Or you could do what I did and sneak in an ecology (field work) part into your PhD proposal for molecular biology things. What do you like so far in biology? Specific type of organism? Have you started doing research in someones lab? I am sure you have heard before, but hands on experience as an undergrad is so crucial, and the earlier you start the more time you have to play around and find a lab you like.
Oh I'm going to avoid post grad education if I can, I am not an academic person. I love bacteria, specifically the relationships between them. But ecosystems also mirror this relationship, so macro biology isn't off the table. I really could enjoy anything, as long as life is involved. I'm a B- student in a pretty competitive college, the profs I've build relations with are looking for someone better, and I don't have the stomach for cold emailing (although I probably should just swallow my pride and go for it.)