Fig. 11

An example of the importance of historical information in identifying the underlying dynamics of a trend. a Shows a clear trend toward increased complexity over time within a hypothetical group of organisms. In this case, the minimum has increased along with the maximum and the average. It is possible for this pattern to occur if less complex groups of related species (“clades”) happen to go extinct and new lineages that originate happen to be more complex (b). By contrast, this could also result from a driven trend in which consistent increases in complexity occur within each component lineage (c). These two very different scenarios can only be distinguished by using comparisons of ancestors and descendants within lineages or by inferring ancestral characteristics from phylogenies. Figure from Adamowicz et al. (2008), reproduced by permission of the National Academy of Sciences of the USA. See Purvis and Orme (2005) for a similar discussion related to body size trends