In the history of life on earth, disparity typically precedes diversity. Lewin's question leads us to an even more important question. One rather convincing explanation that has been offered for the pattern is based upon the fact that any major novelty that arises relatively quickly through non-selective processes is likely to be poorly adapted. The more novel the body plan is, the more susceptible it will be to elimination by competition and therefore the more adaptive space will be required for it to become established. In short, competition tends to inhibit the establishment of higher taxa.
Natural selection tends to inhibit major evolutionary change.