  Pathogen transmission and virulence are main evolutionary variables broadly
assumed to be linked through trade-offs. In well-mixed populations, these
trade-offs are often ascribed to physiological restrictions, while populations
with spatial self-structuring might evolve emergent trade-offs. Here, we
reexamine a model of the latter kind proposed by Ballegooijen and Boerlijst
with the aim of characterising the mechanisms causing the emergence of the
trade-off and its structural robustness. Using invadability criteria, we
establish the conditions under which an evolutionary feedback between
transmission and virulence mediated by pattern formation can poise the system
to a critical boundary separating a disordered state (without emergent
trade-off) from a self-structured phase (where the trade-off emerges), and
analytically calculate the functional shape of the boundary in a certain
approximation. Beyond evolutionary parameters, the success of an invasion
depends on the size and spatial structure of the invading and invaded
populations. Spatial self-structuring is often destroyed when hosts are mobile,
changing the evolutionary dynamics to those of a well-mixed population. In a
metapopulation scenario, the systematic extinction of the pathogen in the
disordered phase may counteract the disruptive effect of host mobility, favour
pattern formation and therefore recover the emergent trade-off.
