Answer:
Naturalists noted that in contaminated industrial areas, contrary to what happened in non-industrial areas, dark forms predominated over clear ones. Why do dark variants leave more descendants than light ones? As soon as we analyze the ecological niche of this species a little, we realize that there are many interactions of all kinds that can determine the biological efficacy of a moth. The ability to capture food, the avoidance of predators, the success in the mating of the males, the fecundity of the females, etc., can be some of the factors. Perhaps dark-colored females are more fertile than light ones. It seems unlikely, however, that there is a relationship between the degree of pigmentation of the body and the number of eggs that a female can leave. Perhaps dark males are more successful in mating than light males. It's possible. A significant fact is that the soot of the factories kills the gray-light lichens that inhabit the bark of the trees, where these moths spend much of their time, and blacken it. This evidence suggests that perhaps dark forms are less conspicuous in the environment in which they develop (the bark of trees) than clear ones, so that predators (birds) preferentially capture the latter. Following this reasoning, the British H. B. D. Kettlewell carried out a series of experiments that demonstrated that dark forms camouflage themselves better than predatory birds than clear ones in tree bark, being favored by selection. Therefore, the dark color is an adaptation because its carriers survive longer - they are less predatory - than those of light color. The adaptations, those properties of the organisms that so often fascinate us, are those characteristics that increase their frequency in the population due to their direct effect on survival or the number of descendants of the individuals who carry it. Adaptations are thus an intrinsic product of natural selection. What determines that a variant is an adaptation? The ecological context of each population. Thus, while dark forms are favored in contaminated areas, light forms, on the contrary, are mimetic in uncontaminated areas, being here those selected in favor. Adaptation is not an invariable, or absolute, but contingent property, a function of each ecological context. There is no a priori a phenotype, a clear or dark, better. We must always go to the ecological context of each species to know the cause of an adaptation. And this is another essential aspect of Darwinism, the contingency of its products, its dependence on the environmental contexts through which species pass through their evolutionary history and which are unpredictable.
