On age and species richness of higher taxa

New from @tanja_stadler and co:

They were thinking about what models might not have a monotonically positive age-diversity relationship for clades:

Several studies have investigated relations between species richness and ages of higher taxa. Three methodological articles (Magallón and Sanderson 2001; Bokma 2003; Paradis 2003) prominently featuring the idea that E[n] = e(λ − μ)t have together been cited by more than 500 articles. Furthermore, Rabosky et al. (2012) investigated the behavior of a simple model where higher taxa originate under a Poisson process (see also Aldous et al. 2008; Maruvka et al. 2013). They found that such a model was expected to result in positive relationships between stem clade age and species richness, even when rates of species diversification varied among clades, provided that rates within clades were constant through time. As we have shown here, the expectation of a positive relationship between stem age and species richness may be incorrect, as it depends on the particular model of diversification and definition of higher taxa.

Many studies have identified young taxa as “unexpectedly” species rich, but our results show that such patterns can result from the manner in which higher taxa are delimited. For example, under scenarios i-b and ii-b, clades with young stem ages are expected to contain not fewer but more species than clades with old stem ages (table 1). In other words, studies may have incorrectly identified young taxa as unexpectedly species rich because they neglected how taxa were defined, and consequently incorrectly expected young taxa to be species poor.

Here is the model they consider:

Although I understand the focus on ‘higher taxa’ as ‘monophyletic clades’ in the more recent papers on this topic, I somewhat befuddled by the lack of reference to the paleontological literature on this same topic (e.g. Patzkowsky, 1994; Foote, 2011), which allows for paraphyletic ‘para-clades’.

At least in my own experience, the fossil record often shows nested paraphyletic groups replacing one another (in terms of richness), due to factors such as extinction events or key adaptations. For example, this is very common in the graptolites (my group) but many others also. Thus, I think if we really want to talk about the evolutionary dynamics of ‘higher taxa’ we should be discussing the dynamics of paraphyletic groups.

Do we really think the higher taxa in the (extant) groups we study are defined based on some relationship of the age of monophyletic clade or, conversely, are groups defined by morphological distinctiveness and richness, often with an untested potential for monophyly? Me, well, I don’t study extant things so I can’t answer that.

That’s an interesting perspective.

Would you mind pasting links to the papers you cite? I couldn’t find the first one on a quick search. Thanks!

http://www.jstor.org/stable/2401215

http://rsbl.royalsocietypublishing.org/content/8/1/135.short

Well, I kind of made it hard by getting both years dead wrong.

Thanks for that, David. I took a peek and yes, it’s certainly related. Perhaps @tanja_stadler will see it fit to comment.

“Do we really think the higher taxa in the (extant) groups we study are defined based on some relationship of the age of monophyletic clade or, conversely, are groups defined by morphological distinctiveness and richness, often with an untested potential for monophyly? Me, well, I don’t study extant things so I can’t answer that.”

Well I do not want to claim that higher taxa are defined only based on age - in fact they are not. However, there might be a tendency that we define higher taxa implicitly also based on their age. With our paper in Am Nat we wanted to point out that higher taxa definitions which (implicitly) involve age will lead to prior expectations on species richness such as independence of stem age and species richness. Overall, I hope that our work with Folmer Bokma will tricker a discussion on how to model the definition of higher taxa. Such a modelling concept will allow us to investigate what clade age / species richness relationship we expect.

Oh, I concur! Looking back, I was really just commenting on why a particular topic of ‘higher taxa’ analysis seems to be little discussed today (involving paraphyletic taxa), and whether it still warrants discussion (I think it does), which isn’t really a specific criticism of your paper but rather of the current mood of the field.