This Article Figures Only Full Text Full Text (PDF) Supporting Table Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a colleague Related articles in PNAS Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My File Cabinet Download to citation manager Request Copyright Permission Citing Articles Citing Articles via CrossRef Google Scholar Articles by Hunt, G. PubMed PubMed Citation Articles by Hunt, G. Social Bookmarking                What's this?

 Previous Article  | Table of Contents |  Next Article 

From the Cover
PHYSICAL SCIENCES / BIOLOGICAL SCIENCES / GEOLOGY / EVOLUTION
The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages

Gene Hunt^*

Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, MRC 121, P.O. Box 37012, Washington DC 20013-7012

Edited by Michal Kucera, University of Tübingen, Tübingen, Germany, and accepted by the Editorial Board October 18, 2007 (received for review May 3, 2007)

The nature of evolutionary changes recorded by the fossil record has long been controversial, with particular disagreement concerning the relative frequency of gradual change versus stasis within lineages. Here, I present a large-scale, statistical survey of evolutionary mode in fossil lineages. Over 250 sequences of evolving traits were fit by using maximum likelihood to three evolutionary models: directional change, random walk, and stasis. Evolution in these traits was rarely directional; in only 5% of fossil sequences was directional evolution the most strongly supported of the three modes of change. The remaining 95% of sequences were divided nearly equally between random walks and stasis. Variables related to body size were significantly less likely than shape traits to experience stasis. This finding is in accord with previous suggestions that size may be more evolutionarily labile than shape and is consistent with some but not all of the mechanisms proposed to explain evolutionary stasis. In general, similar evolutionary patterns are observed across other variables, such as clade membership and temporal resolution, but there is some evidence that directional change in planktonic organisms is more frequent than in benthic organisms. The rarity with which directional evolution was observed in this study corroborates a key claim of punctuated equilibria and suggests that truly directional evolution is infrequent or, perhaps more importantly, of short enough duration so as to rarely register in paleontological sampling.

gradualism | modes of evolution | punctuated equilibria

The authors declare no conflict of interest.

This article is a PNAS Direct Submission. M.K. is a guest editor invited by the Editorial Board.

This article contains supporting information online at www.pnas.org/cgi/content/full/0704088104/DC1.

*E-mail: hunte{at}si.edu

© 2007 by The National Academy of Sciences of the USA

CiteULike    Complore    Connotea    Del.icio.us    Digg    What's this?

Related articles in PNAS:

In This Issue

PNAS 2007 104: 18345-18346. [Full Text]