Darwin und Embryologie.

Dies ist ein begleitendes Post zu einem Teil meiner Buchbesprechung von Dembskis 'Understanding intelligent design'.

Darwin widmete der Embryologie ein eigenes Unterkapitel und wertete verschiedene Beobachtungen aus diesem Gebiet als sehr überzeugende Argumente für eine gemeinsame Abstammung.
Seine Argumentation beinhaltet eine ganze Reihe einzelner Punkte, die ich hier nicht in ihrer Gesamtheit wiedergeben kann. Ich werde trotzdem versuchen, die Wichtigsten hier wiederzugeben, um Euch einen Eindruck über Darwins Herangehensweise zu vermitteln.

Zunächst zählt Darwin einige Fakten auf:

Einige Strukturen, die im erwachsenen Tier große Unterschiede aufweisen und verschiedenen Zwecken dienen, sind im Embryo noch sehr ähnlich (z. B. Körpersegmente von Insekten oder Vorder-/Hinterextremitäten). Innerhalb einer Klasse sind die Embryos der dazugehörigen Arten ebenfalls sehr ähnlich, auch wenn die erwachsenen Tiere sich stark unterscheiden können.

It has already been stated that various parts in the same individual which are exactly alike during an early embryonic period, become widely different and serve for widely different purposes in the adult state. So again it has been shown that generally the embryos of the most distinct species belonging to the same class are closely similar, but become, when fully developed, widely dissimilar. A better proof of this latter fact cannot be given than the statement by Von Baer that "the embryos of mammalia, of birds, lizards, and snakes, probably also of chelonia, are in their earliest states exceedingly like one another, both as a whole and in the mode of development of their parts; so much so, in fact, that we can often distinguish the embryos only by their size. In my possession are two little embryos in spirit, whose names I have omitted to attach, and at present I am quite unable to say to what class they belong. They may be lizards or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals. The extremities, however, are still absent in these embryos. But even if they had existed in the earliest stage of their development we should learn nothing, for the feet of lizards and mammals, the wings and feet of birds, no less than the hands and feet of man, all arise from the same fundamental form."
[...]
A trace of the law of embryonic resemblance occasionally lasts till a rather late age: thus birds of the same genus, and of allied genera, often resemble each other in their immature plumage; as we see in the spotted feathers in the young of the thrush group. In the cat tribe, most of the species when adult are striped or spotted in lines; and stripes or spots can be plainly distinguished in the whelp of the lion and the puma. We occasionally though rarely see something of the same kind in plants; thus the first leaves of the ulex or furze, and the first leaves of the phyllodineous acacias, are pinnate or divided like the ordinary leaves of the leguminosæ. [pp. 387/388]

Der Grund für die Ähnlichkeit einzelner Strukturen während der embryonalen Entwicklung kann nicht darin liegen, dass sie Anpassungen an ähnliche Bedingungen darstellen, da sich auch ähnliche Strukturen in Embryos finden, die unter sehr unterschiedlichen Bedingungen heranreifen.

The points of structure, in which the embryos of widely different animals within the same class resemble each other, often have no direct relation to their conditions of existence. We cannot, for instance, suppose that in the embryos of the vertebrata the peculiar loop-like courses of the arteries near the branchial slits are related to similar conditions,—in the young mammal which is nourished in the womb of its mother, in the egg of the bird which is hatched in a nest, and in the spawn of a frog under water. We have no more reason to believe in such a relation, than we have to believe that the similar bones in the hand of a man, wing of a bat, and fin of a porpoise, are related to similar conditions of life. No one supposes that the stripes on the whelp of a lion, or the spots on the young blackbird, are of any use to these animals. [p. 388]
Diesen Punkt unterstreicht Darwin noch durch Beispiele von Insekten- und Crustaceenlarven und kommt später noch einmal darauf zurück:
We are so much accustomed to see a difference in structure between the embryo and the adult, that we are tempted to look at this difference as in some necessary manner contingent on growth. But there is no reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all their parts in proper proportion, as soon as any part became visible. [p. 390]

Dies ist die Vorbereitung seines Arguments für common descent:

How, then, can we explain these several facts in embryology,—namely, the very general, though not universal, difference in structure between the embryo and the adult;—the various parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at an early period of growth alike;—the common, but not invariable, resemblance between the embryos or larvæ of the most distinct species in the same class;—the embryo often retaining whilst within the egg or womb, structures which are of no service to it, either at that or at a later period of life; on the other hand larvæ, which have to provide for their own wants, being perfectly adapted to the surrounding conditions;—and lastly the fact of certain larvæ standing higher in the scale of organisation than the mature animal into which they are developed? I believe that all these facts can be explained, as follows.
[pp. 390/391]

