#1 GENES. “EVO-DEVO: The more often that two genes are selected together (at the same time/in the same environment) the more selective advantage there is to strengthening developmental interactions between them. This organisational change causes their expression in the phenotype to be correlated in future (e.g. via an increase in the gene-regulatory connection between them.”
http://link.springer.com/article/10.1007/s11692-015-9358-z