There are far too many areas of focus in biology at the doctorate level to provide anything beyond a cursory glance and some examples here. Biology doctorates, unless they are for the express purpose of teaching, tend to be specialized. As an example, the University of North Carolina has an entire department devoted to Cell and Developmental Biology. That alone sounds like two areas of specialization, but the curriculum further subdivides the academic focus:
"In the first year of study, students elect the area in which to focus: Cell Biology, Neurobiology, Developmental Biology or Anatomical Sciences. Students may select courses...which also familiarize them with one or more of several areas of research interests represented in the Department. These include: Cell Biology, Developmental Biology, Neurobiology, Reproductive Biology, Endocrinology, Molecular Biology, Cell Signaling and Membrane Biology, to name but a few."
