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Reading trees: Phylogenetic pitchforks

Often, one sees phylogenies that include polytomies, nodes with more than two descendent lineages, creating a "pitchfork." This can mean one of two things:

  • Lack of knowledge
    Usually, a polytomy means that we don't have enough data to figure out how those lineages are related. By not resolving that node, the scientists who produced the phylogeny are telling you not to draw any conclusions — and also to stay tuned: often gathering more data can resolve a polytomy.

    a polytomy

    There are many ways that the polytomy above could be resolved. Six are shown below. Only more data can help us decide which is the most accurate representation of the relationships between A, B, C, D, and E.

    six possible polytomy solutions

  • Rapid speciation
    Sometimes a polytomy means that multiple speciation events happened at the same time. In this case, all the daughter lineages are equally closely related to one another. The researchers who have reconstructed the tree you are examining should tell you if they feel that the evidence indicates that this is the case.

    The phylogeny below shows the relationships among the members of a group of fish called cichlids. Cichlid fish speciated quickly after their home lakes formed in Africa, resulting in several phylogenetic polytomies.

    cichlid phylogeny



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Reading trees: A quick review

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Using trees for classification


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