  The capacity to aggregate through chemosensitive movement forms a paradigm of
self-organisation, with examples spanning cellular and animal systems. A basic
mechanism assumes a phenotypically homogeneous population that secretes its own
attractant, with the well known system introduced more than five decades ago by
Keller and Segel proving resolutely popular in modelling studies. The typical
assumption of population phenotypic homogeneity, however, often lies at odds
with the heterogeneity of natural systems, where populations may comprise
distinct phenotypes that vary according to their chemotactic ability,
attractant secretion, {\it etc}. To initiate an understanding into how this
diversity can impact on autoaggregation, we propose a simple extension to the
classical Keller and Segel model, in which the population is divided into two
distinct phenotypes: those performing chemotaxis and those producing
attractant. Using a combination of linear stability analysis and numerical
simulations, we demonstrate that switching between these phenotypic states
alters the capacity of a population to self-aggregate. Further, we show that
switching based on the local environment (population density or chemoattractant
level) leads to diverse patterning and provides a route through which a
population can effectively curb the size and density of an aggregate. We
discuss the results in the context of real world examples of chemotactic
aggregation, as well as theoretical aspects of the model such as global
existence and blow-up of solutions.
