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Hoe ontstaan nieuwe soorten?
Darwins boek heette "Het ontstaan van de soorten", ondankst het feit dat hij deze vraag eigenlijk niet echt behandeld heeft; meer dan hondervijftig jaar later is het nog steeds grotendeels een mysterie hoe soorten precies ontstaan.
Chris Colby, The Talk Origens Archive, Introduction to evolutionary biology.
Huidige verkenningen van de nieuwe genetische en embryonale ontwikkelingsmechanismen - door gebruik te maken van de nieuwe technieken van de moleculaire biologie - zouden de lijst van potentiele mechanismen voor micro- en macro-evolutionaire verandering kunnen vergroten,. Misschien dat we dan zullen weten of het soort micro-evolutionaire onderscheiding dat optreedt als twee dochtersoorten van een groep reproductief geisoleerd raken,ook kan leiden tot nieuwe geslachten, families, of zelfs afdelingen.
Biology, N.K.Wessells (Stanford University) and J.L. Hopson, 1988, page 1074
A large volume with a distinct evolutionist slant. A textbook for biology students.
Oftewel, nu weten we het niet, maar misschien weten we het dan.
Het is niet bekend of macro-evolutie plaatsvindt door niet-geledelijk sprongen, noch is het duidelijk of er unieke mechanismen nodig zijn voor of niet-geleidelijke sprongen of macro-evolutie.
Biology, blz 1075
Dit is dus wat vaklieden zeggen, dit in tegenstelling tot goedbedoelende leken.
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Mutaties kunnen geen nieuwe soorten maken:
"Voor een belangrijk deel zijn puntmutaties verantwoordelijk voor wat soms aangeduidt wordt als micro- of moleculaire evolutie, dat wil zeggen: De gedurende de evolutie steeds verdergaande vervanging van basen in homologe genen en van aminozuren in de daarmee corresponderende eiwitten. Maar ze hebben waarschijnlijk weinig van doen gehad met het veel meer omvattende verschijnsel in de evolutie, dat geleid heeft tot het ontstaan van steeds complexere levensvormen. Puntmutaties schijnen tijdens de hele evolutie met dezelfde frequentie te zijn voorgekomen en vertonen geen correlatie met gebeurtenissen die op een bepaald ogenblik nogal abrupt hebben geleid tot het verschijnen van nieuwe soorten."
Prof Christian de Duve, Nobel Prize in Physiology or Medicine, "De Levende Cel", deel 2.
"Puntmutaties of chromosomale herschikking zijn op zichzelf maar een beperkte bron van variatie voor evolutie, omdat ze alleen maar een functie kunnen wijzigen, of één bepaalde soort functie kunnen veranderen in een andere."
An Introduction to Genetic Analysis, A.J.F. Griffiths, (university of Britisch Columbia) J.H. Miller, (university of California, Los Angeles) D.T. Suzuki, (university of Britisch Columbia) R.C. Lewinton, (Harvard University) W.M. Gelbart (Harvard University) 1996, blz 794
"Bewijs stapelt zich op, dat evolutie van eiwit-sequenties niet de enige of zelfs de belangrijkste basis is van de evolutie van organismes."
Biochemistry, D. Voet, (university of Pennsylvania) J.G. Voet, (Swarthmore College) 1995
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De mutation breeding laat zien dat mutaties geen nieuwe soorten kunnen maken:
Na de bewijzen onderzocht te hebben, concludeerde Lönnig: „Mutaties kunnen een bepaalde soort niet omvormen tot een totaal nieuwe. Deze conclusie komt overeen met alle bevindingen en resultaten van mutatieonderzoek van de twintigste eeuw bij elkaar alsook met de waarschijnlijkheidswetten. De wet van de terugkerende variatie impliceert dus dat genetisch correct gedefinieerde soorten echte grenzen hebben die niet weggehaald of overschreden kunnen worden door toevallige mutaties.”
Wolf-Ekkehard Lönnig, wetenschapper aan het Duitse Max Planck Instituut voor Plantenveredeling.
http://www.weloennig.de/Loennig-Long-Ve ... iation.pdf
In the present paper the history of the rise and fall of mutation breeding as an autonomous branch of breeding research is documented as well as its positive side effects for plant breeding and biology in general. Perhaps the most important generalization on the basis of the total outcome of mutation breeding will be termed “the law of recurrent variation”. It states that “treating homozygous lines with mutagenic agents generates large, but clearly finite, spectra of mutants. This consistently occurs when the experiments are carried out on a scale adequate to isolate the potential of alleles causing phenotypic and functional invisible residual effects of changes in redundant sequences and/or of further chromosome rearrangements, the corresponding saturation curve is asymptotically approaching its limit for the micro-quantitative part of variation.” Also, reasons are given why the law is relevant for heterozygotes and allogamous species as well, and the genetical basis of the law is briefly defined.
