Environmental heterogeneity can tip the population genetics of range expansions

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Abstract

The population genetics of most range expansions is thought to be shaped by the competition between Darwinian selection and random genetic drift at the range margins. Here, we show that the evolutionary dynamics during range expansions is highly sensitive to additional fluctuations induced by environmental heterogeneities. Tracking mutant clones with a tunable fitness effect in bacterial colonies grown on randomly patterned surfaces we found that environmental heterogeneity can dramatically reduce the efficacy of selection. Time-lapse microscopy and computer simulations suggest that this effect arises generically from a local 'pinning' of the expansion front, whereby stretches of the front are slowed down on a length scale that depends on the structure of the environmental heterogeneity. This pinning focuses the range expansion into a small number of 'lucky' individuals with access to expansion paths, altering the neutral evolutionary dynamics and increasing the importance of chance relative to selection.

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All data generated or analysed during this study are included in the manuscript and supporting files. Source data files have been provided for all figures.

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Matti Gralka

Department of Physics, University of California, Berkeley, Berkeley, United States

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0003-4599-1859
Oskar Hallatschek

Department of Physics, University of California, Berkeley, Berkeley, United States

For correspondence
ohallats@berkeley.edu

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0002-1312-5975

Funding

Simons Foundation

Oskar Hallatschek

National Science Foundation

Oskar Hallatschek

National Institute of General Medical Sciences

Oskar Hallatschek

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

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This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.