Here, we compare and contrast the traits and selective forces influencing the evolution of dispersal in marine and terrestrial systems. From this comparison, a unifying question emerges: when is dispersal for dispersal and when is dispersal a by-product of selection on traits with other functions?
Dispersal sometimes seems like one of the “big things” that gets lost in the present trajectory of molecular ecology. We know a lot about how dispersal varies between species, populations, and individuals, but it sure is a tricky set of parameters to include in most modern popgen analyses.
One (of many) reasons why dispersal and gene flow are so difficult to generalize is that the why and how can vary so greatly between organisms. Is that jellyfish “dispersing” or just floating around? Is that frog dispersing because of the density of conspecifics or some other reason?
“Wait, uhh, am I dispersing here or what?” (Image by Sanjay Acharya)
Burgess et al. recently published a review that tackles some of these issues and points out that their are big differences between marine and terrestrial dispersal (surprise!) that mostly get left out of theory. However, a bigger goal of the review is asking scientists to think harder about dispersal as a direct adaption or as a by-product of some other process, and they outline a multivariate model for getting started.
Figure 1 (B) of Burgess et al. (2015)
I’ll leave it up to how adaptationist you are for deciding when a by-product is actually an adaptation. I’m not here to get all spandrel-y. For now, its hard to argue against understanding the complexity that underlies dispersal, whether by land or by sea.
A trait-based approach, focused on selection on traits that influence dispersal, will not only improve our understanding of when dispersal is a direct adaptation versus a by-product, but can also advance the integration of theory and data. Theories of dispersal evolution would benefit from considering the evolutionary causes of movement in general as well as additional agents of selection on the multiple traits that influence dispersal specifically.
Burgess, S. C., Baskett, M. L., Grosberg, R. K., Morgan, S. G., & Strathmann, R. R. (2015). When is dispersal for dispersal? Unifying marine and terrestrial perspectives. Biological Reviews. DOI: 10.1111/brv.12198