Antibiotic resistant bacteria. The idea that bacteria have evolved to be resistant to antibiotics is said to be proof of the grand theory of evolution. However, what has really happened it that bacteria do not develop new features that make them resistant. Within a population of bacteria, there are mutants (degenerates) that have lost the functionality that antibiotics attack. If the antibiotic succeeds in killing all of the bacteria that are not resistant, then the resistant strains can reproduce unchecked.
Bacteria have been found in ancient graves or archaeological digs (prior to the invention of modern antibiotics) and there are antibiotic-resistant bacteria. This shows that these bacteria have not evolved new traits since the introduction of modern medicine. They were already in bacterial populations; they are only able to thrive when the non-resistant bacteria are killed off for them.