Micromutations or macromutations

Micromutations - small genetic changes - are the bread-and-butter of neo-Darwinists and supporters of the synthetic theory, who see them as the basis of speciation. Believers in macromutations take a very different view of speciation. A macromutation is a drastic reorganization of the genotype that, in an extreme case, would produce a new species in a sin gle step - a saltation or jump. It thus introduces discontinuity into the evolutionary process and is a true punctuational event. If macromutations do occur and give rise to new species, then the macromutationists - and Darwin's good friend Thomas Henry Huxley was among their number - may justifiably gainsay Darwin's dictum and declare that Nature does progress by leaps. That is a very big if.

Historically, the notion of macromutations was implicit in the work of the botanist Charles Victor Naudin (1815-1899). In 1867, Naudin cited many examples of 'monstrosities' in the plant kingdom that are viable and durable, and concluded that species are transformed suddenly without transitional forms (cited in Hooykaas 1963, 121). The Dutch plant breeder, Hugo de Vries (1848-1935) embellished the idea. In 1886, he fancied that the evening primrose (Oenothera lamarckiana) he found growing in a field of potatoes had escaped from gardens and had mutated to form a new species. Further observations and laboratory experiments led him to conclude that macromutations do indeed occur and give rise to new species. Armed with these findings and assuming the correctness of Kelvin's estimate of 20 to 40 million years for the age of the Earth, he argued in his book Species and Varieties, their Origin by Mutation (1905) that there was not sufficient time for new species to emerge by natural selection. Instead, he proposed that speciation must occur in one generation by a process of macromutation. Others researchers affirmed de Vries's conclusion. The botanist John Christopher Willis (1922), for instance, averred that new species must evolve from an existing species in one, or at most a few, steps.

The chief advocate of speciation by macromutations was Richard Goldschmidt, a German-American geneticist. Goldschmidt (1940) rejected the efficacy of gene mutations as drivers of evolutionary change, proposing instead that chromosomal mutations were the cause of new species. He allowed that microevolution, caused by micromutations, could produce geographical races, but it could never produce a new species. To explain how new species arose, he envisaged 'systemic mutations' leading to completely new genetic systems in a single macroevolutionary step. These chromosomal rearrangements affect the early stages of embryonic development, and may lead to monstrosities, many of which will be nonviable, but some of which may be 'hopeful monsters' ready to fill a new environmental niche. Goldschmidt's ideas found favour among some palaeontologists. Schindewolf, in his Paläontologie, Entwicklungslehre und Genetik (1936), supported the notion of macroevo-lution by large steps, and in his Grundfragen der Paläontologie (1950a) offered evidence of it from the fossil record. He was brave enough, given the rather conservative synthetic theory of the time, to envisage the first bird breaking out of a mutated reptile's egg. According to Schindewolf, these leaps occur chiefly, but not exclusively, during periods of explosive origination of new types, or what he called 'typostrophes', a term deliberately chosen to be redolent of the word 'catastrophes'. Between the typostrophes are long periods of gradual evolution.

Some palaeobiologists toyed with the views of Goldschmidt and Schindewolf, but the majority would have no truck with them, preferring instead the gradualistic system of speciation. However, they have enjoyed favourable re-evaluation by some scientists. Guy L. Bush (1975), in a review of modes of speciation in animals, maintained that the concept of hopeful monsters might have some biological basis. He and his co-workers found a general correlation between the rate of speciation and the rate of chromosomal evolution within the Vertebrata (Bush et al. 1977). And it has been suggested that bolyerine snakes originated from the Boidae as hopeful, monstrous forms (not, it should be said, forms of extreme monstrosity, rather forms differing enough from the parent form to constitute a new family) (Frazzetta 1970). Olivier Rieppel (2001) showed the evolution of the highly derived adult anatomy of turtles was a macroevolutionary event triggered by changes in early embryonic development, in which early ontogenetic deviation caused patterns of morphological change incompatible with a model of gradual, stepwise transformation. Hopeful monsters may also be important in understanding the macroevolution of higher plants -there is evidence in the plant world for the sudden and punctuational appearance of bizarre somatic structures that happen to have had an adaptive value (van Steenis 1969).

