  Microbial colonies are experimental model systems for studying the
colonization of new territory by biological species through range expansion. We
study a generalization of the two-species Eden model, which incorporates local
frequency-dependent selection, in order to analyze how social interactions
between two species influence surface roughness of growing microbial colonies.
The model includes several classical scenarios from game theory. We then
concentrate on an expanding public goods game, where either cooperators or
defectors take over the front depending on the system parameters. We analyze in
detail the critical behavior of the nonequilibrium phase transition between
global cooperation and defection and thereby identify a new universality class
of phase transitions dealing with absorbing states. At the transition, the
number of boundaries separating sectors decays with a novel power law in time
and their superdiffusive motion crosses over from Eden scaling to a nearly
ballistic regime. In parallel, the width of the front initially obeys Eden
roughening and, at later times, passes over to selective roughening.
