  We study the variation of the dimension of the Bloch-Kato Selmer group of a
p-adic Galois representation of a number field that varies in a refined family.
We show that, if one restricts ourselves to representations that are, at every
place dividing $p$, crystalline, non-critically refined, and with a fixed
number of non-negative Hodge-Tate weights, then the dimension of the Selmer
group varies essentially lower-semi-continuously. This allows to prove lower
bounds for Selmer groups "by continuity", in particular to prove some
predictions of the conjecture of Bloch-Kato for modular forms.
