Evolution: Biology

Basic Ideas: Natural Selection

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In the previous article mutations have been introduced as a source of evolutionary innovation: In this connection it has also been mentioned, that detrimental mutations are wiped out. The removal of disadvantageous changes is called selection. Detrimental mutations are removed by the carrier having less or no offspring at all. That is selection means varying success of reproduction.

In this article, observed examples for the effect of selection are introduced and it is explained, which conclusions can be drawn from this.

The theory of selection

Condition: polyvalent gene pool

The best-known example for selection: the peppered moth

Selection and evolution of more complex forms

Selection: often only claimed

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The theory of selection

The basic idea of the theory of selection goes back to Charles Darwin. Selection takes place, because normally a surplus of offspring is produced, from which in the long term averagely two individuals per set of parents survive. It will normally be the offspring adapted best to the particular environmental factors. Their features survive; the others get lost and are sorted out: selection.

The process of selection has been observed in numerous studies. It has for example been shown, that the beak of finches on the Galapagos Islands becomes enlarged after only a few dry years – in adaptation to the altered food supply.

A second example: Iguanas of the genus Anolis have been marooned on Caribbean islands where they did not exist before. After a few years, changes in the length of the hind legs occurred depending on the vegetation on which the saurian move. The legs were shorter wherever thin branches predominated.

Condition: polyvalent gene pool

As a prerequisite for these kinds of changes a polyvalent gene pool is required, that is the initial population from which the selection takes place has to show not only a certain genetic variety (polymorphism), but also the capacity of variation, that - possibly initiated by environmental stimuli – can cause the observed variation within a few generations.

These processes can be interpreted as follows: Whenever environmental factors change, out of the spectrum of polyvalence other varieties than before are favored. That means that these varieties have a better chance of surviving and can produce more offspring. With this, nothing new is produced, but out of a given spectrum, the appropriate is selected (see pic. 57; pic. 59). Therefore selection cannot be counted as an evidence for upward development, selection is not creative in any way – on the opposite: Selection leads to the situation, that out of the polyvalent gene pool only a part remains for the next generation. “Selection” is not “creation”.

The best-known example for selection: the peppered moth

The spreading of dark forms of the peppered moth (pic. 60) to the disadvantage of light forms is considered a classic example of observed evolution. Between 1850 and 1920, the dark forms of the moth had increased considerably and replaced the light form proportionally. This trend reversed after 1950. Air pollution has been taken as the cause for this shift, because it leads to the disappearing of the light lichen on the bark of trees. This again led to the fact, that resting light forms of the peppered moth were not camouflaged enough anymore and therefore picked by birds more often: selection in action. Even the view was expressed that a “great piece of evolution” was going on, that one was able to “watch the evolution”.

This scenario has been criticized for some time already. The most important objection is, that after detailed field studies the peppered moth nearly never settles on the stem of trees (pic. 60 is posed). Until today, the resting place of the peppered moth has remained unknown. Secondly, the number of light forms reincreased, before the lichen had repopulated the barks. Furthermore, the moths don´t tend to choose surfaces matching their color.

If there should be a connection between the environmental pollution and the frequency of the dark moths, it is far more complicated than expected and up to now not understood. Selection probably took place, but the conditions in detail are quite unclear.

Either way shows a typical case of microevolution. It is not documented, where the dark moths come from. What has been observed, is a shift in the frequency of the different forms of moths – but not the development of new varieties. Even if the dark form did evolve by mutation, it still would be microevolution, because the light forms as well carry the dark-brown pigment melanin, which is responsible for the coloring of the dark form (they have brown spots or bands), so that the dark form could develop without any new formation of structures.

Selection and evolution of more complex forms

Selection enables an adaptation of living beings to variable environmental factors. As to the classical Neo-Darwinism, selection (in combination with mutation) does not only lead to an optimized adaptation, but in the end to evolution of new structures, as well. After all there isn´t any recognizable connection between adaptational processes and evolution of novelties.  If a better adaptation to a changed environment is achieved by selection, it is always connected to a mere variation of an already existing organ or feature. A development of new organs cannot be explained by selection, because selection can only work, where there are already existing functions that can be optimized. And vice versa: Evolution in the sense of developing organs that are more complex does not mean increasing adaptation. Adaptation takes place within a specific organizational level that is not overstepped by selection. “An ameba, a worm, an insect or a non-placental mammal is as adapted as a placental mammal; if they weren´t, they would have died out long ago.” (L. v. Bertalanffy)

A worm, exposed to changing environmental factors, may become a “better” (better adapted) worm, but not an insect. Signs of a change towards an insect no longer mean adaptation as a worm; these attempts would therefore be wiped out by selection (pic. 61). This argument goes as well for hypothetical common ancestors of worms and insects.

Selection: often only claimed

The examples for selection given above (Finches, Iguanas, peppered moth) are based on observations in the field. They obviously only give evidence for microevolution. It is often said, that more complex structures developed by selection, as well as the slight changes of beak size, leg length or wing coloring in the given examples. Often the hypothetical acquisition of new organs is described as “adaptation”, as well. That way it is suggested, that selection (in combination with mutation) can be made responsible for the acquisition of organs – and that the emerging of new structures is basically understood.

Wings for instance would then be an adaptation to flight or a long, sticky, protrudable tongue to nutrition with small insects etc. This way of expressing oneself is common but nevertheless factually incorrect. The term adaptation can only be used in the sense of a fine tuning: An already existing wing might be adaptive to the different requirements of flight. But a wing itself is as much an adaptation to flight as a washing machine is to the washing of dirty clothes.

Here the adequate use of the term is important: Wings are not an adaptation, but equipment for a certain purpose, constructions, by which certain tasks can be fulfilled. Actually, the development of new organs by selection (and other evolutionary factors) never has been observed. Selection is a factor of variation, not of creation.

Translator: Sarah Aziz, 30.01.2009

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Author: Reinhard Junker

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