Understanding Evolution

Phylogenetic pitchforks

The branching pattern of phylogenies provides information about the relationships between organisms.

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

A polytomy Lack of knowledge
Usually, it 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.

There are many solutions to this polytomy; six possible solutions are shown below.

Possible solutions to a polytomy

Rapid speciation
Sometimes, it 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 should tell you if they feel that the evidence indicates that this is the case.

The cladogram below shows the phylogenetic relationship 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 clade

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Cichlid clade after Takahashi, K, Y. Terai, M. Nishida, and N. Okada. 2001. Phylogenetic relationships and ancient incomplete lineage sorting among cichlid fishes in Lake Tanganyika as revealed by analysis of the insertion of retroposons. Marine Biology and Evolution 18(11):2057-2001.

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