Pokemon Go evolutions rely on candies. When you catch or hatch a pokemon, you receive a certain amount of candies. Or if you give a pokemon to Professor Willow, he gives you a candy. Then you need a certain amount of candy to evolve different types of pokemon. You get the same base candies for catching all iterations of a pokemon. So catching a pidgey or a pidgeotto would both give you pidgy candies.

Some of them only need 12 candies, like a weedle to it's next stage. Then you need more from there. Others, you need a ridiculous amount of candies. You need 400 candies to evolve a magikarp into a gyrados! x_X