Another supporting evidence comes from the “generative entrenchment”
theory by William Wimsatt (Wimsatt, 1986; Wimsatt
and Schank, 2004). In this model, the phenotype is considered as a
generative structure. The generative structure of the system has a
characteristic set of causal interactions which can be represented by
the directed graph, see Fig. 1. In this model, nodes with more
downstream connections should have slower evolution rates, since
changes to them affect many epistatic interactions that must be
accommodated during the evolution. Quantitatively, we account the
downstream connections for each gene in gene regulatory networks.
For example, if gene A regulates the expression of gene B, we say gene
A has a downstream connection. We observe that kernels genes in
Fig. 1 have an average of 5.4 downstream connections, while the other