I know the practice of de-clawing crabs led to small claws as a dominant characteristic till it was abandoned, and similarly the illegality of harvesting molting variants of hard-shelled crabs resulted in local populations of permanently quasi-soft-shell crabs. Admittedly that took decades rather than the few years we might see for a lobster size ban, but minor evolutionary tweaks when those phenotypes already exist in the wild don't take long to establish themselves under heavy fishing pressure.
