Common descent

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The publication of On the Origin of Species was an immediate success, and the theory of evolution by natural selection was recognised from the start as one of the greatest triumphs of the human mind. At his death, Darwin was buried in Westminster Abbey, near great thinkers of the past such as Newton and Hume, even though his theory was regarded as a danger to religion.

It has been said that there is a paradox in these honours because Darwin did not invent the idea of evolution nor that of natural selection, as he himself openly states in the "Historical sketch" that he wrote for the 6th edition of On the Origin of Species. As for evolution, Darwin admits without hesitation Lamarck's priority: "Lamarck was the first man whose conclusions on the subject excited much attention. This justly-celebrated naturalist... upholds the doctrine that species, including man, are descended from other species." With equal fairness, Darwin adds that the idea of natural selection had already been proposed by William Wells in 1813 and by Patrick Matthew in 1831 (another precursor was Edward Blyth in 1835).

In reality, what deeply impressed people was not the idea of natural selection as such, but the abyssal divide that emerged between the simplicity of the idea and the enormity of its consequences. With an ordinary mechanism Darwin arrived at extraordinary conclusions, and these were so radical that nobody could remain indifferent. There is therefore no paradox in the success of the book and in the honours bestowed on its author.

Darwin did propose, however, some truly original ideas, and perhaps the most extraordinary of all is the concept of common descent, the theory that all living creatures of our planet derive from a single stock of primordial forms. In Darwin's times, the fixity of species was still the official theory of biology, and generations of past naturalists had built, within that reference system, a grandiose classification scheme that appeared capable of revealing, as Linnaeus put it, "the Plan of Creation".

Dogs and wolves, for example, are different species because they are reproductively isolated, but they have countless other features in common and for this are classified in the same genus Canis (Canis familiaris and Canis lupus). In the same way, tigers and lions are two species of the genus Panthera (Panthera tigris and Panthera leo), as polar bears and grizzly bears are different species of the genus Ursus. Tigers and domestic cats, on the other hand, cannot be put in the same genus, but still have so many characters in common that they are classified, together with lions, in a single family (the Felidae).

Dogs, wolves, tigers, lions, cats and bears, on the other hand, are all characterized by meat-eating structures, and for this are grouped together in the order Carnivora. Animals like bats, monkeys and whales are classified in quite different orders and yet they share with carnivores the ability to feed their young by mammary glands, and all animals that have this property are united in the class Mammalia.

Similar criteria allow us to recognise at least five distinct animal classes - mammals, birds, reptiles, amphibians and fishes - but even in these very different groups it is possible to recognize a common feature. Their embryonic development is organised around a dorsal chord (or chordomesoderm), and this allows us to conclude that the five classes belong to a single phylum (Chordata, subphylum Vertebrata). Vertebrates and invertebrates, in turn, have a number of features that separate them from plants, and for this are grouped together in kingdom Animalia.

As we can see, there are different levels of biological features, and this allows us to recognise seven great categories of living organisms, the so-called taxa or taxonomicalgroups: species, genus, family, order, class, phylum and kingdom (Figure 2.1).

This impressive system had been built by generations of naturalists within the classical framework of the fixity of species, and was a good description of the order that we see in nature, but the description could also be interpreted in a different way. Taxonomical relationships, for example, could be a consequence of ancestral relationships, and in some cases the signs of parenthood were quite evident. That dogs and wolves had a common ancestor, for example, was easily acceptable, and also fairly obvious was a relationship between cats and tigers. But to say that butterflies had something in common with whales and coconuts, and that all derived from a common ancestor, is quite a different thing. It is important to notice that this idea is not an automatic consequence of evolution. According to Lamarck, for example, the diversity of life was caused by the spontaneous generation of countless lines of descent which arose independently and were not linked by heredity. Common descent was too improbable to be taken into consideration, and no one before Darwin had proposed it. Darwin described it briefly in the last chapter of On the Origin of Species with these words:

"I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number. Analogy would lead me one step further, namely, to the belief that all animals and kingdom

ANIMALIA

phylum

Chordata

Arthropoda class

Mammalia Aves Reptilia Amphibia Pisces order

Carnivora Rodentia

Primates family

Felidae

Canidae

Ursidae genus Panthera Felis

Canis

Ursus Ailuropoda species tigris leo domesticus familiariaris lupus arctos maritimus melanoleuca

Figure 2.1 Organisms are classified into seven taxonomic groups, or taxa, which are known as species, genus, family, order, class, phylum and kingdom. Modern taxonomy is still using the binomial terminology introduced by Linnaeus (1758), in which every living form is identified by two Latin names written in italics, the first for the genus and the second for the species. A further subdivision of the seven basic groups is obtained by introducing intermediate categories such as subclasses or superfamilies.

plants have descended from some one prototype. But analogy may be a deceitful guide. Nevertheless all living things have much in common, in their chemical composition, their cellular structure, their laws of growth and their liability to injurious influences. We see this even in so trifling a fact as that the same poison often similarly affects plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild rose or oak-tree. With all organic beings, excepting perhaps genus Panthera Felis

Canis

Ursus Ailuropoda species tigris leo domesticus familiariaris lupus arctos maritimus melanoleuca

Figure 2.1 Organisms are classified into seven taxonomic groups, or taxa, which are known as species, genus, family, order, class, phylum and kingdom. Modern taxonomy is still using the binomial terminology introduced by Linnaeus (1758), in which every living form is identified by two Latin names written in italics, the first for the genus and the second for the species. A further subdivision of the seven basic groups is obtained by introducing intermediate categories such as subclasses or superfamilies.

some of the very lowest, sexual reproduction seems to be essentially similar. With all, as far as is at present known, the germinal vesicle is the same; so that all organisms start from a common origin. If we look even to the two main divisions - namely, to the animal and vegetable kingdoms - certain low forms are so intermediate in character that naturalists have disputed to which kingdom they should be referred... Therefore, on the principle of natural selection with divergence of character, it does not seem incredible that, from such low and intermediate form, both animals and plants may have developed; and, if we admit this, we must likewise admit that all the organic beings which have ever lived on this earth may be descended from some one primordial form."

The description is brief because Darwin did not have other arguments, and for a long time the idea of common descent was regarded one of his weakest hypotheses, because many arguments were against it, and very few appeared to support it. Today the situation is completely reversed, because molecular biology has amassed a vast array of data in its favour. Two of them are particularly important: the fact that virtually all cells use the same genetic code for hereditary information, and the same molecular carrier (ATP) for energy exchanges. The existence of the same mechanisms in processes as diverse as heredity and metabolism, which are the very foundations of life, can only be explained with the parenthood of all present creatures with all past living beings.

This is probably the greatest of Darwin's ideas, and almost a century later, in 1949, one of the founding fathers of bioethics, Aldo Leopold, underlined its enormous value with this comment: "It is a century now since Darwin gave us the first glimpse of the origin of species. We know now what was unknown to all the preceding caravan of generations: that men are only fellow-voyagers with other creatures in the odyssey of evolution. This new knowledge should have given us, by this time, a sense of kinship with fellow creatures; a wish to live and let live; a sense of wonder over the magnitude and duration of the biotic enterprise."

The second mechanism of evolution

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