Rough landscapes – like mountains and valleys – may influence on the fate of mutations arising in a population. Image credit: Simon Matzinger (CC0)
Throughout evolution, countless populations have expanded their territories by invading new areas. This invasion can happen on the scale of kilometers and millennia – such as when humans migrated out of Africa – or millimeters and months, such as the growth of a population of cells in a solid tumor. During this expansion, mutations can occur that can either increase or decrease fitness in the new territory. If a favorable mutation occurs at the edge of the population, then it has plenty of room to expand. Such a mutation has a high chance of becoming established, and so it can have a very strong impact on the genetic makeup of the population. This increase in evolutionary advantage in individuals at the front is called “gene surfing”.
This phenomenon is well known in populations living in “homogeneous” territories, where the new space a population is invading is more or less the same as the one they already occupy – think of the endless flat grasslands of the Siberian steppes. But in reality, many territories are not like that. What happens if the new territory is not completely homogeneous? For instance, if a species’ expansion is impeded by a mountain range with a series of valleys.
Gralka and Hallatschek investigated how such changes in landscape could affect phenomena like gene surfing. Experiments using E. coli as a model system and computer simulations showed that a varied environment – such as roughness akin to a mountain range and valleys, but on a bacterial scale – had a strong influence on the fate of mutations arising in a population. Whether the environment is favorable for expansion or not in the place where the mutation happens becomes much more important than how the mutation itself affects fitness. So, if a beneficial mutation occurs at a cliff-edge, it is not likely to get far. But if it happens at a population edge by the bacterial equivalent of gently rolling hills, there is a much better chance of the mutation taking hold.
The findings suggest that the amount a population can adapt during expansion is limited, and it can even lead to the spread of harmful mutations in a population if they occur in just the right spot. Piecing together these scenarios is important in order to accurately infer the evolutionary history of a species based on mutations present in its genome now. This type of knowledge can also be useful in developing new treatments for cancers, making use of these evolutionary processes to slow or halt a tumor’s expansion.
