Rend. Ftk Acc. La)~cei s. 9, v. 5:311-315 (1994)

Storia de l l e s c i e n z e sper imenta l i . - - Evolution: some reported data and some perso- nal reflections. Nota (*) del Socio M~u~IO BENAZZI.

ABSTVa~Cr. - - Evolution is still a flourishing doctrine, with scientific and social implications. It is sup- ported by new paleontological discoveries. The basic principle of natural selection is analyzed from differ- ent viewpoints, including a physical basis. At present, neo-Darwinism encounters both approval and criti- cism. Molecular biology has opened new perspectives in phylogenetic theories.

KEY WORDS: Evolution; Natural selection; Neo-Darwinism; Molecular biology; Phylogenesis.

R~SSUNTO. - - Evoluzione biologica: riferimento di nuovi dati della letteratura ed alcune personali riflessio- ni. La teoria dell'evoluzione, 135 anni dopo la pubblicazione dell'opera di Charles Darwin sull'Origine delle Specze, ~ tuttora in pieno sviluppo, con importanti implicazioni scientifiche e sociali. Nuovi probanti dati so- no offerti dalla Paleontologia. I1 principio della selezione naturale viene analizzato da diversi punti di vista, anchc su basi flsiche. I1 neo-Darwinismo trova sostenitori ed oppositori. La biologia molecolare ravviva i problemi della tilogenesi. L'autore pensa che l'evoluzione nel mondo biologico sia, come importanza, para- gonabile alia gravit/l nel mondo fisico.

<<I see no good reason why the views given in this volume should shock the religious feel- ing of anyone. It is satisfactory, as showing how transient such impressions are, to remember that the greatest discovery ever made by man, namely, the law of attraction of gravi~, was also attacked by Leibniz as a subversive of natural, and inferentially of revealed, religion>>. Darwin (1859).

Evolution, 135 years after the publication of the Origin of Species by Charles Dar- win, is an accepted theory, which concerns a great number of fundamental questions, including scientific and social implications.

I was encouraged to publish my reflections on some of these topics by recent publi- cations, among which I mention the first-class paper by Jenkins jr. and Walsh (1993) who discovered an early Jurassic caecilian with limbs. The fossils consist of 38 speci- mens, which reveal numerous features, particularly four little limbs, that are lacking in the present-day species. The authors consider the fossils as belonging to a new taxon, namely the family Eocaecilidae, with Eocaecilia micropoda gen. et sp. nov.

The phylogenetic importance of this finding is evident, considering that the caecil- ians are the first Amphibia derived from fishes, namely, the Crossopterigia.

Some taxonomic remarks can now properly be made. The class Amphibia, belong- ing to the phylum Vertebrata, comprises three orders: Apoda or Gymnophiona (that is the Caecilians), Urodela or Caudata, Anura or Salientia. While the last two are widely distributed and have been very well investigated, the caecilians occur only in tropical and semitropical regions. They are limbless, wormlike, elongate, mostly fossorial with degenerated eyes. The fossils illustrated by Jenkins and Walsh (1993) offer therefore a

(*) Presentata nella seduta del 12 marzo 1994.

312 M. BENAZZI

significant contribution to the origin of Tetrapoda, namely the terrestrial vertebrates, owing to the presence of little limbs.

In a recent Symposium on vertebrate evolution edited by Ghiara (1991) there is no mention of the caecilians, although in the Opening Remarks he points out the differ- ences of opinion as for as the origin of the amphibians is concerned. In the same Sym- posium, the paleontologist Azzaroli (1991) notes that the transition from rhipidishian crossopterygian fishes to amphibians still appears rather abrupt, adding that the appar- ently abrupt is due to a gap in the fossil records. Also the paleontologist Pinna (1993) offers a reliable analysis of evolution from fishes to amphibians. He outlines the evolu- tionary history of tetrapods in a classical view, in opposition to the cladistic approach, frequently adopted by recent authors (for a critical review of cladism, cf. Simonetta, 1993).

Beside the paleontological data, it is useful to consider some cytogenetic investiga- tions. This aspect of evolution was previously debated by Morescalchi (1991) and fur- ther interesting data have been added recently by Mancino (1993) concerning the three orders of Amphibia. The percentages of the species investigated are as follows:

Caecilians 16 out of 163 (= 9.8%);

Urodeles 155 out of 357 (=43.4%); Anurans 831 out of 3521 (=23.6%).

Mancino points out that almost all aspects of the biology of the caecilians are poorly studied and the knowledge of cladistic and phylogenetic relationships within the group is still scarce or problematic; but he also evidences the importance and the stimulating effect of paleontological discoveries for the evolutionary history of caecilians, as in the case of the reports by Jenkins and Walsh (1993).

These recent aquisitions on the first steps of vertebrate evolution are noticed only as an example of the lasting fertility of evolutionary studies.

Indeed, the essential aspect of the Darwinian revolution is the principle of natural selection, on which the literature is still very rich. Among recent statements, [ am pleased to mention the one by Bonner (1992, first paragraph), who writes that <<evolu- tion by natural selection is the most useful, the most important, the most all-enveloping concept in all of biology>>.

