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polytomy
In biology, a polytomy is a section of a phylogeny in which the relationships cannot be fully resolved to dichotomies, thus giving many temporally based 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.〔Purvis, A., and T. Garland, Jr. 1993. Polytomies in comparative analyses of continuous characters. Systematic Biology 42:569–575.〕 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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