Vertebrates, Invertebrates and Plants

In order to test whether there was directional selection on size resulting in Cope’s predicted body size increase, the authors measured both size variables and other morphological variables against three different fitness variables; survival, fecundity and mating success (Kingsolver and Pfennig, 2004). When data was plotted on a linear selection gradient, a prominent pattern arose. The other morphological traits were centred around zero (50% above zero with a median of 0.02) and the overall size values were shifted towards the positive side (79% above zero and a median of 0.15)(Figure 2)(Kingsolver and Pfennig, 2004). The difference between the overall size traits and the other morphological traits was found to be quite significant (P< 0.0001)(Kingsolver and Pfennig, 2004). The same pattern was found when only data from the same test individuals was used, when difference taxonomic groups were used and when different fitness

Figure 2. Distributions for body size and other morphological traits on a linear selection gradient in relation to (a) survival, (b) fecundity and ( c) mating success. (after Kingsolver and Pfennig, 2004)
 
measures (survival, fecundity and mating success) were used (Kingsolver and Pfennig, 2004). Since this study was preformed on so many different test groups it is unlikely that the results are an artefact of the group used. The authors of this paper believe that their results show that bigger individuals are more fit and hence show how a microevolutionary process (individual-level selection) can explain a macroevolutionary process (Cope’s Rule)(Kingsolver and Pfennig, 2004).