Why abundant tropical tree species are phylogenetically old

Wang et al. PNAS 110(40): 16039-16043. Why abundant tropical tree species are phylogenetically old.

Will Pearse

Will Pearse

I’ve really come to appreciate Neutral Theory. Great conceptual leaps have come from thinking neutrally, such that drift is something a generation of ecologists now take for granted as part of orthodox ecological theory. I think this paper is a perfect example of the exciting work that can come from predicting evolution from ecology, and it’s something Neutral Theory helps make possible.

That said, I have two (slightly snarcky) criticisms. The first is I don’t think ‘species age’ is necessarily the most useful thing to be working with because a lot of things go into species age, and so it doesn’t make for the best test of many models. For example, extinction of closely related species will increase species age – if your thirty closest relatives die, you look older. The second is using a phylogeny from Phylomatic to make predictions about close relatives, because such phylogenies tend to have less resolution among congeners and close relatives. You can see this problem in the horizontal lines in figure 2a – all the congenerics have the same age because they’re within the same polytomy.

More fundamentally and less snarkily, this paper makes me think about models of speciation. What would protracted speciation look like when plotted like this? Do we find these patterns with species that undergo frequent hybridisation, like oaks? I think one of the great strengths of Neutral Theory is it lets us make predictions about the shape of present-day phylogeny, and more papers like this that move from evolutionary process to ecology are only a good thing.

Lynsey McInnes

Lynsey McInnes

First, apologies for the delay in posting PEGE this week, my fault entirely.Now, onto the paper. This was Will’s pick this week (because he loves Barro Colorado Island) and I gamely went along with it. As ever, I spent less time with the paper than I should, but found it really thought-provoking, if a bit odd in places.I like neutral theory and neutral theory predictions. It appeals to the side of me that doesn’t intuitively know what the ecological differences are among species, I like that you can start from a point where there aren’t any. And it amuses me that a theory can rile people so badly. In addition, I love most studies that attempt to bridge across scales and believe that there is a lot to be done in this area using neutral theory. Sure, we’ve come a long way since Hubbell’s hastily added speciation mechanism in the original book, but I don’t think we’ve exhausted possibilities yet.

I’ve got a confession to make. I’ve long been intrigued by range size, but never got to grips with abundance (my macroecological background rearing its ugly head again). So, the intricacies of this analysis did elude me a bit, but basically the authors find that if they add variable speciation rates to an otherwise neutral model they recover the empirical positive correlation in species age and abundance found for some tropical trees. Without the addition, the relationship should be flat or humped depending on speciation mode (mutation- or fission-, both a bit dodgy). The authors do mention that abundant species are usually also large ranged, so I could perhaps sub in range size for abundance and feel more comfortable (but would likely piss someone off).

The authors discuss how the range size-age correlation is often explained by niche differences, large range species are ecological generalists and buffered from extinction from, e.g., climatic fluctuations. They suggest their neutral model is more parsimonious than invoking niche differences and put forward ways to test this. I have a feeling their model would fall down in the face of these tests. There is also the chicken and the egg issue that there is plenty of evidence that most (all?) large range species are (at least now) more generalist than small range species…they have to be just to occupy really large ranges.

So, there is something circular at looking at ages, abundances and ranges in a neutral setting. One somehow needs to look at species as they speciate (and then they are all young) and the hunt to find reasons why some species are old (bad/unwilling to speciate) and especially how some species are old AND large ranged/abundant seems to remain unanswered. Models such as those in this paper and related ones might be the way to go, but it is going to be hard to avoid circularity.

One other thing that is dodgy and difficult to get around is how to define a species’ age. We don’t often know what species budded off from which or whether a split was more ‘even’ and this impacts on what is ‘old’ or not. Resetting age at every node of a phylogeny seems a naive (but understandable) way to go about things (and lets not even get into what extinction does to these ages).

Oops, this came out as a bit of stream of consciousness. Great paper for mulling over and I welcome all attempts to bridge macro and micro, evolutionary and ecological scales of analysis. The world is not neutral, but it seems like there is a lot to be gained from starting from the premise that it is.


About will.pearse
Ecology / evolutionary biologist

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