Effect a proposed explanation for development of Insect domatia

Anglo-Saxon authors have remained faithful to the Darwinian concept of evolution, though from time to time some black sheep tried timidly to shake the edifice and the dogma of Darwinism. Only some very rare and original "froggies" (meaning here French biologists) still dare to preach in favour of Lamarckism, which emphasised inheritance of acquired characters as an important factor in the working of evolution. Not only because Lamarck was a Frenchman, but also for the pleasure to be different and to annoy their neighbours! It has been also a kind of patriotism that Frenchmen have always been defiant to the dogmatic English.

Darwinian dominance was not the same during the last century in France. Most of the zoologists in the Universities and the Museums were openly or secretly believing in the transmission of the acquired characters. Gould (1980) pejoratively named attempts to solve the problems, which could not be addressed effectively by Darwinism, as "the shades of Lamarck", sometimes with disconcerting shades. Among Lamarck's disciples very few subsist today; they speculate on events in the time parameter, a parameter not directly accessible at present, while the Darwinians have found another explanation, subtle indeed, and always theoretically plausible. Today to be Lamarckian is rare, and is regarded old fashioned, but to be fixist is even rarer. People say that in the Land of Darwinism some rare fixists subsist even in the bowels of the Natural History Museum, the former British Museum. That was around the mid-thirties, that a director of the French Museum of Natural History, Paul Lemoine, wrote: "Evolution is a dogma, in which his priests do not believe, but that they maintain for the people". It was printed at the head of the French Encyclopedia. That was a big scandal then and the biologists of that time, P. P. Grasse and George Teissier called the zoologists to a meeting to discuss openly the matter. Lemoine was a sophist and maintained his point of view, just to be different.

From time to time, rare biologists pretend that they could prove Lamarckism by experiments on bacteria, protozoa or viruses (Anonymous, 1981), but more recently on gerbils. An old experiment: a French amateur entomologist, around the forties, used to cut the mediodorsal horn of a Sphinx caterpillar and pretended to obtain a significant proportion of hornless caterpillars in the next generation. Correctness of his findings is doubtful, but only for fun the experiments could be repeated. Cutting the mouse tail for generations to prove Lamarckism is wrong is absurd; it is a mutilation, not a gradual change by evolution.

Jeannel, the French coleopterist, was Lamarckian, as he believed openly in the inheritance of acquired characters, because he dealt a lot with cave insects, and he found a Lamarckian explanation satisfying (Jeannel, 1950). He could not see any other explanation adequate to account for degeneration of some organs and of hypertrophy of others resulting from their non-use or over-use in the cave environment, for example, the blindness of the cave insects, the physogastry of certain beetles, the lengthening of their antennae, the discoloration of their bodies, and the elongation of their legs. In Cuenot's classical books (1932, 1925; Cuenot and Tetry, 1951), though all Darwinist, sometimes appear the shades of Lamarck. One book, written by Wintrebert (1962), entitled "The Living Being Creator of its Evolution", is purely Lamarckian, as well as the one written by Hovasse (1950). As the Phoenix, Lamarckism is rising always from its ashes, and Wintrebert defended what he called the chemical Larmarckism or as Jean Rostand (1958, 1962) used to say "the intelligence of the inanimate matter".

During PJ's youth, he was a student of L. Cuenot, of P. P. Grasse and others, and at that time they were all officially Darwinian, as well as Georges Teissier. That was the same Teissier who proved in Roscoff that wingless Drosophila survived better on a roof exposed to wind than winged ones. From that experiment, the loss of wings on mountains and islands was theoretically explained. Rabaud, another crypto-Lama-rckian, succeeded to stop the creation of a chair of genetics in Paris until 1947. He was materialist, but against any finalism in evolution. It was also in fashion at that time to negate the mimetism. That was the situation with home naturalists, and also of some outside France. The reason was not specially to be Lamarckian (Lamarckism does not explain the mimetism), but to be anti-Darwin. In the modern world, the minds have evolved, and there are even sociobiologists among the young French workers.

