transgression

Rosas U, Barton NH, Copsey L, Barbier de Reuille & Coen E 2010 Cryptic variation between species and the basis of hybrid performance. PLoS Biol 8:e1000429.

• the species lie on a plateau in gene expression-morphology space, so that the variation has no detectable phenotypic effect
• phenotypic differences can be revealed by shifting genotypes off the plateau through genetic crosses
• genomes are free to evolve within an effectively neutral zone in gene expression space

• expression for specific genes may be free to vary during evolution within particular bounds
• such variation may have little phenotypic effect when each locus is considered individually
• the collective effect of variation across multiple genes may become highly significant