutionary unit? What should be the bases for comparison? How can the direction of evolutionary trends among the states of a character be estimated? What principles can be used to infer relative recency of common ancestry from the above considerations? How can these principles be made operational? I have earlier reviewed these questions (Estabrook, 1972) and wish to discuss here only one of them in more depth. This is the question of estimating the direction of trends among the states of a character. I University Herbarium and Botany, University of Michigan, Ann Arbor MI 48109. 2 J thank the Division of Biological Sciences and the University Computing Center for the use of computing facilities. This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 04:41:52 UTC All use subject to http://about.jstor.org/terms 1977] ESTABROOK: DOES COMMON EQUAL PRIMITIVE? 37