Contributions to Zoology, 86 (4) – 2017Jacques J. M. van Alphen; Jan W. Arntzen: The case of the midwife toad revisited

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Kammerer’s views on evolution

Kammerer believed that evolution by the inheritance of acquired characters was far superior a mechanism to evolution by natural selection. In his book “The Inheritance of Acquired Characteristics” (Kammerer, 1924) there is even a chapter titled “The Impotency of Selection” in which he argues that selection can only slightly modify acquired characters. For Kammerer an acquired character was the same as a mutation, however, with the important difference that the new character was acquired by the soma, and had to be imprinted on the germ line to become heritable. He was convinced that all amphibians are phenotypically plastic in reproductive biology. For example, citing Jourdain on Hylodes (now Eleutherodactylus martinicensis (Tschudi, 1838), a Caribbean frog that has direct development and hatches from eggs as froglets, he writes “I am convinced that, with the help of the appropriate measures, one could get any amphibian to show the remarkable reproductive behaviour of Hylodes or at least to approach this behaviour” (Kammerer, 1906:66) (our translation, as elsewhere where the original is in German; the original texts are reproduced in Appendix I). When he did not succeed to induce direct development in the midwife toad (genus Alytes) he says: “Today, however, I do no longer have any doubt, that it is within reach of the possibilities, to force Alytes to develop without metamorphosis and independently of water” (Kammerer, 1909: 523). However, even when Kammerer’s thoughts would have been correct, more important than showing that the reproductive behaviour of amphibians could be modified was to show that such changes could be passed on to next generations. August Weismann (1887) published experiments in which he removed the tails of mice during five generations and showed that this did not result in mice born without tails. From then on, Lamarckists insisted that only adaptive traits could be acquired and a proof of Lamarckian evolution is thus to include evidence that the traits under study are adaptive.

Kammerer was aware of Weismann’s (1893) germ plasm theory, but believed that somatic adaptations, by some unknown mechanism could pass the “Weismann barrier”, would reach the germ-line cells and become heritable. Evidence for such a process would come from data showing that: (1) the reproductive performance of animals placed in a new environment would improve with each reproductive cycle and, more importantly (2) F1 offspring kept in the new environment would perform better than the parents, and later generations would perform better than the F1 . Thus, when he set out to show that the land-breeding midwife toads can acquire adaptations to breed in water, he had to show that they develop all the adaptive features that water-breeding toads and frogs have, and that they would develop these adaptations gradually.

Kammerer (1909) predicted that adaptations to a new environment would slowly disappear when the animals were returned to their original environment. This seems perfectly logic: when animals adapt to a new environment by acquiring new characters, they should as easily adapt again to their former environment. However, this re-adaptation posed a problem for a researcher who wanted to show that an acquired character had become a heritable trait. When the next generation is returned to the original habitat of the parents, it loses the new character, but when the next generation is kept in the new environment, it might show the new character not because it had inherited that character from its parents, but because it has acquired the character itself. Kammerer had two potential solutions for this problem. First, if he could show that the F1 and subsequent generations would be better adapted than the parents, then this would show that an effect on the parents had been transferred to following generations. The second solution was to cross an animal with the acquired character to one from the wild population and study the frequency of the character in the F1 and F2 . If this frequency fitted Mendel’s laws, then, in his view, the character had become heritable. This motivated Kammerer (1911) to perform crossing experiments between wild-type midwife toads and those from his raised-temperature experiments.

Kammerer (1919) claims that there are also newly acquired characters that do not disappear after the animals are returned to the original environment. He does not offer an explanation as why some characters would disappear whereas others would become genetically fixed, except for the idea that atavistic characters would sooner become fixed than real novelties. Citing Weismann, Kammerer 1919:348 writes “We deal here with a set-back of the reproductive mode of ancestors from a long time ago, which had been given up since hundreds of generations, but which had not totally disappeared from the germ plasm and becomes reactivated when the proper stimuli are applied.”.