Gene duplication has been a major mechanism for increasing genomic complexity and variation during evolution. The evolutionary history of duplicated genes has been poorly studied along the vertebrate lineage. Here, we attempt to study that history by analyzing the expression of two members of the Snail family, Snail and Slug [genes—BH], in representatives of the major vertebrate groups. We find a surprising degree of variability in a subset of the expression sites for both genes in different species. Although some of the changes can be explained by neofunctionalization or subfunctionalization, others imply reciprocal changes in the expression of the two genes and the reappearance of expression in sites lost earlier in evolution. Because these changes do not fit easily into current models, we need to invoke additional mechanisms acting on enhancer elements to distribute expression domains and functions of duplicated genes unequally during evolution (99:16841, emp. added).

The researchers’ surprise is clearly evident, and even caused them to expand the study to include other organisms in hopes of getting the desired results. They admitted: “We have extended these observations to cartilaginous and bony fishes, reptiles, and turtles, and found a much higher degree of variability in domains of expression for this gene pair than expected” (99:16841, emp. added).

In an effort to draw attention away from the “surprising” findings, and thereby provide the paper with an evolutionary slant, the authors included several series of comparative embryos reminiscent of Ernst Haeckel’s drawings (for a complete discussion on Haeckel’s Hoax see: “Haeckel’s Hoax—CONTINUED!”). The specimens employed were real, not fakes like Haeckel’s, and even the casual observer can see quite clearly a difference between the various species. The images reveal that the slug and snail genes are present in the tail regions of some of the animals, but totally absent in others. For instance, the slug gene is present in the tail bud mesenchyme [primordial embryonic tissue consisting of mesenchymal cells—BH] of the dogfish and chick embryos, but is completely absent from the zebrafish, lizard, turtle, and mouse. Does this mean the chick evolved from the dogfish, or did this gene magically disappear in amphibians, reptiles, and mammals, only to reappear suddenly in birds? In the conclusion of their results the authors commented:

The present analysis of Snail/Slug duplicated genes during vertebrate evolution reveals a much higher degree of plasticity and complexity than expected and highlights the risk of using expression or function as phylogenetic characters when studying the evolution of gene families (99:16845, emp. added).

As they puzzled over how a function could disappear and reappear, they were forced to conclude:

In this case, we would need to invoke extremely unlikely events as the reversal of degenerative mutations or “de novo” creation of tissue-specific regulatory elements to account for the reappearance of these expression domains after their loss at earlier stages in evolution. Although such events may occur over short time-scales, it has such a low probability in our situation that it could be described as an evolutionary reversal, in the sense prohibited by Dollo’s law on the irreversibility of evolutionary changes. A possibility we cannot formally rule out is that ancestors of all of the species analyzed retained expression of both genes in pnc and tail bud, and that independent enhancer loss by mutation occurred in each lineage. In this case, the distribution of expression patterns we observe would not reflect common descent, but random variation. Once more, this is an extremely unlikely scenario, where in all ancestors both duplicated genes were necessarily expressed at all sites simultaneously (99:16845, emp. added).

While they admitted that the distribution of expression patterns they observed “would not reflect common descent,” this did not stop them from including an evolutionary schematic that tried to tie all of these creatures to the same family tree. They then concluded the study by exploring the possibility that epigenetic [the union of sperm and ovum—BH] factors might explain the data, and suggested the possibility that “rare and not parsimonious events can be observed more than once in the evolution of a single genetic regulatory system” (99:16845).

If it walks like a duck, looks like a duck, and quacks like a duck, it must be a duck—unless of course that duck has something to do with supporting the concept of creation. Then, of course, it must be a fish. After all, if the data “do not fit easily into current models, we need to invoke additional mechanisms.” How sad it is that science is no longer a method of obtaining knowledge by reporting and analyzing the data, but rather has become a realm where only the scientific religion of evolution can be propagated and defended.

Locascio, Annamaria, Miguel Manzanares, Maria J. Blanco, and M. Angela Nieto (2002), “Modularity and Reshuffling of Snail and Slug Expression During Vertebrate Evolution,” Proceedings of the National Academy of Sciences USA, 99:16841-16846, December 24.

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