March 2011


Cumulative Spatial Sorting: An Overlooked Evolutionary Contributor

SUMMARY: Geographical edges of species range dispersal are evolutionary focal points.
It's commonly thought that species which reproduce the most (perhaps more precisely stated as genes that facilitate reproduction) over time are the ones which drive evolution. This isn't exclusively true.

Richard Shine (University of Sydney, Australia) and coworkers report on an often overlooked alternative/complimentary evolutionary driving force. Spatial sorting, which is not inherently driven by reproductive success, nevertheless facilitates rapid genetic dispersal.

A modification of conventional evolutionary theory.

The scientists begin their discussion by pointing out that behavior, wing size, and other traits relevant to enhanced species dispersal accumulate, by definition, at the geographical edges of an expanding population. Many genes facilitate this dispersal, e.g. those relevant to speed and endurance.

These species, being in close proximity, breed with each other, their fastest-dispersing offspring breed with each other, and so on. In this manner, future generations disperse more readily than preceeding generations.

This paradigm differs from traditional natural selection. Here, species dispersal is a fundamental evolutionary driving force, although not to the exclusion of survival and reproduction.

The scientists developed a theoretical model demonstrating that cumulative spatial sorting can drive dispersal probability at geographical edges in the absence of traditional natural selection. However, in my opinion, their strongest argument is an analysis of exerimental research suggesting the existence and importance of spatial sorting for evolution.

They cite many examples, e.g. the flight muscles of butterflies with expanding ranges are more highly developed, and invasive Australian toads at geographical dispersal edges possess more endurance, longer legs, and other dispersal-favoring traits.

The scientists make it very clear that it's difficult to completely experimentally disentangle spatial sorting from traditional natural selection. However, the former may explain many observations, without a need for the latter.

For example, dispersal-inclined populations may inherently appear in neighboring habitats, as opposed to the common hypothesis of convergent adaptation. Furthermore, dispersal-inclined populations may leave a given location, thereby inherently leaving behind a population not adapted to dispersal, as opposed to the common hypothesis of dispersed populations being unfit for survival.

Implications and future directions.

To me, the scientists' most useful nugget is an additional tool for countering people who "disprove" evolution by claiming that complex traits cannot come about via generational selection. Rather than explaining this question through the concept of gradual beneficial intermediate changes, one can reasonably hypothesize that dispersal may inherently facilitate genetic mixing of distinct populations.

In other words, multiple lineages readily interact with one another. These lineages present distinct evolutionary solutions that compete with one another for overall fitness.

Evolution clearly occurs in space as well as the more commonly understood concept of time. Determining the relative importance of both is a topic of future experimental and theoretical research (by someone far more qualified than me).

NOTE: The scientists' research was funded by the Australian Research Council.
Shine, R., Brown, G. P., & Phillips, B. L. (2011). An evolutionary process that assembles phenotypes through space rather than through time Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1018989108