Rise and fall of phylogenetic lines
Repeated rise and fall of entire phylogenetic lines (clades) are a characteristic and very conspicuous feature of the evolution of fauna and flora over long periods of time. Lines whose members completely dominated both in the number of species and in the sizes of populations in various environments and occupied diverse niches in various ecosystems in a certain period either completely died out in the subsequent period or left only a very few highly specialized species. Their key positions in the ecosystems were then occupied by other phylogenetic lines, whose members had been of only marginal importance until that time. Typical or, at the very least, the best known examples consist in the rise and fall or trilobites or the rise and fall of the dinosaurs in the terrestrial ecosystem of mammals.
In studying these processes of the “rise and fall” of dynasties, an explanation is mostly sought on the basis of differences in the adaptation of the members of the individual phylogenetic lines to certain factors in the environment and also of the differences in the ability to adapt to changes in these factors. The first explanation, that the members of one line force out, in competition, the members of another line, does not seem very probable. In most well-documented cases, the fall of the original dynasty occurs before the rise of the new dynasty (Benton 1996). The alternative that the rise and fall of the dynasties is caused by the inability of a certain line to adapt to a change in the environment is supported particularly by the fact that the rise and fall of dynasties occurs at times of mass extinction and thus at a time of major changes in the environment, which are currently mostly considered to have been caused by an environmental catastrophe of major extent (see XXII.5.3.1). However, the fact that the extinction frequently mostly affects the members of environmentally diversified lines makes this common explanation less credible. It is understandably possible that, following a major change in the environment, the members of an originally not very important line gain competitive advantage in some types of environments. However, it is not very probable that they would gain a similar advantage in all types of environments and while utilizing all possible environmental niches. It seems more probable (at least to me personally) that the temporary victory of a certain line over other lines is a result of species selection. The temporal connection between the rise and fall of dynasties and periods of mass extinction can most readily be explained in that, in the period following a period of mass extinction, the ability of the members of a certain phylogenetic line to rapidly undergo speciation and thus fill all the empty or newly formed niches is of key importance for survival. There are, of course, also other possible explanations, such as the one suggested by the viral theory of background extinction (XII.6.5).