In addition, arguments are presented why the overoptimism and euphoria at the beginnings of the period of mutation breeding are to be evaluated in connection with the basic assumptions of the synthetic theory of evolution − i.e. the assurance that mutations and selection constitute the entirely sufficient explanation of the origin of all species and higher systematic categories of the plant and animal kingdoms alike. This point established, the question is discussed whether the finite nature of the mutant spectra found in plant breeding research might also have repercussions on the present theory of the origin of species.
Providing an affirmative answer of the applicability of the law of recurrent variation not only to cultivated plant and animal lines but also to species in the wild, the statements and assertions of the synthetic (=evolutie) theory as quoted below will have to be revised.
The Law of Recurrent Variation and Selection Limits
Mutations are thought to be the ultimate basis for evolution by natural selection. So, let’s have a look at the question of whether mutations could have provided the raw materials for natural selection for the origin of all species and life forms of the earth. Having investigated the question for about 35 years now including the work with collections of mutants of two model plant species (the pea and the snapdragon - more than 1 million plants), I have come to a conclusion strongly differing from the modern synthesis concerning the potential of mutagenesis. The results I have summed up in “the law of recurrent variation” (Lönnig, 1993, 1995; Kunze et al., 1997). This law specifies that, for any case thoroughly examined (from pea to man), mutants occur in a large, but nevertheless limited spectrum of phenotypes which - in accordance with all the experiences of mutation research of the 20th century taken together - cannot transform the original species into an entirely new one. These results are in agreement with the statements of several renowned evolutionary geneticists, two of whom are quoted here. Hans Stubbe wrote after a lifetime spent in mutation research (1966, p. 154):
The improved knowledge of mutants in Antirrhinum has provided some essential experience. During the years each new large mutation trial showed that the number of really new mutants recognized for the first time, was steadily diminishing, so that the majority of the genetic changes was already known.
And Gottschalk stated in 1994, p. 180, “The larger the mutant collections are, the more difficult it is to extend them by new mutation types. Mutants preferentially arise that already exist.”
To understand these observations one must clearly distinguish between two levels: first, the level of the phenotypes, and second, the DNA level. On the latter, the potential of missense and nonsense mutations and other sequence deviations is nearly infinite. However, the spectrum of the resulting different phenotypes is not, because the space of functionally valid sequences within a given system of tightly matching regulatory and target genes and correspondingly co-ordinated functions involved in the formation of the finely balanced whole of an organism, cannot infinitely be stretched by chance mutations.
Expectations in mutation breeding
Since the origin of cultivated lines was thought to be indispensably due to the same factors as the origin of species in the wild, it has been reported that an enormous euphoria spread among biologists in general and geneticists and breeders in particular that the time had come to revolutionize the “old” method of recombination breeding by the entirely new branch of mutation breeding (see documentation below).
In other words: provided that mutations had, in fact, produced the raw materials for the origin of all genes and proteins, all physiological processes and anatomical structures of both the animal and plant kingdoms alike, the most surprising successes had to be expected by applying these factors − induced mutations and selection − to animal and plant breeding research.
Also, three different time-lapse methods complementing each other for a complete success in a rather short period of time were at the disposal of the breeders: (a) multiplication of mutation rates, (b) well-aimed recombination and © intelligent selection. Thus, in the USA as well as in several countries of Europe and Asia, the new research branch of mutation breeding was launched
Mutations and recurrent variation 49
in what might be called two waves: the first billow at the end of the 1930s, which was reinforced especially after the Second World War to form a tide in cooperation with the FAO/ IAEA, worldwide.
Mutation breeding some 40 years later
In the following paragraphs we will condense the general results for mutation breeding after several decades of intense research of this branch by directly quoting the authoritative statements of some of the world’s best agronomical and botanical scientists, most of which have actively taken part in mutation breeding themselves.
Thus, some 40 years after its beginnings Simmonds sums up the inclusive results of the enterprise of mutation breeding in his book on the Principles of Crop Improvement (84):
“Earlier overoptimism, to the effect that induced mutations were about to revolutionize plant breeding, has given place to a more sober appreciation of the technique as a valuable supplement to more conventional techniques in certain, rather restricted circumstances. ….ery many programmes failed, especially in the early days of overoptimism, to produce anything useful because they were not fulfilled. Nowadays we see mutation-induction simply as one technique which is occasionally useful in enlarging the genetic base of a programme in a limited and highly specific fashion.”
