Till the end of time, debates and arguments would always ensue in science, among scientists and sometimes between scientists and laymen. While some debates are plausible and quite interesting, some are generally boring and not worthy of being debated at all. It is generally considered a time wasting if one should engage himself/herself in such debate. For example, I will not bother to debate someone that believes that the earth is flat or that vaccines are bad because there are tons of evidence against such beliefs.

^{By Rajatyadavjaora - CC BY-SA 4.0, Link}

One of the debates that always pique the interests of people in general is the one between those that believe that a divine being created the earth and those in it and those that believe that the earth existed as a result of a big bang and living organisms came into it as a result of evolution. A lot of scientists have been crucified in the past by the church simply because the theory of evolution is a total contrast to what the church believes. If the church is still as powerful as what we had in those days, some of us scientists would either have been neutralized or ex-communicated.

Having said all that, it is quite known that one of the theories that support evolution is the theory of natural selection. The role that natural selection plays in the population of living organisms cannot be overemphasized. The theory suggests that organisms or phenotypes (the physical appearance of living organisms) that successfully adapt to the environmental conditions are selected for, while those that could not adapt are selected against. In order words, only the organisms that are strong enough to survive the conditions of the environment survives and reproduce to form the next generation while those that are not strong enough die out of the population.

This theory, which was postulated by Darwin and Wallace represented a hypothesis that is largely based on historical evidence. As far as Darwin is concerned, the time span involved in the evolutionary change of a population was such that it could not be observed directly. If only it were possible for Darwin to directly observe evolution, perhaps the debate between evolution and creationism would have died down a while back.

However, recent changes that came with the industrial, technological and medical revolutions have produced such strong directional and disruptive pressures such that the results of dramatic changes in genotypic and phenotypic characteristics of populations can now be observed within few days.

A good example is found in the introduction of antibiotics in the 1940s which provided a strong selection pressure for strains of bacteria that have the genetic capability of being resistant to the effects of the antibiotics. Bacteria reproduce very rapidly and can produce several generations and millions of individuals within a period of twenty-four hours. Random mutation or genetic recombination may produce a resistant organism within the population which will thrive in the absence of competition from other bacteria that have been eliminated by the antibiotic. Hence, new antibiotics need to be developed in order to eliminate the resistant bacteria. That the bacteria would develop resistance to new antibiotics is not a subject for debate, rather, it is a matter of time.

Another example of selection pressure has been observed with an insecticide known as Dichlorodiphenyltrichloroethane (DDT). When newly introduced, the insecticide was quite effective against mosquitoes and body-lice which were causing (and still causing) malaria and typhus respectively.

Gradually, the insecticide became less effective till the insects became totally resistant to the chemical. Following the development of resistant strains, they spread throughout the population. Unfortunately, the Nobel price awarded to the scientist that discovered the effectiveness of the chemical against insects could not have been withdrawn, could it?

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Natural Selection and the phenomenon of Industrial Melanism

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A classic example of evolutionary change is provided by the response of moth species to the directional selection pressure produced by the atmospheric pollution that accompanied the era of industrial revolution. Within the last hundred years, it has been observed that darkened forms of about 80 species of moths appeared in varying frequencies throughout the United Kingdom; a scientific phenomenon nomenclature as industrial melanism.

^{By Ben Sale from UK - [1931] Peppered Moth (Biston betularia) - f.carbonaria, CC BY 2.0, Link}

Before the year 1848, all reported forms of the peppered moth appeared creamy-white with black dots and darkly shaded areas. In 1848, a black form of the moth was recorded in Manchester and by 1895, 98% of the peppered moth population in Manchester was black. The black or melanic form arose by a recurring random mutation but its phenotypic appearance had a strong selective advantage in industrial areas.

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Kettlewell's experiment

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The most plausible explanation for the strong selective advantage was given by H. B. D. Kettlewell. According to him, the moths fly by night and during the day, they rest on the trunk of trees. The normal form of the moth is extremely well camouflaged as its colouration merges with that of the lichens growing on the tree trunks. With the spread of the industrial revolution, sulphur dioxide pollution from the burning of coal killed off the lichens growing on trees in industrial areas, leading to the exposure of the tree barks which were further darkened by soot deposits.

Kettlewell conducted a study in the 1950s during which a known number of marked, light and dark forms of moth were released into two areas - the first was a polluted area near Birmingham where 90% of the moth's population were black and the second was an unpolluted area in Dorset where the dark form of the moth were rarely found. On recapturing the moths using a light trap, Kettlewell could only retrieve about 34% and 6% of the amount of marked dark form released in Birmingham and Dorset; and about 16% and 13% out of the marked light form released to Birmingham and Dorset respectively.

Using cine-film, Kettlewell was able to demonstrate that robins and thrushes feed on the moths, hence the low percentages of retrieval. This serves as a form of natural selection commonly referred to as selective predation. It acts as a selection pressure on the distribution of the dark (melanic) and light (non-melanic) forms of the moths.

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Explaining the outcomes

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Results from Kettlewell's study showed that the melanic form of the moth had a selective advantage in industrial areas over the non-melanic form, whereas the lighter form(non-melanic) had a selective advantage over the dark form (melanic) in non-polluted areas. Subsequent research has demonstrated that the colouration of the dark form is due to the presence of a dominant melanic allele.

Thank you all for reading.

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References

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britannica

the conversation

ask a biologist

wikipedia

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