Use of 'subject to' in this case
This was partially because in analyzing his data he used probability theory, an alien subject to most biologists of the time.
Here, to can indeed be replaced by for, meaning that probability theory is a subject that wasn't well-known to the biologists. It is used as a noun, and is categorized as "alien".
Use of 'subject to' in other cases
However, subject to can also be used in other cases, for example:
This new set of rules is subject to last-minute changes.
Where it means that the rules might change, and where you could replace it with prone to.
He is subject to his wife's will.
Where it conveys submission. In these, it's used as a subject's attribute and not as a noun any more.