Back when my older brother started his PhD degree I asked him what it meant to be a doctor in something other than medicine. I don't recall the exact wording he used, but the idea he portrayed was that you take a field, a narrow and specific field, and you specialise in it to a level at which when you are done, you have become one of the ultimate experts in that very specific field.
For instance if you are working with combustion physics, you might be one of the leading experts in efficient 2 cylinder, ultra-light engines made out of refined aluminium... Alternatively if you are into neuroscience you might be an expert on a particular neurotransmitter re-absorption in a particular zone of the brain following heavy exercise (or whatever, hopefully you get the point). It might be an opinionated view of a PhD but I feel it's a common way to look at a PhD degree; a certification of expertise.
Fast-forward 15 years... I am about half-way in my PhD studies in the highly interdisciplinary field of bioinformatics, where statistics, mathematical modeling, physics, molecular chemistry and programming boil together with cell biology, to top it all you typically have a theme spice, which in my case is cancer biology. I have a growing feeling that I am getting stretched thinner and thinner by the day, instead of becoming increasingly competent in a specific field, I become semi-competent in increasingly many fields.
That being the case I am not sure I (or others like myself) will fit the "definition" above. I would appreciate some perspective as to how one should be seeing highly interdisciplinary PhD studies and the development (as a scientist and a professional) that graduate studies entitles. Subsequently, how should one go about to profile him/her-self to future employers, seeing as there is no one natural field to pursue, but rather many different ones.