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: