  The birth/death process with mutation describes the evolution of a
population, and displays rich dynamics including clustering and fluctuations.
We discuss an analytical `field-theoretical' approach to the birth/death
process, using a simple dimensional analysis argument to describe evolution as
a `Super-Brownian Motion' in the infinite population limit. The field theory
technique provides corrections to this for large but finite population, and an
exact description at arbitrary population size. This allows a characterisation
of the difference between the evolution of a phenotype, for which strong local
clustering is observed, and a genotype for which distributions are more
dispersed. We describe the approach with sufficient detail for non-specialists.
