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Darwin's “Tree of Life”

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Darwin's “Tree of Life” Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education DARWIN’S “TREE OF LIFE” mon descent. Finally, he demands that text- books treat universal common ancestry as PHYLOGENETIC TREES unproven and refrain from illustrating that n biology, a phylogenetic tree, or phyloge- “theory” with misleading phylogenies. ny, is used to show the genealogic relation- Therefore, according to Wells, textbooks Iships of living things. A phylogeny is not should state that there is no evidence for com- so much evidence for evolution as much as it mon descent and that the most recent research is a codification of data about evolutionary his- refutes the concept entirely. Wells is complete- tory. According to biological evolution, organ- ly wrong on all counts, and his argument is isms share common ancestors; a phylogeny entirely based on misdirection and confusion. shows how organisms are related. The tree of He mixes up these various topics in order to life shows the path evolution took to get to the confuse the reader into thinking that when current diversity of life. It also shows that we combined, they show an endemic failure of can ascertain the genealogy of disparate living evolutionary theory. In effect, Wells plays the organisms. This is evidence for evolution only equivalent of an intellectual shell game, put- in that we can construct such trees at all. If ting so many topics into play that the “ball” of evolution had not happened or common ances- evolution gets lost. try were false, we would not be able to discov- THE CAMBRIAN EXPLOSION er hierarchical branching genealogies for ells claims that the Cambrian organisms (although textbooks do not general- Explosion “presents a serious chal- ly explain this well). Referring to any phylo- lenge to Darwinian evolution” genetic tree as “Darwin’s tree of life” is some- W (Wells, 2000:41) and the validity of phyloge- what of a misnomer. Darwin graphically pre- netic trees. The gist of Wells’s argument is that sented no phylogenies in the Origin of Species; the Cambrian Explosion happened too fast to the only figure there depicts differential rates allow large-scale morphological evolution to of speciation. If anyone deserves credit for occur by natural selection (“Darwinism”), and giving us “trees of life,” it is Ernst Haeckel, that the Cambrian Explosion shows “top- who drew phylogenies for many of the living down” origination of taxa (“major” “phyla” groups of animals literally as trees, as well as level differences appear early in the fossil coining the term itself. record rather than develop gradually), which WELLS’S SHELL GAME he claims is the opposite of what evolution ells uses phylogenetic trees to attack predicts. He asserts that phylogenetic trees the very core of evolution — com- predict a different pattern for evolution than Wmon descent. Wells claims that text- what we see in the Cambrian Explosion. These books mislead students about common descent arguments are spurious and show his lack of in three ways. First, Wells claims that text- understanding of basic aspects of both paleon- books do not cover the “Cambrian Explosion” tology and evolution. and fail to point out how this “top-down” pat- Wells mistakenly presents the Cambrian tern poses a serious challenge to common Explosion as if it were a single event. The descent and evolution. Second, he asserts that Cambrian Explosion is, rather, the preserva- the occasional disparity between morphologi- tion of a series of faunas that occur over a 15– cal and molecular phylogenies disproves com- 20 million year period starting around 535 mil- 11 Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education lion years ago (MA). A fauna is a group of claiming that this proves that the fossil record organisms that live together and interact as an is complete enough to show that there were no ecosystem; in paleontology, “fauna” refers to a precursors for the Cambrian Explosion ani- group of organisms that are fossilized together mals. This claim is false. His evidence for this because they lived together. The first fauna “well documented” Precambrian fossil record that shows extensive body plan diversity is the is a selective quote from the final sentence in Sirius Passet fauna of Greenland, which is an article by Benton et al. (2000). While the dated at around 535 MA (Conway Morris, paper’s final sentence does literally say that 2000). The organisms preserved become more the “early” parts of the fossil record are ade- diverse by around 530 MA, as the Chenjiang quate for studying the patterns of life, Wells fauna of China illustrates (Conway Morris, leaves out a critical detail: the sentence refers 2000). Wells erroneously claims that the not to the Precambrian, but to the Cambrian Chenjiang fauna predates the Sirius Passet and later times. Even more ironic is the fact (Wells, 2000:39). The diversification