Additionally, Leibenguth describes the overall results of mutation breeding in his work Züchtungsgenetik (Genetics of Breeding) as follows (40):
“Almost all mutants distinguish themselves by negative selection values. According to observations in cereals and legumes the proportion of mutants being suitable for breeding amounted to 0.5 to 1 percent of the genotypes selected in these experiments. Besides, often a negative effect on other components of the pleiotropic spectrum of characters has been found that diminishes the breeding value of a positive mutant. Thus, nowadays the original aim to substitute the time-consuming and expensive methods of recombination breeding by “mutation breeding” is not up-to-date anymore. Mutation breedings is viewed to be less an autonomous method of breeding than an occasionally used supplement to traditional methods.”
Already some years before, Micke had stated that “one has to accept the fact that only a very small fraction of induced mutants (certainly less than 1 %) has ever been found suitable to enter yield trials and eventually only 1 % of those evaluated passed the official tests and obtained approval for commercial utilization” (67).
Over and above, Leibenguth adds that mutation breeding cannot be successfully applied to animal husbandry at all, because, “In contrast to plants, animals are genetically more severely balanced. Hence, all kinds of mutations are even more frequently lethal and more strongly diminishing vitality and fertility in animals” (40). Hence, according to all the evidence achieved so far by experimental investigations (and later also by careful considerations in theoretical genetics) there is absolutely no future for mutation breeding in animals − not to speak of severe ethical problems involved in the artificial mutagenesis of birds, mammals and other animals capable of feeling pain.
In plant breeding less than 1 percent of all the induced mutants have been chosen as possible candidates for further investigations. Of these again only 0.5 to 1 percent have passed the necessary further field trials until they were found suitable for commercial use. Thus, in plant breeding the average proportion of negative or useless mutants to positive ones is smaller than 10,000 : 1. Making calculations on the basis that only 0.5 percent of all induced mutation were suitable for further investigations and that again only 0.5 percent displayed a positive selection value for the breeder, this proportion is about 40,000 : 1. An approximate mean value of 25,000 negative (or useless) mutants to 1 being positive should therefore not to be an unrealistic calculation for plant breeding.
As to the genetically even more severely balanced animals, the state of affairs has been so arduous that no realistic numbers have been produced, which could provide the basis of similarly approximate calculations regarding the proportions of negative (or useless) mutants to positive ones in animal husbandry. If − as an educated guess − one multiplies the proportionate number of disadvantageous mutations by the factor of 10, the result would already be some 100,000 to 400,000 negative (or unavailing or neutral) mutants to 1 useful for breeding research.
It was on the basis of such experiences often made over dozens of years that almost all commercial breeding stations in the USA and Europe have deleted mutation breeding from their research programmes.
A significant concrete example may back up this point: at the end of the 1960s it was still widely believed that it was possible to improve crop proteins by mutation breeding. After some one and a half decades of intensive efforts and extraordinary financial input, Micke and Weindl comment (68):
“Our programme on the improvement of grain protein has now come to an end. …uring the 14 years of the programme it had to be recognized that the matter is more complicated and that there are some mutual limitations of quantity and quality.”
Poehlmann has summed up the overall results of mutation breeding in agreement with the authors quoted above as follows (76):
“One can only conclude that the results from mutation breeding in varietal development of the major field crops have been rather meager in relation to the efforts expended.”
Peter von Sengbusch concurs by the following observation (82):
Mutations and recurrent variation 51
“In spite of an enormous financial expenditure, the attempt to cultivate increasingly productive varieties by irradiation, widely proved to be a failure.”
Also, the distinguished plant breeders Fischbeck, Röbbelen and Stutzer are in accord with these statements (25):
“The objectives of practical plant breeding, to achieve new opportunities of a gradual and continuous amelioration of tried and tested breeding varieties could…not be realized.”
And especially concerning the neo-Darwinian concept of “micro-mutations” these three authors continue (25):
“Also, the modified concept of a direct use of so-called “micro-mutations” remained unsuccessful, because achievable breeding progress by this method distinctly lagged behind useful variation, which could be developed from the broad stream of conventional recombination breeding.”
Yet, perhaps one of the most astounding facts in the history of genetics appears to be the enormous gulf between the optimistic descriptions of mutants by so many authors active in plant breeding research during that period of time and the later “widely spread disappointment regarding mutation breeding” (66) due to the disconcerting reality, i.e. the meagre results obtained. Confirming the observation of a rather strange distance between hypotheses and reality, Micke continues his assessment after his calculations quoted above (explaining the relatively few useful mutants achieved in mutation breeding) as follows (67):
“In contrast to such rare achievements there have been innumerable ‘promising mutants’ reported in innumerable publications, which never seem to appear again on the stage after their first presentation. Nevertheless, there remains a respectable number of mutants which even the self-critical breeder or geneticist have seriously considered as progressive and of which only very few so far have contributed to the development of better crop cultivars.
