Monday, June 28, 2010

Time and Cladograms (1)

Because we are so used to the convention of time running from left to right on diagrams, we assume that a cladogram (like the one below) indicates that the creatures on the left of the diagram came before the creatures on the right. But this is not what a cladogram based on cladistic analysis shows. It only shows relationships of characteristics between creatures.

Here are some references to time and cladograms:
"The following phylogenetic results are taken from Senter (2007) "A new look at the Phylogeny of Coelurosauria (Dinosauria: Theropoda)."[3] This cladogram does not represent time, but a crude estimate of the time can be inferred from morphological changes [differences]. The first coelurosaurs were similar to the coelurids Tanycolagreus and Coelurus, and differed only sightly from other early tyrannosauroids Dilong and Eotyrannus. The two most significant separations between subgroups are those between the Paraves [Eumaniraptors] and other coelurosaurs and between the paravian clades Avialae and Deinonychosauria." [Avialae and Deinonychosauria are the two major subgroups within Eumaniraptora].

"Cladistic analysis is a powerful method of determining evolutionary relationships between organisms, but one weakness is its inability to consider time as part of the equation."
"Firstly, we construct phylogenetic hypotheses by looking at the distribution of characters: the distribution of taxa within time is effectively irrelevant. It doesn't matter that some maniraptorans are geologically younger than the oldest known birds: phylogenetic analyses that have good sampling across taxa and morphology still show that those maniraptorans are more basal than birds." (Darren Naish)

I will expand on this topic in the next post.

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