Krabby evolves into Kingler at level 28.
Machop evolves into Machoke at level 28. Machoke evolves into Machamp at level 36.
Magnemite evolves into Magneton at level 30.
Magikarp evolves into Gyarados at level 20, just like in the original Pokémon games.
Mankey evolves into Primeape at level 28.
Meowth evolves into Persian at level 28.
Nidoran evolves into either Nidorino or Nidorina at level 16, then either Nidoqueen or Nidoking at level 36.
Oddish evolves into Gloom at level 21, then Gloom becomes Vileplume at level 36.
Omanyte evolves into Omastar at level 40.
Paras evolves into Parasect at level 24.
Pidgey evolves into Pidgeotto at level 18. Pidgeotto evolves into Pidgeot at level 36.
Pikachu evolves into Raichu at level 22.
Poliwag evolves into Poliwhirl at level 25. Poliwhirl evolves into Poliwrath at level 36.
Ponyta evolves into Rapidash at level 40.
Psyduck evolves into Golduck at level 33.
Rattata evolves into Raticate at level 20.
Rhyhorn evolves into Rhydon at level 42.
Sandshrew evolves into Sandslash at level 22.
Seel evolves into Dewgong at level 34.
Shellder evolves into Cloyster at level 36.
Slowpoke evolves into Slower at level 37.
Squirtle evolves into Wartortle at level 16. Wartortle evolves into Blastoise at level 36.
Spearow evolves into Fearow at level 20.
Staryu evolves into Starmie at level 36.
Tentacool evolves into Tentacruel at level 30.
Tyrogue (one of the only Generation 2 Pokemon in this game) Evolves into either Hitmonchan or Hitmonlee at level 20, and at random.
Venonat evolves into Venomoth at level 31.
Voltorb evolves into Electrode at level 30
Vulpix evolves into Ninetales at level 36.
Weedle evolves into Kakuna at level 7, then Kakuna evolves into Beedrill at level 10.
Zubat evolves into Golbat at level 22.