Arrow Worms Miss the Mark in Darwin’s Tree

first_imgNature this week1 claims that “The origins of the arrow worms have long been obscure, but molecular studies are finally bringing the true evolutionary position of these beautiful marine predators into sharper focus.” (Emphasis added in all quotes.)  Arrow worms, or Chaetognatha, are “strikingly beautiful marine animals.” writes Maximilian J. Telford.  “Their transparent, slender bodies appear under the microscope like darting shards of glass.”  Despite their visual transparency, their place in the evolutionary “tree of life” has been cloudy: “In the 160 years since Darwin described them as remarkable for ‘the obscurity of their affinities’, arrow worms have been placed by different authors in a myriad of positions in the animal kingdom.”    The confusion has centered on whether they are protostomes (first mouth) or the more advanced deuterostomes (second mouth), because while arrow worms seem to exhibit deuterostomy in their developmental stages, the genetics don’t allow them to be fitted in with deuterostomes.  New phylogenetic studies of mitochondrial DNA have led some molecular evolutionists to place them into an unexpected position in the tree: members of an ancestral group that preceded the protostome-deuterostome split.  Could it be that protostomes descended from a deuterostome-like creature?  Evolutionarily, that seems backwards:Ignoring the possibility of convergent evolution, arrow worms and deuterostomes must have inherited their deuterostomy and other embryological similarities from a common ancestor (Fig. 2).  The new positioning of the arrow worms means that this common ancestor also gave rise to the protostomes.  This gives us an important insight into the early evolution of the protostomes, because it implies that they must have evolved from a creature that had deuterostome-like aspects of embryogenesis.  This result also leaves us with a problem of nomenclature, given that some of the protostomes are, embryologically speaking, deuterostomes….So why are the arrow worms turning out to be so hard to place within the animal kingdom?  The short answer seems to be that the arrow-worm genes studied so far have been subject to more rapid evolutionary change than those of other species.  The result is that, on an evolutionary tree, arrow worms form a long branch, and such long-branch species are notoriously problematic.  Unfortunately, the simplest solution to the long-branch problem – finding a species of arrow worm with a more normal rate of evolution – seems impossible, because all of the 100 or so living species appear to suffer from the same problem.  The unavoidable conclusion is that yet more genetic data need to be gathered in the expectation that the picture will continue to become clearer.  1Maximilian J. Telford, “Evolution: Affinity for arrow worms,” Nature 431, 254 – 256 (16 September 2004); doi:10.1038/431254b.This article illustrates many of the persistent sins of the Darwin Party when faced with uncooperative data.Stretch the rates of evolution to fit the data to the preconceived belief.Change the nomenclature as needed.Invent terms like convergent evolution to whitewash weaknesses in the story.Assume complexity arose even farther back in time, and say it just emerged somehow.Conjure up imaginative ancestors for which there is no fossil record.Speak of evolution in matter-of-fact, eyewitness news terms.Spin each new damaging piece of evidence positively: say it provides new insight into evolution.Use the phrase must have as glue to hold the story together.Claim that future evidence will make the picture clearer (apparently 160 years was not enough).Assume evolution no matter what the evidence says.  Allow no negotiation about the “fact” of evolution.Reread the article above with this list in mind, and decide with your own eyes, guilty or not guilty?(Visited 36 times, 1 visits today)FacebookTwitterPinterestSave分享0last_img

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