関: polytomous

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/06/28 17:17:54」(JST)

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For other uses, see polychotomy.

Cladograms containing polytomies

A polytomy /pəˈlɪtəmi/, meaning many temporal based branches, is a section of a phylogeny in which the evolutionary relationships can not 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[edit]

Two types of polytomies are recognised, soft and hard polytomies. 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 to recognize this.

On the contrary, 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. While some researchers doubt that hard polytomies occur at all^{[citation needed]}, there is little reason to assume that they don't. In particular situations they may even be not uncommon, for example when a species that has rapidly expanded its range and/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 species.

Recognizing hard polytomies[edit]

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.

External links[edit]

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

English Journal

Evaluating the phylogenetic signal limit from mitogenomes, slow evolving nuclear genes, and the concatenation approach. New insights into the Lacertini radiation using fast evolving nuclear genes and species trees.

Mendes J1, Harris DJ2, Carranza S3, Salvi D4.
Molecular phylogenetics and evolution.Mol Phylogenet Evol.2016 Jul;100:254-67. doi: 10.1016/j.ympev.2016.04.016. Epub 2016 Apr 15.
Estimating the phylogeny of lacertid lizards, and particularly the tribe Lacertini has been challenging, possibly due to the fast radiation of this group resulting in a hard polytomy. However this is still an open question, as concatenated data primarily from mitochondrial markers have been used so
PMID 27095169

Analysis of a Rapid Evolutionary Radiation Using Ultraconserved Elements: Evidence for a Bias in Some Multispecies Coalescent Methods.

Meiklejohn KA1, Faircloth BC2, Glenn TC3, Kimball RT4, Braun EL4.
Systematic biology.Syst Biol.2016 Jul;65(4):612-27. doi: 10.1093/sysbio/syw014. Epub 2016 Feb 10.
Rapid evolutionary radiations are expected to require large amounts of sequence data to resolve. To resolve these types of relationships many systematists believe that it will be necessary to collect data by next-generation sequencing (NGS) and use multispecies coalescent ("species tree") methods. U
PMID 26865273

Molecular characterization and phylogeny of some mazocraeidean monogeneans from carangid fish.

Tambireddy N, Gayatri T, Gireesh-Babu P, Pavan-Kumar A.
Acta parasitologica / Witold Stefański Institute of Parasitology, Warszawa, Poland.Acta Parasitol.2016 Jun 1;61(2):360-8. doi: 10.1515/ap-2016-0047.
Polyopisthocotylean monogenean parasites of fishes are highly host specific and have been used as an appropriate model to study the host-parasite co-evolution. In the present study, eight monogeneans of the order Mazocraeidea were characterized by nuclear 28S rDNA sequences and their phylogenetic re
PMID 27078660

Japanese Journal

Molecular phylogenetic analysis of members of Chrysanthemum and its related genera in the tribe Anthemideae, the Asteraceae in East Asia on the basis of the internal transcribed spacer (ITS) region and the external transcribed spacer (ETS) region of nrDNA

Masuda Yu,Yukawa Tomohisa,Kondo Katsuhiko
Chromosome Botany 4(2), 25-36, 2009
… forms a polytomy clade with <I>Ajania</I>. …
NAID 130000130322

食用キノコのスフィンゴ脂質 : III. オオヒラタケ Pleurotus cystidiosus のマイコグリコリピドにおけるヒラタケ属共通の多分岐型糖鎖構造

糸乗前,矢野宏治,小島寿夫 [他],黒田由紀子,伊藤將弘,山本憲二,杉田陸海
滋賀大学教育学部紀要. 3, 自然科学 57, 1-21, 2007
Edible fungi,mushrooms,are a popular food in Japan and over fifteen cultured mushroom species are available at the food markets.Recently,constituents or ingredients of edible mushrooms have drawn atte …
NAID 110006869236

食用キノコのスフィンゴ脂質 II . エリンギ, Pleurotus eryngii の多分岐型ガラクトースを含有したマイコグリコリピドの構造

糸乗前,卯田昌代,矢野宏治 [他],青木一弘,羽田紀康,山本憲二,竹田忠紘,杉田陸海
滋賀大学教育学部紀要. 3, 自然科学 55, 15-30, 2005
Three mycoglycolipids (glycoinositolphosphosphingolipids) were isolated from the edible mushroom, Pleurotus eryngii (the king oyster mushroom)by successive column chromatography on ion exchange Sephad …
NAID 110004633850