Evolution at the Edge of Expanding Populations

Area of Science:

• Evolutionary Biology
• Population Dynamics
• Mathematical Biology

Background:

• Predicting the evolution of expanding populations is crucial for managing biological threats like invasive species and cancer metastasis.
• Organisms often evolve trade-offs between reproduction and dispersal, with some paying a growth cost for faster dispersal.

Purpose of the Study:

• To identify a simple rule determining when selection favors faster dispersal despite a reproductive cost.
• To explain experimental observations of bacterial evolution towards faster dispersal with a growth penalty.

Main Methods:

• Analysis of a two-species model based on the Fisher equation to define population expansion rate (v) from growth (r) and dispersal (D).
• Mathematical derivation of the condition for a faster mutant to invade an established population.
• Numerical simulations and time-course experiments with swarming bacteria.

Main Results:

• Population expansion rate (v) is determined by the interplay of growth and dispersal, independent of carrying capacity.
• A mutant invades if its expansion rate (v2) exceeds the established population's rate (v1), defining the maximum growth cost a faster mutant can sustain.
• Faster dispersers can succeed even under less favorable conditions, though delayed.

Conclusions:

• The expansion rate acts as a form of "traveling wave fitness."
• This fitness metric, combined with trade-offs, can predict the evolutionary trajectory of expanding populations.
• The "survival of the fastest" is favored when dispersal rate outweighs growth cost.