I don't think it would change much, because the Earth layers are all differently dense, with the Inner core being the most dense and then the outer core, inner mantle, outer mantle, then the crust. Thefore even if the mantle was thicker it will still go in that order due to it density.
The earth would no longer have the mobile surface it has now, gradually eroding flat and water-covered. The sea might eventually evaporate into space or freeze.
D. Learned behaviors do not directly affect natural selection because they are not inherited. This is true because learned behaviors can be taught to the offspring but do not come within them the way instincts do.
