Can EVSs evolve over time? How?

No, Effort Values (EVSs) do not evolve over time, but they can be increased, decreased, or redistributed through specific methods.

Effort Values, or EVs, are a mechanic in the Pokémon games that help determine a Pokémon's final stats. They do not 'evolve' in the traditional sense, but they can be manipulated over time through various methods. Each Pokémon can gain a maximum of 510 EVs, with a maximum of 252 EVs in any one stat. These values are gained through battling other Pokémon, with each defeated Pokémon providing a specific type of EV.

For example, defeating a Pikachu will give your Pokémon 2 Speed EVs. This means that the more Pikachu your Pokémon defeats, the faster it will become. However, this doesn't mean that your Pokémon's EVs are 'evolving'. Rather, they are being increased through effort and training.

There are also ways to decrease or redistribute your Pokémon's EVs. Certain berries, for example, can decrease a Pokémon's EVs in a specific stat. This can be useful if you've accidentally given your Pokémon too many EVs in a stat that isn't beneficial to it. Additionally, the use of certain items, such as the EV-reducing berries or the Reset Bag in Pokémon X and Y, can completely reset a Pokémon's EVs, allowing you to redistribute them as you see fit.

In conclusion, while EVs do not 'evolve' over time, they can be increased, decreased, or redistributed through specific methods. This allows you to customise your Pokémon's stats to better suit your battle strategy.