Evolution: Biology

Basic Ideas: Breeding

Content

This article introduces methods, aims and some results of breeding. Furthermore conclusions are drawn concerning the efficiency of evolutionary mechanisms.

Classical breeding methods

Recombination

Aims of breeding

Results

Breeding – a model for natural processes?

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In the last two articles Mutation and Selection have been introduced as factors of evolution. Both mutation and selection evidently take place in nature as well as they can be reproduced experimentally. In classical breeding research naturally occurring processes of mutation, recombination and selection are copied and accelerated by calculated intervention. This works by increasing the mutation rate, by selective crossing and artificial selection (pic 62).

Processes that mostly happen very slowly in nature can be accelerated enormously by breeders´ activities. Therefore studies of breeding are of great interest for evolutionary research. Changes can be achieved in much shorter a time, so that the potentials and limits of the species own variability can be estimated more precisely.

Classical breeding methods

In the article about Mutation it has already been mentioned, that mutations do not only occur spontaneously in the wild but can also be caused artificially. This way it is tried to raise the range of variation (mutation breeding). This however has only been conditionally successful (see below). Also selection can take place artificially. Natural selection (caused by environmental factors) is replaced by man as a breeder who selects from the multitude of forms within one species those varieties that are useful for his purpose. That is he sorts out the part of the existing diversity that does not serve his breeding aim – this exactly is selection. Furthermore the breeder crosses selected individuals with the desired qualities to optimize these very qualities. It sometimes is possible to achieve a promising combination of features by a clever crossing of forms with different favorable features: combinational breeding.

Recombination

The combination of genes is also called recombination. Recombination happens within every sexual reproduction. Different mechanisms of the formation of the gametes and the fertilization lead to a new combination of the genome. This process is comparable to the shuffling of playing cards. The cards stay the same all in all, but they are combinable in various ways. The breeder can try to support certain combinations by calculated crossing.

Aims of breeding

Aims of breeding are for example an increase in grain size or enlargement of fruit in general, loss of the spindle fragility of grain, enhancement of contents (e.g. sugar in sugar beets), simultaneousness of fruit ripeness, improvement of taste or storability and so on (see pic. 63).

Results

Many of the aims and results of breeding make sense from an economical point of view, but are unsuitable in the wild, for instance the loss of spreading organs (pic. 63), the simultaneousness of fruit ripeness, the loss of poisonous substances or filled blossoms (lower fertility; see pic 64). These examples show losses that can be of advantage under certain circumstances. But in these as in other cases no new formation of structures occurs. The difference between wild and cultivated forms usually is of quantitative nature, e.g. the development of bigger fruits, a larger amount of grain, more stalks per plant, a larger amount of pigments (see pic.64) etc. In other cases a feature may experience some variation, so that in a certain sense a new quality occurs, but even this new quality requires an already existing complex initial situation (for example the acquisition of poison resistance; see article about Mutation). Some variations again are based on the fact that in some cases the development only proceeds until the juvenile state. The skull of the Pekinese for instance shows a juvenile form (pic.65).

Mutation research has shown, that the range of mutation cannot be extended infinitely but is exhausted after a certain time so that the same mutations occur again and again (rule of the recurrent variation; see article about Mutation.  Besides that the artificially caused mutations often are identical with the naturally (spontaneously) occurring ones. This is why mutation research is hardly of significance any more. Obviously the range of mutation cannot be extended infinitely.

Crossings, often interspecific crossings, can produce new combinations of features and with these new varieties. Here existing genetic capacity is used and combined – there is no generation of new capacity.

Selection, too, does not lead to new varieties over and over again, but to the limits of variability. The number of eggs laid by hens, the milk production of cows or the concentration of sugar in sugar beets cannot be increased infinitely. And obviously these kinds of increases do not lead to new structures in the sense of macroevolution. Extreme varieties and one-sided specialization can be shaped by artificial selection, but when the capacity of selection (the initially existing genetic variety) is exhausted, no further changes are possible this way. Continued artificial selection leads to a reduction of variability and to specializations. At last specializations and one-sided adaption lead to evolutionary dead ends.

Breeding – a model for natural processes?

Breeding research only partly provides a model for evolution and is certainly no example for macroevolutionary processes. The breeder mostly selects after totally different criteria than nature; usually breeding aims are counterproductive for the general survivability and competitiveness. This is why breeding forms often are less vital than the wild form, in a way that they not seldom only survive under the breeder´s hand.

Still the mechanisms of variation are quite the same as the processes happening in the wild. In that sense breeding can be looked upon as a model for in the wild occurring changes very well. Nevertheless breeding processes cannot be taken as a model for macroevolution, because the generation of new structures has never been achieved by breeding.

Translator: Sarah Aziz, 13.05.2008

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Author: Reinhard Junker

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