In evolutionary biology, Cope’s rule refers to the trend towards increasing body size in a lineage over geological time periods. Put simply, the rule says that members of a lineage get bigger over time. The rule has received mixed support, applying to some lineages (such as fossil mammals), but suffering from inconclusive evidence in others (Mesozoic birds, living mammals).
To explain the occurrence of the trend in some lineages, two explanations can be put forward:
- A passive mechanism: body size evolution passively drifts away from a lower boundary, an idea favored by S.J. Gould, among others.
- An active drive: here, selection drives an increase in body size through the competitive advantages of being bigger.
Studies on the active drive hypothesis are quite rare and mostly focused on invertebrates. Now, a study published in The American Naturalist provides a phylogenetically informed test on the active drive mechanism in extinct mammals, where active drive was assumed to lead to the replacement of smaller species by larger ones which are more common, show a more widespread distribution and longer evolutionary lifetime.
The authors composed a species-level tree of 554 mammal species, ranging from a 200g mustelid to an eleven ton proboscidean. After determining whether Cope’s rule applied to the data (see figure 1), the researchers looked at data about the range size, commonness and duration (in an evolutionary sense) of the species involved.
Figure 1: Phylogenetic tree showing changes in size. Red lines refer to clades that show an increase in size, purple lines to clades that decreased in size, and black lines to clades which lack a significant change in size. Red circles indicate an increase in the body size of both the smallest and largest species in the group, orange circles indicate an increase in the body size of the largest member, but not the smallest one, resulting in an increase in size spectrum, and white circles refer to a decrease in size in both the largest and smallest member.
(Source: Raia et al., 2012)
They found that Cope’s rule did indeed apply to their data. Furthermore, it seems that this average increase in size might be explained by a niche specialization. For example, among ungulates the acquisition of grass feeding was accompanied by an increase in body size. The data suggests that small and unspecialized species are ‘replaced’ by larger species, who show a decreased range and shorter evolutionary lifetime, contradicting the active drive hypothesis. Then why is the world not full oflarge, specialized species? As the authors note:
That the world is nevertheless not full of large specialists is explained by the lack of very specialized morphotypes early in the history of animal clades, by the negative relationship between body size and diversification, and by the positive relationship between extinction risk and both body size and range size.
Another interesting fact is that most of the peaks in body size increase occur alongside changes in the climate, which hints at a possible role for Bergmann’s rule, which states that a drop in temperature results in an increase in size.
Overall, the authors conclude:
The acquisition of new morphotypes (and ensuing invasion of new ecospaces) — which Cope recognized, even without having the morphotype concept at hand — can, in a contemporary framework, be comfortably attributed to the exploration of new resources and habitats by specialized forms. The early dominance in evolving clades of unspecialized small-sized forms and the subsequent niche expansion provided by new specialized morphotypes of larger size appears to us to be the most likely fundamental driver of Cope’s rule of size increase in mammals.
Reference
Raia, P.; Carotenuto, F.; Passaro, F.; Fulgione, D. and Fortelius, M. (2012). The American Naturalist. 179(3). Published online 19 January. doi:10.1086/664081.
