A key innovation is a characteristic that lets a species use the environment in a way that is sufficiently different from its relatives and other existing species so that many new ways of using the environment become possible. For example, when wings evolved, the species with wings could fly and therefore use the environment in different ways. The evolution of the flower allowed many new ways of reproduction in the environment. It is important to note here that the definition of a key innovation is a little fuzzy. You might ask how much differently a characteristic should let a species use the environment for it to be a key innovation.
Body size can affect either the probability of speciation or extinction in a group.
So both sympatric and allopatric speciation may be more likely for species with smaller body size, resulting in a higher rate of speciation in small species. Extinction rates, in contrast, may be higher in species with large body size. Large organisms require more resources from the environment and are likely to exist in populations of fewer individuals as a result. They may thus be more subject to genetic drift, which can result in non-adaptive evolution, so they may not be as well adapted to their environments. If species with small body size have higher rates of speciation or lower rates of extinction than do species with large body size, we would predict there to be more small species than large species.