Dispersal capacity predicts both population genetic structure and species richness in reef fishes

Riginos et al. 2014 Dispersal capacity predicts both population genetic structure and species richness in reef fishes American Naturalist 184:52-64

Distance, distance, everywhere. Figure 2 from

Distance, distance, everywhere. Figure 2 from Rignios et al.

Will Pearse

Will Pearse

This was a strange paper for me to read in some ways, because it harks back to some things that I probably should know something about: fish dispersal (honestly!), diversification, and phylogenetic analyses. The basic idea is that fish that brood their larvae, as opposed to releasing them into the water and letting them do their thing, have more spatial population genetic structure.

It may not come as much surprise to you that species that spray their genetic material with gay abandon have less genetic structure, but this is a pretty comprehensive investigation and is nice for that. It was, however, a surprise (to me) that there isn’t much phylogenetic signal to genetic differentiation. Genetic differentiation is something I would expect to ‘play out’ in the context of a whole host of other ecological factors, all of which will likely exhibit wildly different rates and kinds of evolution. Thus perhaps a single, simple thing like a lambda value isn’t really ever going to capture that level of complexity. Similarly, I feel like there may be another paper coming after this one examining the species richness of the families in more detail. More complex model-fitting exercises might have helped the authors weigh in a little with the Rabosky et al. radiation literature that they reference, but doing so would probably be a lot of work, so I can understand why they might want to leave that for another day!

The authors mention there is likely to be variation in these patterns across space, and I think no one would disagree with that. My personal thought was that this variation should be mapped onto the oceanographic conditions and the timing of reproduction: the currents around reefs are notoriously variable and strong (just ask a diver!) and I would be very surprised if it was easy to account for all of this in a single analysis. Equally, the timing of reproduction could be important since direction and speed of currents change so frequently (and often reliably) throughout the year. Of course, it’s been a very long time since I pretended to know something about the ocean, so I’m likely very wrong about this. All in all… it sounds like it’s a good time to be working on diversification in reef fish!

Lynsey McInnes

Lynsey McInnes

I was really looking forward to this paper. I’m not quite sure what I was hoping for, but I guess I was hoping for big insights underpinning the final line of the abstract: ‘our findings provide a compelling case for the continuity between micro- and macroevolutionary processes of biological diversification and underscore the importance of dispersal-related traits in influencing the mode and tempo of evolution.’

I’m not sure I came away quite satisfied, but that is probably due to my unrealistically high expectations rather than any fault of the authors. In short, they find that reef fishes that hold tight to their eggs have both higher genetic differentiation, or structure, and more species per clade. They infer this is due to less gene flow in benthic guarding species so that populations do not homogenise, instead they can diverge and ultimately speciate.

My disappointment probably stems from reading the wrong paper. I think I was excited to read how they quantified differentiation, but this is lifted from an early paper of the same authors: Effects of geography and life history traits on genetic differentiation in benthic marine fishes published a little while ago in Ecography that I’m definitely going to go read now (and probably should read before posting this post).
I do believe that dispersal-related mechanisms must often underpin diversification patterns and the results documented here do support this idea. I still wonder if we will ever hit upon a more general way to understand the shape of this relationship. For instance, here the fishes possess a handy binary trait that makes classifying them as good and bad dispersers easy. Other analyses have found, using e.g. wing length as a more continuous proxy for dispersal ability, that intermediate dispersers are the most diverse because they move enough to get away from where they were born, but not too well that they constantly homogenise. Is there any way of making this more general? One could argue that Fst/other genetic measures provide this general measure, but what are the species’ traits that actually underpin variation in these measures. Does there need to be a general measure? How general should it be?

Really, I imagine that we probably still need to work out what dispersal is, to actually consider intra-specific variation in dispersal ability and the potential for dispersal ability to evolve because we can really get to grips with how it actually influences diversification propensity only if we are all the same page as to what it is.

I also wonder if the current interest in dispersal ability stems from the fact that its a catch-all trait that nicely links ‘nature of the landscape’ extrinsic factors and ‘ability to move across landscape’ intrinsic factors. Perhaps our problem is that dispersal ability is a stupid trait to consider as it means something different and encompasses different things in different taxa. Maybe we need to build up from smaller building blocks (as here with benthic guarding as a trait)?

Speaking as a budding population geneticist, we should probably also work out if the ways we quantify population differentiation are adequate and look closely at that nasty time-scale between a population differentiating and two or more populations being reproductively isolated and happy to be called new species.

As ever, we need more good studies such as this one, combined we better comparative studies across taxa. Lots to do, go go go.

One more thing – Happy New Year! I’m (Will) very sorry that this post is so late – I’m just about to start a new job in a new continent and all of that has made the Christmas period a bit fraught. All of this running around has made me be a nightmare in a lot of ways, and sadly PEGE took an unexpected holiday break while I handled all of that. Sorry for the bother, and thank you for reading!



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