There is clearly the need for an expansion of evolutionary concepts in various social circles. I am not aware about their learning in Italian high schools: after the unhappy abolition (1923) of Natural History as an independent subject of teaching - of which evolution would represent the conclusive synthesis -, a probable decrease of interest has progressively taken place. Possibly, the basic knowledge of evolution is lectured by teachers of philosophy, rather than by experts in biological sciences, when considering the moral implications of Darwinism.

Aside from the schools, our Academy has offered recent publications on evolution. I am pleased to remember the article by our late colleague G. Montalenti (1985) and, in the same treatise, the one by Omodeo (1985) concerning the definition of a living organism.

E V O L U T I O N : S O M E R E P O R T E D D A T A A N D S O M E P E R S O N A L R E F L E C T I O N S 313

I cannot forget the books by R. Dawkins: The Selfish Gene (1976) and The Blind Watchmaker (1987). They offer a personal interpretation of natural selection as a cre- ative force toward an improving structural complication, distinguishing biology from physics.

But I wish now to attempt an evaluation of the rank of evolutionary theory in com- parison with other natural sciences, particularly physics. I was stimulated to such trial by the book of our late colleague Ageno (1972). Chapter 5 concerns biological evolu- tion and fundamental principles of physics, expressed particularly by the principle of uncertainty and the second law of thermodynamics. Ageno states that these principles have their firm foundation on experimental ground, while ~evolution seems to be char- acterized by the gradual fulfilment of a more and more complex level of organization and the emergency of new qualities and capacities~). Taking into account the basic con- cept of selection of new characters, Ageno points out that mutations are casual events, in the sense of the uncertainty principle, and that their occurrence always agrees with the consequences of the second law of thermodynamics. The trend of evolution through natural selection toward more and more complex levels of organization is, however, submitted to a limit, beyond which no evolutionary advantage can be ob- tained. Ageno postulates that the complexity may be a relative optimum, according to the relative pathways followed in the past by the species considered. I believe that these conceptions, elaborated by a scientist who dedicated his activity mainly to the physical basis of biological phenomena, deserve attention.

In recent years the term neo-Darwinism has been introduced as an expansion of evolutionary thought, expressed by the so-called ~modern synthesis~. This theory, founded essentially on the principle of natural selection, attempts to explain all aspects of biological evolution.

However, the power of natural selection is not unlimited, as acknowledged by Dar- win himself with the famous expression: ~Furthermore, I am convinced that natural se- lection has been the main but not exclusive means of modification,.

Conceptions contrasting neo-Darwinism have recently been put forward by various scientists and are based on different motivations. The most radical may be the one by Lima-de-Faria, who entitled his book (1988) Evolution without Selection. The evidence for distinguishing the so-called ~autoevolution>~ from neo-Darwinism seems to me rather speculative. More attention is deserved by his statement No. 6: ~recent studies on DNA evolution show the canalized character of mutational. According to Lima-de- Faria, in fact, mutation is not random, but is directed. I observe that canalization is a common attribute of populations showing a high degree of uniformity for various char- acters, that are assumed to be strongly associated with fitness. This is an epigenetic question perhaps already taken into account by Darwinism.

Evolution has also greatly involved several geneticists: this topic was thoroughly treated by Barigozzi (1993).

Very far from the casualist ground of Darwinism are, instead, the finalistic concep- tions of evolution (the doctrine of ~final cause~). In this regard, I only mention the sci-

314 M. BENaZZt

entific writings by the French naturalist and philosopher Pierre Teilhard de Chardin (1881-1955), whose ideas were divulgated in Italy by Galleni (1987).

Another important chapter concerns the time and modes of animal evolution. A new approach was represented by the theory of <<punctuated equilibria,>, due to the pa- leontologists Eldredge and Gould. In our Academy this theory has been well illustrated by Capanna (1992): therefore, here, I only summarize that, according to the two au- thors, animal species are endowed with a genetic property which eliminates from the populations foreign variation: this determines a stasis or equilibrium. Evolutionary change occurs, however, in peripheral population groups, demes subjected to strong se- lection. Thus: long periods of stasis are alternated by short periods of speciation. The theory of punctuated equilibria has been strongly criticized by neo-Darwinists (see Ca- panna, 1992), but very recently Gould and El&edge (1993) have published a conclu- sive report on their theory, evidencing that as a complement of phyletic gradualism, sta- sis represents a meaningful and predominant pattern within the history of species, and recasting macroevolution as the differential success of certain species within clades.

The word macroevolution induces me to ponder over many events of my life and to cast my mind many years to back, particularly when I was attracted by Haeckel's ge- nealogical trees. Now, the development of molecular biology offers a wealth of infor- mation, allowing reliable phylogenetic deductions within restricted taxa, such as genus, tribe, etc. However, phylogenetic frameworks inferred on a molecular basis have been extended to the full animal kingdom. As an example, I recall the Monophyletic Origin of the Metazoa: An Evolutionary Link with Fungi by Wainright et al. (1993) and also the paper by Morris (1993), who suggests a monophyletic origin of the Animal King- dom.

As a concluding remark, I suggest, as a possible postulate, that ,evolution in the liv- ing world may be considered equivalent to gravity in the physical world,>.

ACKNOWLEDGEMENTS

I am very grateful to Dr. Peter Christit~ for revision of the manuscript.

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Professore Emerito di Zoologia Via dell'Ordine di S. Stefano, i1 - 56013 IMaI~INA i ) l PISA PI