The chief merit of Lamarck was to express for the first time clearly the idea of evolution as a reality in nature. That is what even the fiercest disciples of Darwin recognize, even if the Lamarck's basis of the evolutionary mechanism theory remains unprovable. Darwin, who did not like Lamarck and criticised him openly, accepted some of his theories.

There is, however, a small problem, which still remains difficult to explain, and that J. Mark Baldwin (1896) named a "new factor of the evolution", and which was baptized later as the "Baldwin effect". Cuénot discussed that in details in his books (namely 1925 and 1951) and later on S. J. Gould (1980) recalled the matter in "Panda's Thumb". Really the Baldwin effect has been often used to explain the inexplicable or not easily explicable. If the Lamarckian explanation is appealing, we can also use the adaptive effect of the natural selection on a great number of generations. Another problem, which is not raised here and remains also difficult to explain is the Hopkins' principle, which states that chemical experience (food selection habit) acquired by the larva of an endopterygote insect can be transferred through the pupal stage to the adult and to the next generation (Van Emden et al, 1996). There are also Darwinian interpretations of the principle, and no unambiguous evidence for it has ever been obtained.

Baldwin effect is a sequential process in which characters, acquired under the effect of the environment, are assimilated by genetic factors, i.e. they become heritable. It is Hovasse (1950) who baptized the concept as "Baldwin principle", which was preferred to "Baldwin effect". In brief, the selection would be based on the aptitude to acquire new characters in agreement with the phenocopies, i.e. the somations provoked by the environment. Baldwin effect or genetic assimilation was the reason to question the Darwinian theory several times in Cuenot's writings.

A good interpretation of the Baldwin effect is the one given by Mayr (1974): "The situation where, due to an appropiate modification of the phenotype, an organism can stay in favourable environment, until the selection has achieved the genetic fixation of its phenotype". This interpretation was in some ways a tempting reconciliation between Larmarckism and neo-Darwinism. Huxley (1942), himself a passionate Darwinian, believed in the Baldwin effect, to which he attributed the formation of races.

According to Simpson (1953), who did not believe much in the Baldwin effect, the effect itself would comprise three steps:

(1) The organisms react with the environment so as to produce behavioural, physiological or structural changes, which are not hereditary (somatic), and which show some advantages for survival, i.e. adaptative for the individuals.

(2) There happen mutations in genetic factors; such mutations, which produce hereditary characters similar to those mentioned above, having the same adaptive advantages

(3) Mutant genetic factors, as mentioned under (2) are selected by the natural selection process, and show the tendency to spread among the population through the generations. The result is that those non-hereditary adaptations become hereditary.

The classic cases of the callosities of the ostriches, rheas, emus, or phacocheres, which appear already in the embryos (Cuenot, 1925; Cuenot and Tetry, 1951), are typical cases, which need deep consideration. "I don't know any clearer and at the same time more favourable example to the Lamarckian thesis than the thickening of the plantar sole and the mammal callosities", wrote Cuenot. Really, the adaptation is flagrant, but no more surprising than other adaptations. A long natural selection can perhaps explain the callosities for the fixation of acquired characters. Evidently some questions remain unanswered. Nothing is perfectly clear in this area. It is evident also that the mite pockets are present in the reptile embryos (see the chapter "Pocket mites") as well as the domatia

(dwellings) of ants or mites among plants existed before the arrival of ants and persist in greenhouses where ants or mites don't exist.

An example among the Acacia: the horned Acacia of East Africa harbour ants and also symbiotic beetles. The horns are stipular spines, sometimes very big and variable from tree to tree. The explanations about their origin vary: Lamarckian (Beccari), galls (Jeannel), preadaptation (Schnell) and coevolution (Janzen). Hocking (1970, 1975) gives a Baldwinian interpretation!

Only the Central-American or East-African Acacia have those enormous inflated spines and are regularly inhabited by ants. Among the Central American Acacia, the differentiations are so big that the tree provides the protein rich food called trophosomes or the beltian bodies, and sugars from extra-floral nectaries. Among the East-African Acacia, only lodging is offered to ants with a bit of nectar. The ants compensate by rearing coccids. Australian Acacia, the most numerous and the most diversified, have never offered something similar to ants, perhaps because the big browsing mammals were absent from the continent. Ants, not specialized there, use to collect nectar from the trees. How then should we explain the occurrence of stipular spines in two different parts of the world, the adaptation to certain ants and even the providing food to the insect guests?

