Francesc Mestres, professor of genetics at the University of Barcelona
The simulation you have just use and that has allowed you to see the effect of natural selection, is based on a real case. The XIXth century British naturalists picked up specimens of the lepidopteron Biston Betularia for their entomological collections. This way it is possible to know exactly how in terms of abundance different forms of this specie has changed, the clear form (whitish mottling) and the dark form (melanic).
The clear form dominates in natural environments....
In the XIXth century England the environment was not contaminated, so when the cleared butterfly stopped on the surface of the trees, because the bark of those was also clear and was covered with lichens (also gray), butterflies were very well camouflaged. In contrast, dark form highlighted much respect the colour of the surface of the trees. In this sense, clear form was favoured by natural selection because his perfect camouflage. So natural predators of the butterfly Biston Betularia (birds such as robins, Erithacus rubecula) easily detected the dark form and devoured it preferentially.
.... and the dark form predominates when there is contamination
This environmental situation changed dramatically in the mid-nineteenth century with the industrialization through steams that used coal as fuel. This generated a much polluted environment, so lichens from the bark of the trees died, and a soot layer was deposited on its surface. Therefore, the bark of trees became black. For this reason the melanic form (black) of Betularia Biston become very well camouflaged in this new environment, while clear form become very apparent. For this reason birds started predating the clear form, and that's why dark form becomes the most abundant because it was being favoured by natural selection. This situation continued until late 60s of the XXth century, when anti-pollution measures promoted by the British Government, made trees surfaces gradually whitened, so selection again favoured the clear form (well camouflaged) against the dark one.
The selection change when environment changes
It is importantly to highlight that this example allows observing natural selection in action (the principal mechanism of evolutionary change, as Darwin suggested). Also note that selection change when environment changes. First, in a non-polluted environment clear forms were favoured by selection because they were well adapted (camouflage). Then, as conditions change towards a much polluted environment, selection benefited dark shapes as they were better adapted. The new environmental change (pollution reduction) reversed again environmental conditions, so that clear forms were one more time better adapted (was not detected again by predators) and was then favoured by natural selection.
We must also consider that when a type of individuals is favoured by natural selection, automatically another group is being non-selected. Always that selection is favouring a particular colour (light or dark), the other is at a disadvantage.
A simplified example
Another important detail is that the present example is useful to highlight evolutionary mechanisms, but as all models are, it is a simplification. In this sense, in the simulation we just considered the individuals variability (if they camouflage they have less probabilities to suffer predation). Variability is a fundamental factor on which natural selection can act. But we must keep in mind that it is not the only factor, and that selection can act over much more levels: fecundity, fertility and mate choice, program production of gametes, and so on. Consider the following case: if we have a light coloured Biston Betularia butterfly, over a non contaminated bark trees will camouflage well, but if it is sterile their genes will no pass to next generation and therefore this specimen will not be favoured by natural selection.
Basically, the concept of natural selection is just a differential reproduction: an individual or groups of individuals better adapted than the others in a population will leave to next generation a higher number of descendents. What is being favoured by natural selection is leaving more descendents than others. This situation can be reached by different ways, for example, living longer or having a higher fertility.
Selection examples
Natural selection can be not just visualised in nature, its effect can be also quantified. In this sense, the effect of natural selection has been estimated in many species and in many different situations. The case of Biston Betularia has been extensively studied in a quantitative level and the effect of selection has been possible to measure (which is technically known as the biologic efficacy and the selection coefficient). Among the classic examples of natural selection, we have the progressive resistance of the insects against insecticides or bacteria against antibiotics. On men, the natural selection has been able to analyze with diseases such as falcemic anemia. Finally we must not forget also that artificial selection carried out by humans for centuries on domesticated plants and animals, it is just an example of how selection acts at a level of the nature.
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