One point to start.

The way a fish looks , acts and lives is often due to plasticity (adaptation/environment) not genetics. Every thing is in play. Plasticity has been shown to effect sunfish (Lepomis) shape, size, feeding and behavior in some cases. Also shown to effect internal organs and digestion (energetics) and in RES and possibly PS pharyngeal morphology (crusher plates). Over long periods plasticity can change species (genetics).