Mel and Norma Gabler are continuing their criticisms of evolution in hearings on evolution in Texas public schools. They now propose that "minimal changes necessary in science/biology textbooks" should "acknowledge incongruities between anatomical and molecular homologies" and that "one relational tree often lacks predictive value for the other. Molecularly, crocodiles group more closely with chickens. . . ."
It has long been familiar to paleontologists that birds diverged early from primitive reptiles. This conclusion is based upon the fossil record and upon anatomical homologies. For example, Romer lists anatomical similarities in "the dinosaurs, pterosaurs, and crocodiles—and in the primitive birds (which seem clearly derivable from the archosaurs)" (1966).
The anatomical and fossil evidence therefore actually leads to a prediction of molecular homology between birds and crocodiles and, perhaps, other primitive reptiles. A telephone call to the Gablers' office by Dr. Martin Meltz of the University of Texas at San Antonio produced a response that their statement referred to α-hemoglobin sequence homologies. These have been discussed by Dr. Max Perutz, whose research with hemoglobin contributed so greatly to the beginnings of the science of molecular evolution and who was awarded the Nobel Prize in 1962 for this research, together with Sir John Kendrew, whose studies were on myoglobin. The Gablers find themselves in distinguished company. Unfortunately, they are out of step.
Some years earlier, Holmquist and I compared amino acid sequences of cytochromes c for birds, rattlesnakes, and turtles. The turtle is anatomically more "primitive" than snakes and closer to crocodilians than to snakes in many respects. The divergences are shown in TABLE I.
Percentage Differences in Amino Acid Sequences of Cytochromes c and α-Hemoglobins
(Condensed from Jukes and Holmquist, 1972; and Perutz et al. 1981)
Average of Percentage Differences
|Birds : turtles||7.5|
|Chicken : crocodilian||47|
|Birds : rattlesnakes||18.0|
|Chicken : viper||57|
|Turtle : rattlesnake||21.0|
|Crocodilians : viper||64|
We concluded that the evolutionary rate of change of vertebrate cytochrome c is species-dependent as well as time dependent. The results with cytochrome c parallel those with α-hemoglobin. Birds are related more closely to the primitive turtle than to snakes. Snakes have evolved more rapidly than turtles.
Perutz and coworkers compared α-hemoglobin amino acid sequences in three crocodilians (caiman, Mississippi alligator, and Nile crocodile) with those of other species, including chicken and viper (1981). They found that "crocodilian sequences are most similar to those of the birds, consistent with fossil evidence suggesting that birds arose from archosaurian reptiles" and that the number of amino acid differences between crocodilians and viper (lepedosaur reptile) "are much larger than between the crocodilians and the birds." These results are in TABLE I. Perutz and colleagues point out that the results support the conclusions of Romero-Herrera et al., based upon myoglobin, that the "rates of change differ in different branches of the phylogenetic tree."
Contrary to the Gablers, molecular and anatomical homologies correspond and are mutually predictive. A summary of the above findings could well be included in science biology textbooks in support of evolutionary theory.
Jukes, T. H., and Holmquist, R. 1972. "Evolutionary Clock: Nonconstancy of Rate in Different Species." Science, 177:530-532.
Perutz, M. F.; Bauer, C.; Gros, G.; Leclercq, F; Vandercasserie, C; Schnek, A. G.; Braunitzer, G; Friday, A. E.; and Joysey, K. A. 1981. "Allosteric Regulation of Crocodilian Haemoglobin." Nature, 291: 682-684.
Romer, A. S. 1966. Vertebrate Paleontology, third edition. Chicago: University of Chicago Press, pp. 136-138.