Current phylogenetic trees that include Ecdysozoa and, to a lesser extent, Lophotrochozoa are grossly incorrect. Because DNA neucleotide sequences are subject to selection and have had different rates in different groups the direct calculation of rates from comparative differences produces flawed evoutionary trees.
This can be shown graphically by comparing two branches of a phylogeny using the different assumptions involved. The determination of when and how fast the differences in longevity develop is subject to error also, but, in the case of the descendants of pogonophorans, may rapidly move to shorter life cycles as soon as the transition from abyssal to shallow depths occurs. Thus very few new species may make the connection of ancient pogonophorans to those that became the earliest shallow water deuterostomes.
The first figure illustrates two types of error made in calculating phylogenies, generation time error and calibration error.
Generation time error is the one that produces incorrect branching in a phylogeny. The figures illustrate generation time error for two species assuming that half the change occurs in each line as in the 5 and 5 of the right figure of each pair. When 90% of the change occurs in the right branch, the estimated time from origin at the ancestral node is nearly doubled. With pogonophorans having generation times several thousands of years longer than modern non-abyssal species, the node where deuterostome phyla branched off is clearly during Pre-Cambrian times as is also suggested by the Paleozoic emergence of chordates.
Calibration error is one that can cause overestimates or underestimates in the time of divergence in two lines from the ancestral node. The two right figures keep the generation time error intact, if it is in error. The calibration error occurs when the time per nucleotide change is based on a calibration species whose rate of change is different from the species to which the rate is applied. Selection of a calibration species or a rate of change is not likely to be a major problem when closely related species are studied.
If you think about the result of the generation time error introduced by the central position of pogonophorans in the protostome, deuterostome radiation, it is not surprising that their relatively unchanged DNA shows affinity with widely diverse phyla.
A similar problem is probably operating in the nematodes being a significant group in the erroneous group, Ecdysozoa. The small size of nematodes is probably a selective reason nematodes have a single gene per gene family and thus a more rigid selection producing relatively unchanged genotypes over a long. time. Thus the "long-branch attraction" is not recognized as the error compounded in the Ecdysozoa concept.
The last figure illustrates one possible view of the development of two species having different longevity from a common ancestor. It is perhaps worthless as an illustration because the common ancestor of related species, one with a one year life cycle, the other with a two year life cycle, could very well have resulted from all the change in one line, or both from greater change from some extreme value. Rates of change are also unlikely to be so uniform.
Until I stumbled across what seemed to be an unlikely ancestral role of the pogonophorans I would have been been very likely joining my peers in welcoming the Ecdysozoa and Lophotrochozoa. I hope I have included enough information in the blogs on this site to help my peers make the same transition I have made.
Joseph G. Engemann June 16, 2014
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