Biology is not capitalized because it does not come from a proper noun. Such as English. English is capitalized because it comes from the proper noun "England" but biology does not come from a proper noun, so it stays lower case.
It needs to be capitalized, Biology, because it's a subject that you learn about in school. Like Math, Science, English, Art, P.E. (well that one is all capital, but I think you get the point). I hope this helps. =^p
