  Multilevel selection and the evolution of cooperation are fundamental to the
formation of higher-level organisation and the evolution of biocomplexity, but
such notions are controversial and poorly understood in natural populations.
The theoretic principles of group selection are well developed in idealised
models where a population is neatly divided into multiple semi-isolated
sub-populations. But since such models can be explained by individual selection
given the localised frequency-dependent effects involved, some argue that the
group selection concepts offered are, even in the idealised case, redundant and
that in natural conditions where groups are not well-defined that a group
selection framework is entirely inapplicable. This does not necessarily mean,
however, that a natural population is not subject to some interesting localised
frequency-dependent effects -- but how could we formally quantify this under
realistic conditions? Here we focus on the presence of a Simpson's Paradox
where, although the local proportion of cooperators decreases at all locations,
the global proportion of cooperators increases. We illustrate this principle in
a simple individual-based model of bacterial biofilm growth and discuss various
complicating factors in moving from theory to practice of measuring group
selection.
