Fig. 13

Trends resulting from processes operating among species. In a, there is a bias whereby new species tend to differ from their ancestors in one direction (in this case, increase in some morphological parameter). It could be that that increases confer some advantage or simply that decreases are constrained or otherwise less likely (or both). The end result is a trend toward increase in the average value of the trait in the clade. In some cases, large-scale evolutionary changes in one direction (for example, the loss of a complex feature) are thought to be irreversible (“Dollo’s Law”), which obviously would create a trend in one direction. However, even more limited forms of irreversibility may be sufficient to generate an overall directional pattern. For example, in b, increases and decreases are roughly equally likely up to a certain point, after which decreases become constrained. This can be thought of as creating a “trap” for lineages that pass a threshold representing a point of no return. This, too, will result in a trend toward an increased average value overall. Figure from Wagner (1996), reproduced by permission of Blackwell. In c, species with higher values of a particular morphological parameter tend to leave more descendant species than those with lower values of the parameter (i.e., more new branches split off those parts of the tree). Over time, this differential speciation results in the production of more species exhibiting higher values than lower ones, thereby generating a trend in which the average increases in the group as a whole. In (d), the rate of new species formation does not differ according to the morphological trait under consideration, but those with higher values for the trait tend to persist longer before going extinct (i.e., those branches of the tree are longer on the time axis). This results in a larger overall number of species with higher values for the trait, and once again generates a trend in which the average for the entire group increases over time. In e, new species exhibit both increases and decreases in the trait compared to their ancestors, but two mass extinction events have occurred (dashed lines) that affected groups with lower values much more than those with higher values. As a result of their greater rate of survival through mass extinctions, lineages with higher values for the trait are more common today than those with lower values such that that there has been an increase overall in the average value of the trait—and therefore a trend