Abstract
Many cellular populations are tightly packed, such as microbial colonies
and biofilms, or tissues and tumours in multicellular organisms. The
movement of one cell in these crowded assemblages requires motion of
others, so that cell displacements are correlated over many cell
diameters. Whenever movement is important for survival or growth, these
correlated rearrangements could couple the evolutionary fate of
different lineages. However, little is known about the interplay between
mechanical forces and evolution in dense cellular populations. Here, by
tracking slower-growing clones at the expanding edge of yeast colonies,
we show that the collective motion of cells prevents costly mutations
from being weeded out rapidly. Joint pushing by neighbouring cells
generates correlated movements that suppress the differential
displacements required for selection to act. This mechanical screening
of fitness differences allows slower-growing mutants to leave more
descendants than expected under non-mechanical models, thereby
increasing their chance for evolutionary rescue. Our work suggests that,
in crowded populations, cells cooperate with surrounding neighbours
through inevitable mechanical interactions. This effect has to be
considered when predicting evolutionary outcomes, such as the emergence
of drug resistance or cancer evolution.
