Differential biology is the study of everything that distinguishes individuals, or the biology of individual differences. It is personality—all the aspects of being that make you you—in its most catholic sense.
Despite Darwin’s rejection of species as types and his re-founding of biology on population thinking (cf. Mayr, The growth of biological thought), evolutionary studies long looked only to explain species universals, not why individuals within a species should vary. While genetic and theoretic models have explored the dynamics leading to such variation and other biologists have been cognizant of each creature’s unrivaled singularity, only now are we developing the tools and datasets for evolutionary studies of individual differences in wild populations.
Does the world need yet another sub-discipline? You tell me. This work on personality is already being carried out in a number of places, primarily as differential psychology and behavioral ecology. What I wish to add is a bit of evolutionary quantitative genetics. Evolutionary psychology already partly spans this territory, but takes its cue from palæontology. Too much for my taste. I am trying to understand variation as it exists now, not just its history.
I owe the term, and many fruitful discussions on the biology of difference, to Alex Weiss.