The first explanation, which comes to the mind, it is the preadaptation of those structures, occupied later on by the ants. It is Schnell's thesis (1970). The second is the coevolution of plants and ants, dear to Janzen, which could explain that way the close adaptation between the American Acacia and their hosts. All those theories have been explained in detail in Jolivet (1986; 1996 a and b).

Hocking (1975) invokes the Baldwin effect. Here goes how he explains this evolution. The Acacia spines, in Africa, according to him, originally were Homoptera galls, probably from aphids. Certain galls would have had sometimes a rough resemblance with poplar galls, even with a histological similarity. The sensitivity of the plant to this initial stimulus would have been selected in view of the big advantages that it confers to both ants and to plants. While ants got lodging and readily available food, plants got protection by lodging ants against herbivorous animals. If the threshold of the response by the plant is such that development of a gall takes place only in presence of an insect, then galls would be of different types due to diversity of the gall producing Homoptera. The diversity of shape of the ant lodging Acacia galls then becomes understandable. According to Hocking, this explanation amounts to the Baldwin effect, and is similar to the one used to explain the early development of callosities in ostriches.

This explanation, for us, is not a good one. It is a far-fetched argument. Either it has been a coevolution between Acacia and ants, or there has been a preadaptation of stipular horns, since these spines can develop easily without ants and without any kind of stimulus. We must also reject a modification of the stipules under the effect of the ants, as believed by Beccari and Delpino, modification becoming later on hereditary. It is not so simple as that.

Hopkin's explanation to the problem, as mentioned earlier (see van Emden et al, 1996, cited above), is debatable. According to this explanation, adults of polyphagous and oligophagous herbivorous insects show a tendency to deposit their eggs on the host plant, on which they were previously reared as larvae. Certain experiments with the vinegar fly Drosophila support this assumption. But how is the plant's preference in the larvae transferred to the adult insect? Suitable experiments should provide an answer.

In choosing among all these interpretations, time remains the main hurdle, the "deus ex machina". However, we must say that Baldwin effect seems to offer an explanation as to the origin of certain fixed adaptations, even if the explanation offered remains to some extent vague and tortuous. Lamarck-ism has never been proved. Animals, living in dark, do not lose their eyes; they lose some of their function, at least on a short time. Experiments of Kammerer on Proteus were pure lies, also the theories of the sinister Lysenko. We have seen that natural selection and Darwinism explains very well the microevolution. For the macroevolution, it is another matter. Not ignoring the factor time, over millions of years, we must accept the classical explanation of Neodarwinism. Gould had added the concept of punctuated equilibrium, but since we have seen probably a very small part of the fossils which exist, particularly among insects, we have not been able to present an acceptable tree of evolution, and to visualise if some events in the past were due to the Baldwinian effect or to any other reason.

References

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Baldwin, J. M. 1896. A new factor in Evolution. The American Naturalist 30: 441451 and 536-553.

Cuénot, L. 1925. L'adaptation. Doin, Paris: 420 pp.

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Hocking, B. 1970. Insect associations with the swollen thorn Acacias. Trans. R. Ent. Soc., London 122 (7): 211-255.

Hocking, B. 1975. Ant-plant mutualism: evolution and energy. In Gilbert and Raven (eds.) Coevolution of Insects and Plants. Univ. Texas: 78-89.

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Mayr, E. 1974. Behavior programs and evolutionary strategies. American Scientist 62: 650- 659.

Rostand, J. 1958. Aux sources de la Biologie. Gallimard, Paris.

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Problèmes généraux. I. Les Flores. Les structures. Gauthier-Villars, Paris: 409-441.

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Douloumpaka, S. 1996. Hopkins' host selection principle, another nail in the coffin. Physiological Entomology 21: 325-328.

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