You need to become an expert in something, but you need not let it take over who you are (though it may do so for a few years).
My primary focus (and interest) is in *pond scum*, but there seems to be little demand for such a field right now. So I am prepared to study just about anything that keeps me close to the pond scum field. Hopefully I will have time for some nightlighting reseach in the scum I really like, or wait 20 years for a job opening.