Examples of microevolution
Scientists have actually observed many cases of natural selection in the wild:The size of the sparrow
Science has documented many examples of the evolution of resistance of pests to pesticides, weeds to herbicides, and pathogens to medicines all of which are cases of microevolution by natural selection. In the case of antibiotic resistance, for example, a bacterial strain's huge population size and short generation time mean that natural selection acts quickly. In each bacterial generation, new mutations and gene combinations are generated. If any of these confer resistance to a drug to which the bacteria are exposed, natural selection will favor those gene versions. Over the course of many bacterial generations (a small fraction of a single human lifetime), the bacteria adapt to our defenses, evolving right out from under our attempts to rid ourselves of them.
This general scenario has played out many, many times. Just a few examples include:
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