  Environmental heterogeneity can drive genetic heterogeneity in expanding
populations; mutant strains may emerge that trade overall growth rate for an
improved ability to survive in patches that are hostile to the wild type. This
evolutionary dynamic is of practical importance when seeking to prevent the
emergence of damaging traits. We show that a sub-critical slow-spreading mutant
can attain dominance even when the density of patches is below their
percolation threshold and predict this transition using geometrical arguments.
This work demonstrates a phenomenon of ''assisted percolation'', where one
sub-critical process assists another to achieve super-criticality.
