Evolution: Biology
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Evolution: Biology | |
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Basic Ideas: Mutation | |
InhaltThis article explains which kinds of mutation are known, furthermore some examples are introduced and the validity concerning the development of new structures is discussed.
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Introduction – terms in overview |
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Important terms in causal evolution research are gene pool, recombination, selection and genetic drift (pic 53). The gene pool – a virtual quantity – includes all genetic varieties (allele) (=Zustandsformen eines Gens) within a single species. These varieties can be mixed in the offspring by crossbreeding: recombination. Mutation means an erratic, spontaneously occurring or artificially caused change of the genotype that may also lead to changes in the phenotype. Selection means the picking-out of those individuals that are adapted best to the environment and stands for varying success of the individual’s reproductive success. An accidental change of the gene pool, not caused by selection, is called genetic drift. |
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Mutation research |
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Mutations are the only known source of evolutional changes. In the end, all other factors are only effective under the condition of mutative possibilities of change. This is why mutation research takes on a key role in causal evolution research (=Ursachenforschung). In the field Mutations occur spontaneously, that is without recognizable reason (as an example: pic. 55), but they can also be caused artificially under laboratory conditions, such as by treatment with chemicals, radiation or extreme temperature. However, it is not predictable in which genes (=Erbfaktoren, Abschnitte auf der DNS) the mutations appear and in which “direction” the change will go. One must distinguish between point mutations, where only a single unit of the DNA is changed, and chromosome mutations (pic. 56), where whole parts of the DNA get lost, are turned, doubled or fitted in elsewhere. Even whole chromosomes or complete chromosome sets can be doubled by mutation. |
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Decade-long experiments with the fruit fly Drosophila brought forth hundreds of differently changed flies – but never (not even after a long period of time) a complex new structure was produced even remotely. The artificially caused mutation lead for instance to an altered eye- or body color, altered bristles, diversely formed eyes or deformed wings (pic. 57). Sometimes whole parts of the body appear at wrong places, e.g. legs instead of antennae or wings instead of halteres (pic. 57, down right). The mutations seen here are within the limits of microevolution, because only a variation or recombination of already existing components took place – not a development of new ones. The mutations did not lead to new species and the fruit flies can still crossbreed. So mutations mostly lead to defects, which is also the case with plant mutations. Concerning the intense mutation research on the thale cress Arabidopsis, Meinke et al. (1998) ascertained, that „several thousand mutants of Arabidopsis defective in almost every aspect of plant growth and development have been identified over the past 20 years.“ on the other hand there are mutations that stay without phenotypic consequence (neutral mutation). |
Advantageous changes? |
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Now the argument goes that disadvantageous mutations do not matter in the process of evolution, because they are eliminated again by selection. The decisive aspect is said to be, that mutations were advantageous from time to time. These mutations would are assumed to have prevailed within the populations and the process could be continued by further positive mutations. This argumentation is basically correct. Still the occurring of advantageous mutations is not equivalent to the development of new genetic information, but normally also goes back to defects, which turn out to be positive under certain environmental factors. An example shall help to illustrate this: Some of the insects that live on islands exposed to strong winds have vestigial wings or even none at all (pic. 58). For the indigenous insects this is an advantageous change, because a violent storm can blow them far out to the open sea while flying. If the wind does not turn, the insects mostly will not have the energy to fly back by their own and will die. So in this case it is better if the insects are unable to fly from the beginning. Also, the loss of motility is bearable, because there normally are fewer natural enemies on the islands than onshore. So all in all, the loss of wings is advantageous. Alone, it will not do any good to the understanding of evolution, because the advantage is based on a loss, that means the reduction of an organ – no upward development. Moreover, the loss of flight is positive only under specific circumstances; it is disadvantageous elsewise and such mutants are eliminated by selection immediately. The following conditions are known from biochemistry. If by means of mutation an enzyme can metabolize a new substance (new substrate affinity), this normally is at the expense of the (former) specificity of the substrate affinity: The exact coordination between enzyme and substrate gets lost. Newly acquired toxitolerance or antibiotic resistances can be based on metabolic defects that prevent the toxin from being infiltrated into essential metabolic processes. We can state that mutations that are only “positive” under certain circumstances do not explain an evolution of new structures, because they imply losses. So it is not the question whether or not advantageous mutations exist, but rather if new genetic material and new structures develop. |
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The “recurrent variation” |
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A certain phenomenon has been observed throughout decades and various organisms: After some time the same mutations show up again and again. Quite often, already existing mutations just reoccur. Plant breeders found out, that artificially caused mutations show the same features and characteristics as the spontaneous mutations of wild plants. The number of new types of mutants decreases with further mutation experiments. The geneticist W.-E. Lönnig summarized this phenomenon as the “rule of recurrent variation”. Until now, over 5000 hereditary deviations are known from the human genome. Examples of recurrent variations of grain are thick ears, early ripeness, the missing of wax or pigments, shortened awns or mildew resistance. As a reason for the rule of recurrent variation of mutations, Lönnig suggests, that there is only a limited number of hereditary factors, which can lose their function partially or totally without being lethal. The recurrent variation of mutations indicates the outlined - rich but limited – changeability of living beings. The variety of mutations stays within the borders of |
Mutations in developmental genes |
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The understanding of homeobox genes has been of some importance recently. Homeobox genes stand quite at the beginning of the ontogenetic (=concerning the individual development) development cascade of whole organs. Therefore mutations in homeobox genes can have an enormous effect, for instance to the extent that whole organs are expressed in other parts of the body (see pic. 57, bottom right). But even homeobox genes are not capable of explaining the development of life´s structures, because they are only effective within already existing subsequent structures and development cascades. |
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Result |
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Generally, mutation research has shown until now, that no new structures develop by mutation. They rather show effect within already existing complex structures or metabolic processes. The spectrum of mutations stays within the scope of microevolution. |
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Introduction – terms in overview