There is a trade-off between size and number, due to finite amount of resources that can be allocated to some combination of size and number. One might think it would be adaptive to get rid of the sepals, etc sooner than that, in order to conserve plant resources. But, due to physiological constraints on the formation of an abcission zone, for example, the plants are constrained to retain the showy floral structures past the time they are useful. Also example on host plant shifts to illustrate that adaptive evolution can be constrained by a lack of genetic variation. Maintenance of genetic variation -Lack of genetic variation can constrain adaptive evolution.
Selection maintains the genetic variation.