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When looking broadly at the tree of life, my feeling is that, after a divergence there is almost systematically one taxon that speciates way more than the other.

(There will have some approximations below it is not really the point of my question to discuss whether the urochordata is a sister group to the vertebrates or not.) Let's look at the kingdom Animalia for example:

  1. Chaonflagellata vs "the others"
  2. Sponges vs "the others"
  3. Cnidaire and cténaire vs "the others"
  4. Protostomia vs Deuterostomia (both are fairly equally diverse diverse! Some might not be happy with this claim!)...let's continue with the Deuterostomata
  5. echinodermata vs chordata
  6. cephalocordata vs vertebrata

etc ...

My questions:

  • Is there really a systematic difference in diversity between the branches?
  • Is this difference can be explained by chance?
  • Is it only a bias due to out ability to recognize more species in animals that look more like us (or a bias due to something else)?
  • Does it make sense to say: the more diverse is a taxon, the more likely it is that it gets even more diverse (because each species might have an equal chance to speciate or something like this)?
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Neat question. I'm tempted to assume that this must be related to where we recognize splits as being? I.e. its an artefact of our typology, rather than a property of the external world. It bears thinking about, though! You might use "key trait" or "key innovation" as a search term (I think I remember Galis gives a good introduction) –  Oreotrephes Jul 26 '13 at 0:32

1 Answer 1

up vote 2 down vote accepted

Bond and Oppell address your question points one, two and three empirically by looking for what they call "unbalanced bifurcations" as a sign of adaptive radiations in a well-resolved phylogenetic tree (spiders), and find that the number of unbalanced bifurcations does indeed exceed what they would expect to occur by chance.

As you mention and as I suggested in the comment, there are some serious problems that arise in trying to answer your question. Many of these are well presented in Cracraft, including assuming that higher taxa are comparable with one another, making arbitrary choices about what rank of taxa to compare, ignoring counterexamples, and qualifying rather than quantifying diversity. I think that Bond and Oppell effectively address most of these problems through rigorous, apriori definitions.

There's a diversity of opinions and evidence from biological systems on your fourth question point. Bond and Oppell connect these significantly unbalanced bifurcations to key traits (like orb-weaving) that open up new adaptive zones. I suppose therefore they might say that there's a fundamental factor (a key trait, and therefore open adaptive zones) driving diversification, and therefore might not agree with you that diversification would necessarily beget more diversification (at least, not in the absence of open adaptive zones). For instance, in Hawaiian tetragnathid spiders, ecomorph niches on an island are filled either through immigration from another island or through adaptive diversification (Gillespie). No ecomorph is represented by two sympatric species, so a niche filled by immigration will presumably not spur adaptive radiation. However, species radiations in Andean lupines (Hughes and Eastwood) and plethodontid salamanders (Kozak et al) seem not to have been accompanied by any key traits or particular ecological opportunity, and could theoretically be driven by the self-reinforcing mechanism you propose. As a related point, I suppose more diverse lineages could be less likely to go extinct; which could support diverse lineages becoming more diverse.

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Thks a lot for this excellent answer! –  Remi.b Jul 30 '13 at 6:37

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