Petal number is a robust trait in flowering plants. Crucifer flowers have four petals but Monniaux et al. show that petal number varies from zero to four in Cardamine hirsuta.
They mapped quantitative trait loci controlling stochastic variation in petal number and found that both the mean and uniformity of this trait share a common genetic basis. They suggest that this genetic variation, which is available to alter the mean and variance of petal number in C. hirsuta, may be cryptic in species such as Arabidopsis thaliana with robust petal number.
Marie Monniaux, Bjorn Pieper, Angela Hay, 2015, 'Stochastic variation in Cardamine hirsuta petal number', Annals of Botany, vol. 117, no. 5, pp. 881-887 http://dx.doi.org/10.1093/aob/mcv131