  Developmental bias plays a major role in phenotypic evolution. Some
researchers have argued that phenotypes, regulated by development, can only
evolve along restricted trajectory under certain scenarios, such as the case
for mammalian molar size ratios. However, this view has been challenged.
Broadly speaking, sources for phenotypic variation remain largely unknown. The
study here presents a generalized Inhibitory Cascade Model and explains that
the original model described only means of phenotypes resulting from selection
when viewed under a higher taxonomic scope. Consequently, I propose the
combined property of development: certainty, when the prior intersegmental
inhibition is strong, and uncertainty, when the opposite holds. This property
potentially not only explains counterintuitively high levels of developmental
instability, but also plays an essential role in generating phenotypic
variation.
