Yes, it has. It has been observed both in the lab and in nature. It would be impossible to list all confirmed examples of microevolution, but here are a few.
1. Bacteria have acquired resistance to viruses [Luria, S. and Delbruck, M. (1943) "Mutations of bacteria from virus sensitivity to virus resistance." Genetics 28: 491-511.].
2. Bacteria have developed resistance to antibiotics as well [Lederberg, J. and Lederberg, E. M. (1952) "Replica plating and indirect selection of bacterial mutants." Journal of Bacteriology 63: 399-406.].
3. Bacteria have also developed the ability to synthesize new amino acids and DNA bases [Futuyma, D. (1998) Evolutionary Biology. Third edition. Sunderland, MA, Sinauer Associates, p. 274].
4. Bacteria have also developed the ability to grow at temperatures that initially killed the population [Bennett, A. F., R. E. Lenski, et al. (1992) "Evolutionary adaptation to temperature. I. Fitness responses of Escherichia coli to changes in its thermal environment." Evolution 46: 16-30.].
5. In E. coli, we have observed by artificial selection the development of an entirely novel metabolic system including the ability to metabolize a new carbon source, the regulation of this ability by new regulatory genes, and the evolution of the ability to transport this new carbon source across the cell membrane [Hall, B. G. (1982) "Evolution on a petri dish: The evolved b- galactosidase system as a model for studying acquisitive evolution in the laboratory." Evolutionary Biology 15: 85-150.].
6. Protistans have developed the ability to use nylon and pentachlorophenol (which are both unnatural manmade chemicals) as their sole carbon sources [Okada, H., S. Negoro, et al. (1983) "Evolutionary adaptation of plasmid-encoded enzymes for degrading nylon oligomers." Nature 306: 203-206. ; Orser, C. S. and Lange, C. C. (1994) "Molecular analysis of pentachlorophenol degradation." Biodegradation 5: 277-288.].
7. Insects have been observed to become resistant to insecticides [Ffrench-Constant, R. H., N. Anthony, et al. (2000) "Cyclodiene insecticide resistance: from molecular to population genetics." Annual Review of Entymology 45: 449-466.].
8. Animals and plants have been observed to acquire disease resistance [Carpenter, M. A. and O'Brien, S. J. (1995) "Coadaptation and immunodeficiency virus: lessons from the Felidae." Current Opinion in Genetics and Development 5: 739-745.; Richter, T. E. and Ronald, P. C. (2000) "The evolution of disease resistance genes." Plant Molecular Biology 42: 195-204].
9. Crustaceans developed new defenses to predators [Hairston, N. G., Jr. (1990) "Fluctuating selection and response in a population of freshwater copepods." Evolution 44: 1796-1805.].
10. Amphibians develop tolerance to habitat acidification [Andren, C., M. Marden, et al. (1989) "Tolerance to low pH in a population of moor frogs, Rana arvalis from an acid and a neutral environment : a possible test case of rapid evolutionary response to acidification." Oikos 56: 215-223.].
11. Mammals acquire immunity to poisons [Bishop, J. A. (1981) "A neo-Darwinian approach to resistance: Examples from mammals." In Genetic Consequences of Man Made Change. J. A. Bishop and L. M. Cook. London, Academic Press.].
These kinds of experimental observations show us that species can adapt and change readily to variations in their environment, which is a testament to the design ingenuity of the Almighty. What ALL of these changes have in common is that they are changes WITHIN a given genetic code. So far, there has been no observation of changes OUTSIDE of a genetic code.