This experience has been disappointing to many, to those who worked with mutations and expected optimistically fast ‘break-throughs’ as also to those who watched the many mutation activities sceptically but nevertheless hoped that results would make the difficult task of plant breeders easier, at least in particular areas.”
Micke also pointed out that neither the application of different mutagenic agents, nor various degrees of dosages, nor diverse modifying measures were able to revise the overall results: “The ultimate hope of obtaining more of the ‘better’ mutants has not been fulfilled” (67) (see also note 1 at the end of the paper).
According to the premises of the synthetic theory, explaining the origin of the entire world of organisms predominantly by selected mutations, a worldwide revolution in plant breeding research had been expected in the late 1930s, which was reinforced by Nobel laureate Josef H. Muller in 1946 especially for first decades after the Second World War.
However, due to the fact that:
(a) “many programmes failed…to produce anything useful”,
(b) “almost all mutants distinguish themselves by negative selection values”,
© “all kinds of mutations are even more frequently lethal and more strongly diminishing vitality and fertility in animals”,
(d) the overall results “have been rather meager in relation to the efforts expended”,
(e) “in spite of an enormous financial expenditure… widely proved to be a failure”,
(f) “the objective of practical plant breeding…could not be realized” neither by “macro-mutations” nor by “micro-mutations”,
(g) none of the modifying measures applied could help fulfilling “the ultimate hope of obtaining more of the ‘better’ mutants”,
- the overall result was that these strong anticipations concerning a revolution in plant breeding, accompanied by an intense euphoria especially among geneticists and agronomical scientists after the Second World War, ended up in a worldwide failure and breakdown of mutation breeding as an autonomous branch of breeding research in the 1980s at the latest in most Western countries.
The status of mutation breeding today is that of “an occasionally used supplement to traditional methods”, just “occasionally useful in enlarging the genetic base of a programme in a limited and highly specific fashion”.
To answer the question, what this “limited and highly specific fashion’ could essentially consist of, one should be aware of the fact, that mutations usually produce weaker or non-functional alleles of wild-type genes. Such mutagenic effects can be useful in plant breeding research when, for example, some of a plant’s secondary metabolites are disadvantageous for human consumption. If the gene functions necessary to produce such metabolites can be switched off by mutations without greater pleiotropic shortcomings for the plant as a whole, such a mutant could be interesting for further breeding.
In fact, Reinhold von Sengbusch (the father of Peter), perhaps Germany’s most successful plant breeder of the 20th century, summed up the essence of plant breeding by stating that − apart from polyploidy − the transformation from the wild to the cultivated plant is genetically characterized mainly by the fact that the features of the wild plants are dominant and those of the cultivated lines are recessive (83). Usually, recessiveness means losses of gene functions (for a documentation, see 43). As inactivations are the most common effect, which ‘normal’ mutations and/or mutations generated by transposons are exerting on genes (thus producing recessive alleles), the inference can be made that − as far as gene inactivations are important for breeding − mutations might “occasionally” still be relevant for some further progress (see also some reviews on transposons, where these points are further discussed: 1, 2, 10, 11, 38, 43, 54 - 56).
Yet, in our age of molecular genetics, tools are being developed that should increasingly allow directed mutagenesis to inactivate genes coding for undesirable second plant metabolites thus substituting conventional mutation breeding by accidental mutations probably entirely in the near future.
Nu dus nog meer bewijs dat mutaties niet in staat zijn om macro evolutie te bewerkstelligen.
Het “mutatie-fokken” is afgeschaft omdat men tegen een muur aanliep waar mutaties niet doorheen konden breken. Hoe verder men door muteert hoe vaker dezelfde mutaties gaan voorkomen, nieuwe kunnen niet meer voorkomen, omdat die steeds vaker het organisme zullen gaan vermoorden.
Dus waar komen nieuwe soorten vandaan?
De “wetenschap” weet het niet.
“Darwins boek heette ”Het ontstaan van de soorten“, ondankst het feit dat hij deze vraag eigenlijk niet echt behandeld heeft; meer dan hondervijftig jaar later is het nog steeds grotendeels een mysterie hoe soorten precies ontstaan.”
Chris Colby, The Talk Origens Archive, Introduction to evolutionary biology.