Seine folgende Erklärung beinhaltet zunächst, dass Modifikationen, die das Aussehen des erwachsenen Tieres betreffen, nicht notwendigerweise auch schon für den Embryo/das Jungtier von Nutzen sind. Solange z. B. ein Jungvogel noch von seinen Eltern gefüttert wird, benötigt er noch keinen spezialisierten Schnabel. Die Ausbildung dieser Modifikation früher in der Entwicklung wird also durch nichts begünstigt (oder könnte sogar negativ sein). Darum, wenn man eine gemeinsame Abstammung annimmt, sollten sich die frühen Entwicklungsstufen ähnlicher sehen als die erwachsenen Tiere.

Um dies zu überprüfen, vermaß Darwin verschiedene domestizierte Hunde-, Pferde- und Taubenrassen, von denen mit ziemlicher Sicherheit angenommen werden konnte, dass sie jeweils aus einer einzigen Art hervorgegangen war. Er konnte bestätigen, dass die sehr jungen Tiere (3 Tage nach der Geburt bzw. 12 Stunden nach dem Schlüpfen) noch sehr ähnlicher in ihren Proportionen waren als die erwachsenen Tiere.

Dies übertrug er dann auf natürlich vorkommende Arten:

Let us take a group of birds, descended from some ancient form and modified through natural selection for different habits. Then, from the many slight successive variations having supervened in the several species at a not early age, and having been inherited at a corresponding age, the young will have been but little modified, and they will still resemble each other much more closely than do the adults,—just as we have seen with the breeds of the pigeon. We may extend this view to widely distinct structures and to whole classes. The fore-limbs, for instance, which once served as legs to a remote progenitor, may have become, through a long course of modification, adapted in one descendant to act as hands, in another as paddles, in another as wings; but on the above two principles the fore-limbs will not have been much modified in the embryos of these several forms; although in each form the fore-limb will differ greatly in the adult state. [p. 393]

Darwin wusste natürlich noch nichts über die genetischen Grundlagen der Vererbung und wenn es darum geht, dieses spätere Auftreten von Modifikationen in der individuellen Entwicklung zu erklären, liegt er einigermaßen falsch. Wenn ich ihn richtig verstehe, stellt er sich die Embryonalentwicklung als eine Art Programm mit verschiedenen Stufen vor, die durchlaufen werden und neue Modifikationen werden einfach "hintendrangepackt". Aus einer Urform mit dem Programm 1-2-3 werden durch verschiedene Modifikationen dann verschiedene Spezies mit den Programmen 1-2-3-4, 1-2-3-5 oder 1-2-3-6.

Diese Sichtweise erscheint einem heute natürlich ziemlich naiv, aber wenn man sich vor Augen führt, dass Darwin nichts von Genen oder DNA wusste, dann finde ich seine Leistung, mit einem plausiblen Modell aufzuwarten, wirklich bewundernswert, auch wenn er nicht ganz richtig lag.

Er war in sofern sogar schon relativ "modern", als ihm auch bewusst war, dass auch einzelne Stufen reduziert, modifiziert oder im erwachsenen Tier revertiert werden können (für letzteres gibt er als Beispiel parasitär lebende Crustaceen an, deren Embryonalform "komplexer" aufgebaut ist als die adulte Form). Zudem war ihm auch bewusst, dass auch frühe Entwicklungstufen neu entstehen könnten.

Ich gebe im Folgenden seine abschließende Bemerkungen zu diesem Unterkapitel wieder, weil es noch einmal verdeutlich, was Darwin aus der Untersuchung von embryonalen Formen schloss. Ich gebe den ganzen ziemlich langen Abschnitt wieder, hebe aber meiner Meinung nach besonders wichtige Passagen besonders hervor.