These studies have somewhat softened Goldschmidt's original conception of a hopeful monster. The utterly monstrous forms envisaged by Goldschmidt would be most unlikely to find a mate and produce fertile offspring, so even if they arose, they could not perpetuate themselves. In other words, '. . . a single hopeful monster might survive and be well adapted, but it could never contribute to evolution unless another hopeful monster of the other sex appeared with which it could reproduce and contribute progeny to the next generation' (Kutschera and Niklas 2004, 261).

Richard Dawkins (1996, 87-96) drew informative analogies with 'Boeing 747' macromutations and 'Stretched DC8' macromutations. Fred Hoyle once said that the evolution of a complex structure such as the eye by natural selection is about as likely as a hurricane creating a Boeing 747 as it whirls through a junkyard. Dawkins feels that Goldschmidtian macromutations have the same level of improbability. On the other hand, a stretched DC8 is like a DC8 only longer, in the same way that a giraffe is, in effect, an okapi with a longer neck. Dawkins does not rule out a macromutation that would lead, for instance, to a sudden elongation of neck length (though he does not think this is what happened in giraffe evolution). Such punctuational change, he argued, would build on existing complexity, unlike the changes in a Boeing 747 macromutation, which would produce a new complexity. If Stretched DC8-type macromutations do occur, then they may result from chromosomal rearrangements, much of the pioneering work on which Michael J. D. White (1978, 1982) carried out. This raises the possibility that chromosomal transformation plays a key role in speciation (e.g. Volkenstein 1986; Sites and Moritz 1987). For this reason, and given the findings mentioned below, it may pay to treat warily any declaration that 'the concept of macromutations as a distinct class of genetic events constituting the main mechanism of speciation appears today implausible' (Hoffman 1989, 112).

A new line of research provides essentially micromutational mechanisms for creating new species that do differ substantially in important particulars from their parents. The focus of study is Hox genes, which play a regulatory role, guiding embryonic development by switching other genes on and off. Their action provides alternative mechanisms for evolutionary change that may lead to incremental changes in morphology. The summation of such changes over long periods would produce differences in Hox gene function between taxa comparable to the effects of gross homeotic mutations, without the need 'to invoke the selective advantage of hopeful monsters' (Akam 1998). Recent studies suggest that Hox protein mutations with large effects of phenotypes played an important role in invertebrate evolution (Ronshaugen et al. 2002). Six-legged insects diverged from a crustaceanlike, multiple-limbed arthropod ancestor some 400 million years ago. Experimental evidence showed that the transition was abrupt and the result of relatively simple changes in regularity genes of the homeotic (Hox) gene family, which encode DNA-building proteins that profoundly affect embryonic development. The researchers used laboratory fruit flies (Drosophila melanogaster) and a crustacean - the brine shrimp (Artemia franciscana). They modified the Hox gene Ultrabithorax (Ubx), which suppresses 100 per cent of the limb development in the thoracic region of fruit flies, but only 15 per cent in Artemia. Had the same mutation occurred naturally, it would have would have allowed the crustacean like ancestors of Artemia, with limbs on every segment, to lose their hind legs and diverge 400 million years ago into the six-legged insects (Figure 6.4). The implications of this work go far beyond insects because the Hox gene family is ancient, highly conserved, and found in arthropods (insects, crustaceans, chelicerates, myriapods), chordates (fishes, amphibians, reptiles, birds, and mammals), and has analogues in plant and yeast species. The Hox gene family seems decisive in understanding the evolution of developmental processes and patterns, perhaps allowing viable microevolutionary steps towards 'hopeful monsters' with macroevolutionary alterations in body shape (Ronshaugen et al. 2002; see also Carroll 2005).

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  • Mattalic
    What are micromutants and example?
    9 months ago

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