You might have done it before: you find cockroaches inside your apartment or house, and so you employ an insecticide in an attempt to kill what you might subjectively perceive to be repulsive creatures. Perhaps the insecticide kills or disables the insect. Perhaps not—maybe the pests in your residence seem unaffected. In the latter case, the roaches would appear to have genetic immunity to insecticides. While using whatever means of killing them that works might achieve a certain goal in the short-term, it reportedly is already having a significant evolutionary impact on roaches. This is not the kind that can necessarily be observed merely from seeing them, but the kind that can be observed as people continue their efforts to kill roach invaders using familiar means.
As people eliminate roaches that do not have genetic resistance to insecticide sprays, the ones that do have natural immunity become a higher and higher proportion of the overall roach population in the area(s); the latter are the ones that survive and pass on their genes to the next generation, after all, which further cements resistance to insecticides across the local members of the species, triggering an evolutionary shift with its accompanying ineffectiveness of the insecticides. While this might worry people who abhor roaches, it does give some people a very direct way to observe changes in a species that could occur, in this case, in a relatively short timeframe since the life expectancy of roaches in America is around 1-2 years. As opposed to a creature like humans, roach populations can evolve more significantly within decades.
No, this alone could not produce a "new species" in one generation or even several; the roaches would still be roaches. What it does exemplify is how the genetics of a current species could be greatly affected by "natural selection" very quickly, so that the subsequent generations more broadly have a given trait. Indeed, this is an example of what is sometimes called "micro-evolution", the phenomenon of a species undergoing minor changes, like bacteria developing antibiotic resistance, due to increasing allele frequency (and the increase of corresponding physical traits), that do not on their own result in a new species. In contrast, macro-evolution is the concept of a species gradually evolving into a new kind of organism that might have little physical resemblance to its ancestral predecessors. This is the kind people mean when they speak of organic molecules giving rise to unicellular microbes in the oceans which slowly transitioned into land-dwelling, multicellular life forms.
Oftentimes, people who reject macro-evolution do not object to and even outright support or simply do not care about micro-evolution. What they really tend to be after is opposing macro-evolution because they think it contradicts base theism (which is utterly untrue [1]) or the particular type of religious theism they hold to on the basis of assumptions—and just to emphasize something vital, it is the necessary truths of logic, not either God or any contingent, scientific phenomena, that are the core of reality [2]. However, people who hold to macro-evolution, like their opponents, very often have no idea what the difference between logical necessity/proof and empirical/hearsay evidence is, and so they just assume evolution of this sort must have happened because of appeals to authority or to a modern zeitgeist, though historical and scientific matters like this are absolutely unverifiable with human limitations [3]. They are just as stupid as the people who reject evolution as if it was logically impossible in itself (that is, as if it contradicts the intrinsically true logical axioms almost everyone disregards no matter their stances on evolution) or because they wrongly think it contradicts a particular theistic worldview they have assumed and will hold to out of familiarity or comfort either way.
Very few people are rationalistic skeptics of macro-evolution who know it is logically possible (it does not contradict the inherent truths of axioms), compatible with theism (which is true regardless [4]), and supported at the microscopic level by genetic evidence. In truth, as with all scientific matters, there is no way to prove evolution of a macro scale has or has not happened, though either option is indeed possible and there is a very particular kind of evidence for one of them. It is just that with macro-evolution, the only direct evidence would be related to genetics, which is unobservable macroscopically, without technological aid. All the same, at the micro-scale of shifting traits within a given population of a species, the roaches that invade our homes exemplify how genetics and reproduction can lead to changes in a species that, given enough time, could lead to a very different species indeed.
[3]. Though I have written extensively about this, here are a small handful of articles:
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