On density-dependent diversification and speciation in birds

Posted by Erik Svensson


This coming Wednesday (March 21 at 13.30 NOTE TIME!!!!), we will discuss a paper about the (possible) density-dependent slowdown of cladogenesis in birds with the progress of adaptive radiation. It is a paper that was published relatively recently, although it is not entirely new, in PLoS Biology by Albert Phillimore and Trevor D. Price. The title of the paper is:
"Density-Dependent Cladogenesis in Birds"
The authors have used molecular phylogenies from various avian groups to test the hypothesis that diversification rates decline with the progress of adaptive radiation, and as we are approaching the present. Here is the Abstract to the article, which can be downloaded here (PLoS Biology is a "Open Acess"-publisher, which makes everything easier):

Abstract 

A characteristic signature of adaptive radiation is a slowing of the rate of speciation toward the present. On the basis of molecular phylogenies, studies of single clades have frequently found evidence for a slowdown in diversification rate and have interpreted this as evidence for density dependent speciation. However, we demonstrated via simulation that large clades are expected to show stronger slowdowns than small clades, even if the probability of speciation and extinction remains constant through time. This is a consequence of exponential growth: clades, which, by chance, diversify at above the average rate early in their history, will tend to be large. They will also tend to regress back to the average diversification rate later on, and therefore show a slowdown. We conducted a meta-analysis of the distribution of speciation events through time, focusing on sequence-based phylogenies for 45 clades of birds. Thirteen of the 23 clades (57%) that include more than 20 species show significant slowdowns. The high frequency of slowdowns observed in large clades is even more extreme than expected under a purely stochastic constant-rate model, but is consistent with the adaptive radiation model. Taken together, our data strongly support a model of density-dependent speciation in birds, whereby speciation slows as ecological opportunities and geographical space place limits on clade growth.

We meet at 13.30 (not 13.00!) to discuss this interesting article.

On ecological speciation, tempo and mode of evolution

The coming week's lab-meeting (16 December 2009), will be the last one for 2009. Due to teaching obligations, I would like the meeting to start somewhat later than usual, at 10.30. The topic of this week's lab-meeting will be speciation, and we will discuss two papers published in 2009 in and Nature and Science (abstracts are provided below):

Phylogenies reveal new interpretation of speciation and the Red Queen

Chris Venditti¹, Andrew Meade¹ & Mark Pagel^1,2

(Nature advance online publication)

Evidence for Ecological Speciation and Its Alternative

Schluter Dolph

Science (2009) 323: 737-741

These two papers are interesting, because they reflect radically different views on the causes of speciation and the drivers of speciation processes. I therefore thought it would be interesting to discuss them with this in mind, and contrast their different underlying viewpoints against each other. Who is correct and who is wrong? Or are both correct, and if so, in what domains?

We will thus meet at 10.30 in "Darwin" on Wednesday 16 December. Any fika-volunteer?

Abstracts follow below:

Chris Venditti¹, Andrew Meade¹ & Mark Pagel^1,2

Phylogenies reveal new interpretation of speciation and the Red Queen

The Red Queen¹ describes a view of nature in which species continually evolve but do not become better adapted. It is one of the more distinctive metaphors of evolutionary biology, but no test of its claim that speciation occurs at a constant rate² has ever been made against competing models that can predict virtually identical outcomes, nor has any mechanism been proposed that could cause the constant-rate phenomenon. Here we use 101 phylogenies of animal, plant and fungal taxa to test the constant-rate claim against four competing models. Phylogenetic branch lengths record the amount of time or evolutionary change between successive events of speciation. The models predict the distribution of these lengths by specifying how factors combine to bring about speciation, or by describing how rates of speciation vary throughout a tree. We find that the hypotheses that speciation follows the accumulation of many small events that act either multiplicatively or additively found support in 8% and none of the trees, respectively. A further 8% of trees hinted that the probability of speciation changes according to the amount of divergence from the ancestral species, and 6% suggested speciation rates vary among taxa. By comparison, 78% of the trees fit the simplest model in which new species emerge from single events, each rare but individually sufficient to cause speciation. This model predicts a constant rate of speciation, and provides a new interpretation of the Red Queen: the metaphor of species losing a race against a deteriorating environment is replaced by a view linking speciation to rare stochastic events that cause reproductive isolation. Attempts to understand species-radiations³ or why some groups have more or fewer species should look to the size of the catalogue of potential causes of speciation shared by a group of closely related organisms rather than to how those causes combine.

Schluter Dolph

Natural selection commonly drives the origin of species, as Darwin initially claimed. Mechanisms of speciation by selection fall into two broad categories: ecological and mutation-order. Under ecological speciation, divergence is driven by divergent natural selection between environments, whereas under mutation-order speciation, divergence occurs when different mutations arise and are fixed in separate populations adapting to similar selection pressures. Tests of parallel evolution of reproductive isolation, trait-based assortative mating, and reproductive isolation by active selection have demonstrated that ecological speciation is a common means by which new species arise. Evidence for mutation-order speciation by natural selection is more limited and has been best documented by instances of reproductive isolation resulting from intragenomic conflict. However, we still have not identified all aspects of selection, and identifying the underlying genes for reproductive isolation remains challenging.