Considerações sobre o mecanismo da evolução

Considerações sobre o mecanismo da evolução

F. G. Brieger

Escola Superior de Agricultura "Luiz de Queiroz", Universidade de São Paulo

RESUMO

O presente trabalho não tem por fim discutir exaustivamente os fatos conhecidos e as hipóteses até hoje formuladas sobre o mecanismo da evolução. Êle apresenta apenas um resumo de alguns pontos, sem entrar numa discussão detalhada da literatura, com o fim de por em relevo principalmente os métodos como podem aparecer expontaneamente novos caracteres. As nossas considerações servem como uma introdução para alguns trabalhos experimentais, que serão publicados a seguir.

A) - Conhecemos até agora os seguintes modos para a obtenção de novos caracteres fenotipos:

1) - Mutação gênica.

2) - Alteração citológica como poliploidia, polisomia e aberrações na estrutura cromossômica.

3) - Recombinação gênica, ou pela combinação de efeitos específicos de gens determinadores, ou pela combinação de fatores complementares ou principalmente pela mudança no conjunto dos modificadores ("Modifier chift").

A existência deste último modo, comprovado numa série de experiências, pode ter dois efeitos : alterar a base genética de certos caracteres, sem provocar novos efeitos fenotípicos, ou então provocar novas ações fenotípicas de determinados gens.

Com respeito à fisiologia do gen, não pode haver dúvida que o conjunto dos modificadores é capaz de alterar a sua ação, provocando novos efeitos e cancelando outros.

B) - As duas principais modalidades de aumentar a freqüência dos novos caracteres são:

1) - Seleção natural.

2) - A flutuação das freqüências ou seleção flutuante ("genetic drift").

C) - Finalmente, discutimos rapidamente os processos indispensáveis para completar o processo de evolução : os métodos de isolamento que garantem a manutenção dos novos fenotipos.

ABSTRACT

The present publication is not intended' to be a complete review of the very extensive literature on the modern aspect of the theory of evolution, but represents mainly an introduction to several papers which will be published simultaneously or shortly afterwards on experimental studies about the evolutionary mechanism. The latter are the results of studies which have been underway for about 15 years, howewer with several and unforeseen interruptions. Owing to special circustances, the work was carried out in three different countries, and the results so far otained have beem published in three languages. Thus their natural sequence may have become obscured, and since a more general paper was published as far back as 1929, I thought it necessary to bring my publications again more or less into line. This explains why I have always made references to my own papers, even where I might have cited other authors.

Another reason for the present publication is the scarcity of Brazilian publications of a more general nature which may keep the student informed about progress of scientific knowledge.

A complete review of the literature on evolution may be found in the books by DOBZHANSKY, GOLDSCHMIDT, FISHER, HURST, etc.

The three main aspects of the mechanisme of evolution are discussed:

a) The apearance of new heritable characters,

b) their selection, and

c) their preservation.

a) Three processes are generally mentioned as possible sources for obtaining new characters. Mutations play no doubt a very important role in evolution, inspite of their low frequency and their lack of direction. Cytological aberrations seem to be of much less importance. Autosomy and autopolyploidy do not produce, as a rule, new characters, causing generally only an alteration of proportions (Fig. 2 and 3). Polyploidy is of special importance as a mechanism for restoring fertility in species hybrids and to reestablish the conditions for normal and complete chromosome pairing. Thus euploid and aneuploid segregates may become established, as in the case of Erophila (Fig. 1) studied by WINGE (1940).

The result of genetical recombination is discussed with more detail, since I believe this method to be of most importance in the evolutionary process. Several cases of the interaction of special genes from diferent species or forms are cited. But the effect caused by changes in the modifier background seem to me of still more importance. The first clear case of the effect caused by a "modifier shift" upon single gen action seems to have beem discovered by BRIEGER (1929) (Fig. 5) in species crosses of Nicotiana and an identical case was reported later by HOLLINGSHEAD (1930) in Crepis. Various cases of selection for modifiers, showing the effect of the modifier shift, in Lebistes (WINGE), Antirrhinum (BRIEGER) and Zea (BRIEGER) are discussed. Both progressive and regressive evolutionary series and also the origin of ecotypes may best be explained by means of a "modifier shift", instead of returning to the hypothesis of the acumulation of rare chance mutations.

b) One may distinguished two processes of selection: directed seletion, which may be either natural or artificial, and fluctuating seletion or "genetic drift". The existence of the former in nature and experiments is well established and thus reed not be discussed anymore. The existence of fluctuating selection is difficult to prove experimentally, though there exists a large amount of indirect evidence. An artificial experiment with cards is used to show the nature and speed of fluctuating selection (Fig. 9).

c) The mechanisms of isolation are discussed rapidly, and a classification given, which combines several poinst of DOBZHANSKY'S classification (1941 pg. 256-7) with poitns discussed by BRIEGER (1930c):

I - Prohibition of crossing during the vegetative phase:

A - Geographical separation. B - Ecological separation. C - Separation in time.

II - Prohibition of crossing during the reprodutive phase:

A - Impossibiliy of copula or pollinisalion.

B - Impediments in the movements of spermatozoids or the growth of pollen tubes.

C - Absence of fertilization.

III - Hybrid sterility:

A - Imperfect vegetative development of the hybrids.

B - Gametic sterility of hybrids.

Finally the main dificulty is mentioned of explaining the evolution of these isolating mechanism which so far remains an unsolved problem.

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LITERATURA

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Datas de Publicação

• Publicação nesta coleção
  26 Fev 2013
• Data do Fascículo
  1944