But is there an evolutionary pressure in this direction because of the decrease in need? That would be Lamarckian then. I wonder if certain traits need evolutionary pressure to stay stable, and otherwise degenerate in some specific direction.
Many (most?) traits require energy to develop and maintain. A stronger muscle will need more energy so there's always pressure to reduce it. That pressure may just be countered by pressures in other directions.
Something something opportunity cost? If you can allocate your next bit of energy and protein to something with better bag-for-buck than a jaw muscle then that should be an evolutionary advantage.
More expected (over ensemble of specimens) evolutionary "fitness" per unit of whatever you need to expend to make a jaw muscle a bit stronger. The buck is small because we're doing optimization calculus.
One thing to consider is that some things might not be genetic, but environmental. I think jaw muscles are one such thing and short-sightedness might be other. First caused by food we eat and the other by less natural light and less time spend outside in childhood.
