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Cladograms B and C contain polytomies, where more than one branch descends from a single node.

A polytomy /pəˈlɪtəmi/, meaning many temporal based branches, is a section of a phylogeny in which the evolutionary relationships cannot be fully resolved to dichotomies. In a phylogenetic tree, a polytomy is represented as a node which has more than two immediate descending branches. They present an analytical problem, but can usually be better studied by utilizing more flexible phylogenetic network software such as SplitsTree, which can represent some polytomies as median networks.

Soft polytomies vs. hard polytomies

Two types of polytomies are recognised, soft and hard polytomies.^[1] Soft polytomies are the result of insufficient phylogenetic information: though the lineages diverged at different times – meaning that some of them are closer relatives than others – the available data does not allow recognition of this.

In contrast, hard polytomies represent more than two (three or more) speciation or lineage divergence events occurring from one same common ancestor, i.e. without their ancestral population, lineage or gene pool evolving in between, the resultant daughter species are equally distant from each other. In particular situations they may be common, for example when a species that has rapidly expanded its range or is highly panmictic undergoes peripatric speciation in different regions.

An example is the Drosophila simulans species complex. Here, the ancestor seems to have colonized two islands at the same time but independently, yielding two equally old but divergently evolved daughter species.

Recognizing hard polytomies

As DNA sequence evolution is usually much faster than evolution of complex phenotypic traits, it may be that genetic lineages diverge a short time apart from each other, while the actual organism has not changed if the whole ancestral population is considered. As few if any individuals in a population are genetically alike in any one population – especially if lineage sorting has not widely progressed – it may be that hard polytomies are indeed rare or nonexistent if the entire genome of each individual organism is considered, but rather widespread on the population genetical level if entire species are considered as interbreeding populations (see also species concept).

Note that "speciation or lineage divergence events occurring at the same time" refers to evolutionary time measured in generations, as this is the only means that novel traits (e.g. germline point mutations) can be passed on. Also note that in practical terms, our ability to distinguish between hard and soft polytomies is limited: if for example a kilobase of DNA sequences which mutate approximately 1% per million years is analysed, lineages diverging from the same ancestor within the same 100,000 years cannot be reliably distinguished as to which one diverged first.

In that respect, it is also important to realize that founder effects and genetic drift may result in different rates of evolution. This can easily confound molecular clock algorithms to the point where hard polytomies become unrecognizable as such.

References

^ Purvis, A., and T. Garland, Jr. 1993. Polytomies in comparative analyses of continuous characters. Systematic Biology 42:569–575.

External links

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

English Journal

A sequential cognitive diagnosis model for polytomous responses.

Ma W1, de la Torre J2.
The British journal of mathematical and statistical psychology.Br J Math Stat Psychol.2016 Nov;69(3):253-275. doi: 10.1111/bmsp.12070.
This paper proposes a general polytomous cognitive diagnosis model for a special type of graded responses, where item categories are attained in a sequential manner, and associated with some attributes explicitly. To relate categories to attributes, a category-level Q-matrix is used. When the attrib
PMID 27317397

Calcium Channel Blocker Use and Risk of Prostate Cancer by TMPRSS2:ERG Gene Fusion Status.

Geybels MS1,2, McCloskey KD3, Mills IG3, Stanford JL4,5.
The Prostate.Prostate.2016 Oct 18. doi: 10.1002/pros.23267. [Epub ahead of print]
BACKGROUND: Calcium channel blockers (CCBs) may affect prostate cancer (PCa) growth by various mechanisms including those related to androgens. The fusion of the androgen-regulated gene TMPRSS2 and the oncogene ERG (TMPRSS2:ERG or T2E) is common in PCa, and prostate tumors that harbor the gene fusio
PMID 27753122

Long and irregular menstrual cycles, polycystic ovary syndrome, and ovarian cancer risk in a population-based case-control study.

Harris HR1,2, Titus LJ3, Cramer DW4,5, Terry KL4,5.
International journal of cancer.Int J Cancer.2016 Oct 6. doi: 10.1002/ijc.30441. [Epub ahead of print]
Long and irregular menstrual cycles, a hallmark of polycystic ovary syndrome (PCOS), have been associated with higher androgen and lower sex hormone binding globulin levels and this altered hormonal environment may increase the risk of specific histologic subtypes of ovarian cancer. We investigated
PMID 27667654