Vertebrates, Invertebrates and Plants

Beyond explaining a mechanism for Cope’s rule, Kingsolver and Pfennig (2004) went on to discuss why animals don’t appear to increase in size infinitely. They believe that there must be some opposing selective force that will work against size increase once the organism gets too big. More specifically they thought that the constraint on size may be due to genetics correlations between the opposing traits and body size. They tested body size against development time as well as other life history traits and found that when plotted on a linear

Cephalaspis sp.
selection gradient, the other life history traits were symmetric around zero (46% below zero and a median of 0.01) whereas development time was shifted towards the negative (84% below zero and a median of -0.145)(Kingsolver and Pfennig, 2004). Since there were only a few studies which had measured selection on both size and development time, these results may not have been totally accurate.