From the remarks just made we can see how by changes of structure in the young, in conformity with changed habits of life, together with inheritance at corresponding ages, animals might come to pass through stages of development, perfectly distinct from the primordial condition of their adult progenitors. Most of our best authorities are now convinced that the various larval and pupal stages of insects have thus been acquired through adaptation, and not through inheritance from some ancient form. The curious case of Sitaris—a beetle which passes through certain unusual stages of development—will illustrate how this might occur. [...] Now, if an insect, undergoing transformations like those of the Sitaris, were to become the progenitor of a whole new class of insects, the course of development of the new class would be widely different from that of our existing insects; and the first larval stage certainly would not represent the former condition of any adult and ancient form.
On the other hand it is highly probable that with many animals the embryonic or larval stages show us, more or less completely, the condition of the progenitor of the whole group in its adult state. In the great class of the Crustacea, forms wonderfully distinct from each other, namely, suctorial parasites, cirripedes, entomostraca, and even the malacostraca, appear at first as larvæ under the nauplius-form; and as these larvæ live and feed in the open sea, and are not adapted for any peculiar habits of life, and from other reasons assigned by Fritz Müller, it is probable that at some very remote period an independent adult animal, resembling the Nauplius, existed, and subsequently produced, along several divergent lines of descent, the above-named great Crustacean groups. So again it is probable, from what we know of the embryos of mammals, birds, fishes, and reptiles, that these animals are the modified descendants of some ancient progenitor, which was furnished in its adult state with branchiæ, a swim-bladder, four fin-like limbs, and a long tail, all fitted for an aquatic life.
As all the organic beings, extinct and recent, which have ever lived, can be arranged within a few great classes; and as all within each class have, according to our theory, been connected together by fine gradations, the best, and, if our collections were nearly perfect, the only possible arrangement, would be genealogical; descent being the hidden bond of connexion which naturalists have been seeking under the term of the Natural System. On this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. In two or more groups of animals, however much they may differ from each other in structure and habits in their adult condition, if they pass through closely similar embryonic stages, we may feel assured that they all are descended from one parent-form, and are therefore closely related. Thus, community in embryonic structure reveals community of descent; but dissimilarity in embryonic development does not prove discommunity of descent, for in one of two groups the developmental stages may have been suppressed, or may have been so greatly modified through adaptation to new habits of life, as to be no longer recognisable. Even in groups, in which the adults have been modified to an extreme degree, community of origin is often revealed by the structure of the larvæ; we have seen, for instance, that cirripedes, though externally so like shell-fish, are at once known by their larvæ to belong to the great class of crustaceans. As the embryo often shows us more or less plainly the structure of the less modified and ancient progenitor of the group, we can see why ancient and extinct forms so often resemble in their adult state the embryos of existing species of the same class. Agassiz believes this to be a universal law of nature; and we may hope hereafter to see the law proved true. It can, however, be proved true only in those cases in which the ancient state of the progenitor of the group has not been wholly obliterated, either by successive variations having supervened at a very early period of growth, or by such variations having been inherited at an earlier age than that at which they first appeared. It should also be borne in mind, that the law may be true, but yet, owing to the geological record not extending far enough back in time, may remain for a long period, or for ever, incapable of demonstration. The law will not strictly hold good in those cases in which an ancient form became adapted in its larval state to some special line of life, and transmitted the same larval state to a whole group of descendants; for such larvæ will not resemble any still more ancient form in its adult state.
Thus, as it seems to me, the leading facts in embryology, which are second to none in importance, are explained on the principle of variations in the many descendants from some one ancient progenitor, having appeared at a not very early period of life, and having been inherited at a corresponding period. Embryology rises greatly in interest, when we look at the embryo as a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class. [pp. 394-396]

Im engeren Sinn lag Darwin auch mit der Ähnlichkeit heutiger embryonaler Formen mit der adulten Form eines gemeinsamen Vorfahrens falsch. Die ancestrale Form einzelner Strukturen kann man u. U. aber durchaus am Embryo erkennen. Im anschließenden Unterkapitel zu rudimentären Formen gibt Darwin selbst einige Beispiele:

What can be more curious than the presence of teeth in fœtal whales, which when grown up have not a tooth in their heads; or the teeth, which never cut through the gums, in the upper jaws of unborn calves? [p. 397]

Bei all diesen Tieren kann man aufgrund der Embryonalform davon ausgehen, dass ihre Vorfahren Zähne hatten.

Ich habe hier den letzten Abschnitt in seiner Gänze wiedergegeben, weil er sehr schön zeigt, wie differenziert Darwin argumentiert und auch seine Skepsis gegenüber Agassiz deutlich wird, der die embryonic resemblance gleich als ein "Gesetz der Natur" verstanden wissen will.

[Quelle aller Zitate: Darwin, C. R. 1872. The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. 6th edition; with additions and corrections. Eleventh thousand.]