When learning a Science speciality or a Profession, you learn the basics during boot - and that's Science 0, which covers all the bases - biology, chemistry, physics, mathematics, even statistics.
Thus a Traveller with Science (astrophysics) 1 must have learned Science 0 at some point, so she'd have Science (physics) 0 at the very least. Similarly, someone who learns Science (biochemistry) 1 would have Science (biology) 0 and Science (chemistry) 0 in order to specialise.
In Cosmopolite, I namedropped Social Science (mathematics), it being about the only place where I could put that speciality - but it's not needed here, because a mathematician can now have Science (mathematics) 1, which makes me very happy because it means I can have a Traveller like Charles Epps from the TV show Numb3rs, using maths to solve problems.
You could have a scientist who's really into the biology of the edible plondak with Science (edible plondak biology) 5, but he'd be kind of useless if called upon to work out why some crop plants on Regina are dying because he'd only have Science (biology) 0 to count on unless he'd chosen Science (biology) 1 or more during chargen or studied it during down time.
Actually, I'd love it if there were adventures for a Traveller who kept finding himself in demand from Patrons for some really out there non-combat specialities such as mathematics or soil science.