contin- that the conclusion of the paper directly refutes ues through the Burgess shale fauna of Canada Wells’s claim that the fossil record does not at around 520 MA, when the Cambrian faunas support the “tree of life.” Benton et al. (2000) are at their peak (Conway Morris, 2000). Wells assessed the completeness of the fossil record makes an even more important paleontological using both molecular and morphological error when he does not explain that the “explo- analyses of phylogeny. They showed that the sion” of the late Early and Middle Cambrian is sequence of appearance of major taxa in the preceded by the less diverse “small shelly” fossil record is consistent with the pattern of metazoan faunas, which appear at the begin- phylogenetic relationships of the same taxa. ning of the Cambrian (545 MA). These faunas Thus they concluded that the fossil record is are dated to the early Cambrian, not the consistent with the tree of life, entirely oppo- Precambrian as stated by Wells (Wells, site to how Wells uses their paper. 2000:38). This enables Wells to omit the Wells further asserts that there is no evi- steady rise in fossil diversity over the ten mil- dence for metazoan life until “just before” the lion years between the beginning of the Cambrian explosion, thereby denying the nec- Cambrian and the Cambrian Explosion (Knoll essary time for evolution to occur. Yet Wells is and Carroll, 1999). evasive about what counts as “just before” the In his attempt to make the Cambrian Cambrian. Cnidarian and possible arthropod Explosion seem instantaneous, Wells also embryos are present 30 million years “just grossly mischaracterizes the Precambrian fos- before” the Cambrian (Xiao et al., 1998). sil record. In order to argue that there was not There is also a mollusc, Kimberella, from the enough time for the necessary evolution to White Sea of Russia (Fedonkin and Waggoner, occur, Wells implies that there are no fossils in 1997) dated approximately 555 million years the Precambrian record that suggest the com- ago, or 10 million years “just before” the ing diversity or provide evidence of more Cambrian (Martin et al., 2000). This primitive primitive multicellular animals than those seen animal has an uncalcified “shell,” a muscular in the Cambrian Explosion (Wells, 2000:42– foot (Fedonkin and Waggoner, 1997), and a 45). He does this not by producing original radula inferred from “mat-scratching” feeding research, but by selectively quoting paleonto- patterns surrounding fossilized individuals logical literature on the fossil record and (personal observation; Seilacher, pers. 12 Icons of Evolution? Why Much of What Jonathan Wells Writes about Evolution is Wrong Alan D. Gishlick, National Center for Science Education comm.). These features enable us to recognize Cambrian Explosion, for example, is the first it as a primitive relative of molluscs, even time we are able to distinguish a chordate from though it lacks a calcified shell. There are also an arthropod. This does not mean that the chor- Precambrian sponges (Gehling and Rigby, date or arthropod lineages evolved then, only 1996) as well as numerous trace fossils indi- that they then became recognizable as such. cating burrowing by wormlike metazoans For a simple example, consider the turtle. How beneath the surface of the ocean’s floor do you know a turtle is a turtle? By the shell. (Seilacher, 1994; Fedonkin, 1994). Trace fos- How would you recognize the ancestors of the sils demonstrate the presence of at least one living turtle, before they evolved the shell? ancestral lineage of bilateral animals nearly 60 That is more complicated. Because its ances- million years “just” before the Cambrian tors would have lacked the diagnostic feature (Valentine et al., 1999). Sixty million years is of a shell, ancestral turtles may be hard to rec- approximately the same amount of time that ognize (Lee, 1993). In order to locate the has elapsed since the extinction of non-avian remote ancestors of turtles, other, more subtle, dinosaurs, providing plenty of time for evolu- features must be found. tion. In treating the Cambrian Explosion as a Similarly, before the Cambrian Explosion, single event preceded by nothing, Wells mis- there were lots of “worms,” now preserved as represents fact — the Cambrian explosion is trace fossils (i.e., there is evidence of burrow- not a single event, nor is it instantaneous and ing in the sediments). However, we cannot dis- lacking in any precursors. tinguish the chordate “worms” from the mol- Continuing to move the shells, Wells lusc “worms” from the arthropod “worms” invokes a semantic sleight of hand in resur- from the worm “worms.” Evolution predicts recting a “top-down” explanation for the diver- that the ancestor of all these groups was worm- sity of the Cambrian faunas, implying that like, but which worm evolved the notochord, phyla appear first in the fossil record, before and which the jointed appendages? In his argu- lower categories.
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