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Phylogenetic trees with >2 branches on a node are polytomic, and polytomy can appear on trees for two reasons. Firstly a lack of information in the data prevents proper resolution within a clade, called soft polytomy. Alternatively, hard polytomy may exist, this is when there truly is a simultaneous split from one lineage (species) in to more than two. Yang states that "it may be argued that hard polytomies do not exist" but makes no citation or attempts at theoretical discussion. Are there any papers which discuss the theoretical possibility of hard polytomy and any evidence of it occurring?

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Image from http://biology.fullerton.edu/biol404/phylo/polytomies.html

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I think that changes in features that determine a node may happen in parallel or close together in a sequence. If the fossil record is not complete enough then the cladogram has to contain these stars. From a stochastic statistic point of view, although unlikely, it could actually happen that 2 or more characteristics develop exactly during a similar time frame, so from a philosophical point of view, I disagree with yang.

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    $\begingroup$ I think Yang's wrong too, divergence is not an instantaneous process so it's possible that two (or more) divergence events may be occurring at once within the same population, but it is probably quite unlikely $\endgroup$ – rg255 Nov 3 '14 at 10:55
  • $\begingroup$ @GriffinEvo "but it is probably quite unlikely" - I think rare would be a better word in this case... $\endgroup$ – inf3rno Nov 3 '14 at 13:23
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    $\begingroup$ @GriffinEvo Technically one event of mutation can give rise to only one mutant. But as Chris Stronks said, it is possible that in a given time frame, due to lack of evidence such a result is possible (especially if intermediates perish — perhaps as in case of LUCA). However, it is almost improbable for trees based on molecular phylogeny (Unlikely that a given nucleotide position has more than two polymorphs and there are no differences elsewhere). $\endgroup$ – WYSIWYG Nov 4 '14 at 11:28
  • $\begingroup$ One may imagine the following scenario: different unlinked mutations occur in the germline of a single individual, this individual has at least three offspring, each carrying a different mutation, and each being the ancestor of a new clade. Even if said offspring are not born simultaneously, in terms of cladogram, there will be a single splitting event. $\endgroup$ – bli Oct 13 '17 at 5:48
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You might want to have a look at some of these: Patterns of speciation and limits to phylogenetic resolution, Molecular polytomies, Problems with "soft" polytomies, and Polytomies, the power of phylogenetic inference, and the stochastic nature of molecular evolution.

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    $\begingroup$ Thanks, Do you have time to expand on these links? $\endgroup$ – rg255 Nov 4 '14 at 8:33
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    $\begingroup$ Sorry, no. Besides, I'm not knowledgeable in this area; I just read some abstracts found via Google Scholar. I suspect that essentially all "hard" polytomies are just soft polytomies where evidence to resolve them is hard to acquire. $\endgroup$ – mgkrebbs Nov 4 '14 at 20:20

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