MY NOMINATION FOR coolest article title of 2007 is Travis’ and coworkers’ “Deleterious Mutations Can Surf to High Densities on the Wave Front of an Expanding Population”.1 The idea of mutations surfing is not new, but this phenomenon was previously thought to apply to neutral mutations, while these researchers have discovered it is true of deleterious mutations as well.
Mutational surfing is a phenomenon that occurs when a population is expanding into new territory. At the borders of the expanding population, neutral mutations reach higher frequencies than would normally be expected. This is explained by the founder effect–by chance an individual with that mutation succeeds in colonizing new territory and reproduces, spreading the mutation to its descendants. An amplification occurs since each descendant will breed with others near the border, and since that border was colonized by the parent with the mutation, it’s likely their mate will carry the mutation as well, thus producing offspring likely to carry the mutation. This founder effect results in a neutral mutation reaching higher frequency and spatial distribution than would be expected in a static population.
It’s not hard to see how a neutral mutation could surf, but what about detrimental mutations? Usually these are suppressed by natural selection. But using modelling Travis et al. demonstrate that detrimental mutations are adept surfers as well. They are able to do this because of the nature of the population wavefront. Inside this border, individuals must compete with each other for resources, but at the wavefront an individual can escape this competition by being at the edge of the population, where it is not surrounded by competitors. The result is a slackening of natural selection that allows negative mutations to escape eradication. As the population grows outwards, negative mutations will be slowly eradicated in previously border areas now heavily colonized, and the negative mutation will only survive at the wavefront. This leads to surfing of the detrimental mutation from its original location out towards new territory.
The other effect of this slackening of selective forces is to reduce beneficial mutations to a more neutral status. Losing their selective drive, these beneficial mutations are subject only to genetic drift and may reach unusually low frequencies.
The result of this is an unusual mutation distribution in which beneficial mutations remain in the core of the population and do not travel far from their area of origin, but detrimental mutations spread from their area of origin, leaving the core, and are present on the borders of the population.
The authors additionally found that landscape features can greatly influence the distribution of mutations. Landscape bottlenecks funnel populations into a new area, and mutations arising in the bottleneck or just after it can surf very efficiently to high frequency on the wavefront. Other natural barriers such as rivers and mountain ranges also influence mutation distribution in a more intermediate fashion, making the distribution more heterogenous.
This phenomenon of mutational surfing is not to be confused with the dispersal-competition tradeoff, in which individuals at the wavefront are under selection for traits allowing rapid dispersion to new ground, resulting in a phenotype that may be detrimental in the core of the population. On the other hand, surfing mutations do not affect dispersal ability–they are propagating under genetic drift, and not under selection. An example of the dispersal-competition tradeoff is seen in the example of cane toads in Australia, which are under selection for larger size and longer legs to let them travel farther, but this results in spinal problems as the heavier weight places unusual stress on the skeleton.2
This discovery will be helpful in the study of invasive species, which has become a much more high profile area of research than it was in the past, with the recent introduction of various harmful species into new areas. It may also have application in the preservation of endangered species and their genetic diversity.
- Travis, J. M. J.; Münkemüller, T.; Burton, O. J.; Best, A.; Dytham, C.; Johst, K. “Deleterious Mutations Can Surf to High Densities on the Wave Front of an Expanding Population.” Molecular Biology and Evolution 2007, 24, 2334-2343; doi:10.1093/molbev/msm167.
- Brown, G. P.; Shilton, C.; Phillips, B. L.; Shine, R. “Invasion, stress, and spinal arthritis in cane toads.” Proceedings of the National Academy of Sciences 2007, 104, 17698-17700; 10.1073/pnas.0705057104.