i havent played it in a minute, but each pokemon entry in the pokedex has some tasks you have to do. i think some tasks give more research XP, those are the ones usually most relevant to the pokemon (like evolving eevee into each of its different evolutions).

completing each tier of tasks (again with eevee, one of the tasks is evolve eevee into 1, then 3, then 5 unique evolutions or something) gives research XP

you will have to catch a LOT of the same pokemon over and over, since thats also one of the tasks on almost every pokemon entry besides legendaries. with eevee you'd have to catch enough eevees to evolve into each of its evolutions