Darwin’s Tree of Life was uprooted in the Cambrian explosion
|March 17, 2014||Posted by News under Tree of life, News|
and didn’t get a chance to write anything down.
Further to the fact that enough was known by 2009 to fell Darwin’s “tree of life,” which still flourishes in the vanished world of textbook certainties, here are some comments from Steve Meyer’ Darwin’s Doubt:
Vanderbilt University molecular systematist Antonis Rokas is a leader among biologists using molecular data to study animal phylogenetic relationships. Nevertheless, he concedes that a century and a half after The Origin of Species, “a complete and accurate tree of life remains an elusive goal.” In 2005, during the course of an authoritative study he eventually copublished in Science, Rokas was confronted with this stark reality. The study had sought to determine the evolutionary history of the animal phyla by analyzing fifty genes along seventeen taxa. He hoped that a single, dominant phylogenetic tree would emerge. Rokas and his team reported that “a 5-gene data matrix does not resolve relationships among most metazoan phyla” because it generated numerous conflicting phylogenies and historical signals. Their conclusion was candid: “Despite the amount of data and breadth of taxa analyzed, relationships among most metazoan phyla remained unresolved.”
In a paper published the following year, University of Wisconsin at Madison biologist Sean B. Carroll went so far as to assert that “certain critical parts of the TOL [Tree of Life] may be difficult to resolve, regardless of the quantity of conventional data available.” This problem applies specifically to the relationships of many of the animal phyla, where “[m]any recent studies have reported support for many alternative conflicting phylogenies.” Investigators studying the animal tree found that “ a large fraction of single genes produce phylogenies of poor quality” such that in one case, a study “omitted 35% of single genes from their data matrix, because those genes produced phylogenies at odds with conventional wisdom” Rokas and Carroll tried to explain the many contradictory trees by proposing that the animal phyla might have evolved too quickly for the genes to record some signal of phylogenetic relationships into the respective genomes. In their view, if the evolutionary process responsible for anatomical novelty works quickly enough, there would not be sufficient time for differences to accumulate in key molecular markers, in particular those used to infer evolutionary relationships in different animal phyla. Then, given enough time, whatever signal did exist might become lost. Thus, when groups of organisms branch rapidly and then evolve separately for long periods of time, this “can overwhelm the true historical signal”—leading to the inability to determine evolutionary relationships.
Their article brings the discussion of the Cambrian explosion full circle from an attempt to use genes to compensate for the absence of fossil evidence to the acknowledgment that genes do not convey any clear signal about the evolutionary relationships of the phyla first preserved by fossils in the Cambrian. (pp. 120–21)
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