Industrial melanism
Tutt is credited as the first scientist to explain the phenomenon of industrial melanism in terms of natural selection. Industrial Revolution started in England in the late 18th century. By the early 19th century, most of the industrial regions, particularly in western England, became heavily polluted due to extensive usage of coal. One of the most heavily polluted cities was ridden with black smokes, black rains and black snow that it was literally "jet-black" environment. Before the Industrial Revolution, insects were mostly light coloured. The most notable insect, the peppered moth (Biston betularia) was known to be white with patches of black spots. The first black specimen (of unknown origin) was collected before 1811, and kept in the University of Oxford. The first live specimen was caught by Robert Smith Edleston in Manchester, in 1848. Edleston continued to record the moth population and by 1864, he noted that the black had outnumbered the white moth population. By 1895, 98 per cent of peppered moth in Manchester was the black type, and the increasing black moth population was seen in different parts of England.
In 1890, Tutt analysed the records of insect studies since Edleston's report and found that the increasing number of black insects (peppered moth was not the only insect with increasing melanism) was related to the degree of pollution in industrial cities. At the time there were several and varied hypotheses on the insect melanism. Some scientists posited altitude differences as the cause, while some advocated humidity due to differential rainfall as the source. Nicholas Cooke in 1877 was the first to bring up the theory of natural selection and hypothesised that there was no natural selection but that the insects turned black due to exposure to black fumes in their larval stages. Buchanan White held the opposite view that natural selection could have a role and that black body was a form of protection.
Tutt took up White's view and explained that acquiring a black body was an adaptive advantage since the black individuals remained inconspicuous against the polluted walls and tree trunks where they rested. It would be hard for predators to notice them, whereas predators would have easily picked the light-coloured individuals. In 1891, he gave a full explanation in a book Melanism and Melanochroism in British Lepidoptera, writing:
If we add to these natural conditions [clean forest environment] the presence of chemical fumes, smokes, and other atmospheric impurities which are specially characteristic of large manufacturing districts, we can see how intensified the tendency to melanism would become in such districts. Only those insects that become positively black, would have in many places the slightest chance of escape, and "natural selection" by weeding out the paler specimens, would leave only the darker, and "hereditary tendency" would play its part in the intensification of the colour of the species.
Tutt went on to explain how industrial pollution could produced black environment and thus favoured insect melanism, taking specific example on the moths:
How does rain permanently darken the tree-trunks and other objects on which insects rest? It is well known to the most elementary student of chemistry that rainwater rarely occurs pure, and that in large towns the quantity of impurities is very great. The vast quantity of smoke, gases, fumes, etc., in manufacturing towns, brought down by rain, is scarcely credible, and it is from these impurities I consider the permanent darkening comes. When the water evaporates, the solid matter is left behind, and as a result, the impurities are left to darken the surfaces to which they have been carried by rainwater... [As an example] the intensity of Yorkshire and Lancashire melanism produced by humidity and smoke, is intensified by "natural selection" and "hereditary tendency." As example I would cite the species Amphidasys betularia [former name of Biston betularia, and included five other moths], which, occurring on trees, fences and similar objects would thus be affected.
Tutt did not explain how "protection" was key to the evolution of moths. In 1896, he gave the full scientific foundation for the adaptive benefits for black colouration specifically discussing the peppered moth. By then he knew that the main predators of moths were birds and that bird would easily picked and eat white moths in a polluted area, while they would remain quite safe in a clean lichen-covered trees. He wrote in British Moths :
In our woods in the south the trees are pale and the moth [white specimen] has a fair chance of escape, but put the Peppered Moth with its white ground colour on a black tree trunk, and what would happen? It would, as you say, be very inconspicuous, and would fall a prey to the first bird that spied it out. But some of these Peppered Moths have more black about them than others, and you can easily understand that the blacker they are the nearer they will be to the colour of the trunk of the tree, and the greater will become the difficulty of detecting them. So it really is; the paler ones the birds eat, the darker ones escape. But then if the parents are the darkest of their race, the children will tend to be like them, but inasmuch as the search by birds gets keener and keener, only the very blackest will be likely to escape. Year after year this has gone on, and the selection has been carried to such an extent by Nature that no real black and white Peppered Moths are found in these districts, but only the black kind.
Tutt's concept is known as "selective predation" or "differential predation" hypothesis. The hypothesis was experimentally demonstrated by Bernard Kettlewell during 1953–1956, known as the Kettlewell's experiment. From the experiment, peppered moth story became a "classic" and "textbook" example of natural selection in action. Kettlewell's experiment was further vidicated by Michael Majerus who performed the most elaborate zoological experiment during 2001–2007. Majerus's experiment was considered the ultimate proof of peppered moth evolution that "convincingly show that 'industrial melanism in the peppered moth is still one of the clearest and most easily understood examples of Darwinian evolution in action'."
