The latitudinal species richness gradient in New World woody angiosperms is consistent with the tropical conservatism hypothesis

Kerkhoff, Moriarty & Weiser  (2014) The latitudinal species richness gradient in New World woody angiosperms is consistent with the tropical conservatism hypothesis. PNAS 111:81258130


Figure 4 from Kerkhoff et al. Diagram of ancestral–descendent transitions among different latitudinal zones

Lynsey McInnes

Lynsey McInnes

Yet another latitudinal diversity gradient/phylogenetic niche conservatism paper I hear you cry? Oh yes! I was deeply sceptical upon opening up this paper: more LDG/PNC, patchy datasets only including woody angiosperms, a dodgy family-level phylogeny. But somehow it almost won me over. If nothing else is was pretty brave.

The paper starts with a really clear introduction to the background to the question of what drives latitudinal diversity gradients, how the tropical conservatism hypothesis (lineages originate in the tropics and very few evolve the necessary adaptations to life outside the tropics, those that do then forming a temperate flora with tropical ancestry) is nested within other hypotheses such as ‘differential diversification rates’ or ‘out of the tropics.’ The paper goes on to summarise recent analyses finding evidence for or against tropical conservatism.

I also really appreciated that the authors were open as to the limitations of their dataset (patchy dataset, dodgy phylogeny, geographic restrictions). They conclude that these limitations do not prevent them concluding that they find robust evidence for tropical conservatism: most lineages are tropical, those that are temperate often have tropical ancestors. Done deal.

I’m trying really hard to distinguish between my hangups based on the dataset (e.g., is it ok to look at tropical-to-temperate transitions when the group you are looking at is not monophyletic (i.e., what about all the herbaceous lineages); is it ok to use an incomplete and family-level phylogeny (and only one at that)) and on the methodology (e.g., I did not understand their rerooting method for inferring ancestral states). I would have liked to have seen more extensive analyses that filled in the missing lineages or added ambiguity to their tropicality index and looked at any biases their restricted datasets might introduce. My gut feeling is that when we obtain the holy grail of ‘global, taxonomically comprehensive distributional, phylogenetic, climatic, and ecophysiological data resources ‘ (in the words of the authors), evidence for tropical conservatism will remain or will become stronger, but at the moment perhaps it is still presumptive to barge on pretending the data is adequate (at least without more comprehensive bias testing). Just so you know I also belong to the club of people that have barged on regardless. Maybe we all need to spend more time generating these better resources. Think of the questions we could ask when we get them.

Grumble, grumble.

Why is it so popular to investigate niche conservatism at the moment? Really, its a relatively new bandwagon which, if my rant above is anything to go by, is already ‘going out of fashion.’ What is going on? As the authors mention, the hypothesis has appeal because it integrates ecological and evolutionary drivers of diversity patterns and incorporates elements from a bunch of the most popular hypotheses for explanations behind the latitudinal diversity gradient. It’s delightfully vague in terms of what a niche is, falling back mostly on co-linear climatic factors and it can probably be adapted endlessly to include additional niche elements. All species have a niche, some aspects of which are likely to be conserved among closely-related species, the whole package is very satisfying.

Beyond getting complete data resources, a conclusive ‘yes’ to the tropical conservatism hypothesis also needs more sophisticated methods for ancestral state reconstruction. We get into a circular situation if we are looking for conservatism, but assumes minimal change across the phylogeny. Can fossils help? Can microevolutionary studies help? Can we ever really know?

Final note: extinction. What about extinction?

Will Pearse

Will Pearse

I’m not quite as jaded as Lynsey, but I admit I was somewhat skeptical about this paper; I’ve read so many diversity gradient papers that they’ve started merging in my head (like James Bond films). However, I liked this paper (like James Bond films): they bring their limitations out right away (of course it’s not a perfect dataset, they’re examining the world!) and by sticking to their hypotheses they are able to push the field forward.

Lynsey mentions the woody bias, but woodiness may not be as labile as I used to think. This either means that it’s not a ‘quick-fix’ trait that fluctuates to ‘allow’ species to move quickly (good for this study), or it means that it constrains species’ thermal tolerances and thus can affect long-term evolutionary dynamics (a bad thing for this study). Evolutionary biology is, to an extent, a historical science, and so whatever choices an investigator makes are always going to be pulled apart by someone being awkward and announcing that there was some other factor they didn’t take into account. Equally, there’s a real trend at the moment to be snarky about methods – my snarky comment is that attempting to reconstruct ancestral states over a tree of this size using either a Brownian motion or OU model is going to lead to problems. I’m not sure how much we can trust these ancestral states, but then again I’m not really sure how better they could have been done, because frankly I’m always concerned about ancestral state reconstruction.

However, I buy their hypothesis that younger lineages tend to be temperate. The question now is whether this is because of the general cooling of the Earth over the last 34 million years (as the authors seem to think), or whether it’s because it takes ~34 million years for clades to die out once they move out of the tropics. Paleo-ecologists have been asking questions like this for some time now, but perhaps now that they have a specific time-frame within which to look they have more hope of finding an answer. Fingers crossed!


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