For this tag to apply, the picture should reasonably be about evolutionary lines. That is, the theme connecting characters in the picture should be the fact that they're part of the same family, not that they're part of the same franchise.
For example, post #4134648 technically contains a lot of evolutionary lines, but realistically nobody searching for this tag wants the majority of posts for unpopular pokemons to be everyone-style posts.
For an evolutionary line with 3 or more creatures, not every single one needs to be present. This tag is fine to use if there is focus on, for example, the first stage and third stage of an evolutionary line with the second stage creature not being present.
For Pokemon, Paradox Pokemon (introduced in Scarlet & Violet) are not considered to be part of the same evolutionary line as the Pokemon they are referencing
