As for medicine preventing evolution, that is simply a mistaken belief based on the general poor understanding of evolution common in society. Evolution is measured as the rate of genetic change in a population. Selection *slows* this process - it is a break on evolution - as it limits the rate of genetic change by selecting for or against certain genotypes. In the western world we've greatly reduced the impact of selection - ergo, we as a species are accumulating mutations faster, and thus are evolving faster than before.
Not my area but I know the above is an older hypothesis more recent evolutionary studies use a slightly different hypothesis
http://en.wikipedia.org/wiki/Effective_evolutionary_timeBoth are unproven but the newer hypothesis has had some success where the older hypothesis fails.
Gillman et al. (2010) extended their earlier study on plants [21] by determining whether the effect is also found within highly conserved DNA. They examined the 18S ribosomal gene in the same 45 pairs of plants. And indeed, the rate of evolution was 51% faster in the tropical than their temperate sister species. Furthermore, the substitution rate in 18S correlated positively with that in the more variable ITS. These result lend further very strong support to the hypothesis.[22] Wright et al. (2010) tested the hypothesis on 188 species of amphibians belonging to 18 families, using mitochondrial RNA genes 12S and 16S, and found substantially faster substitution rates for species living in warmer habitats at both lower latitudes and lower elevations.[23] Thus, the hypothesis has now been confirmed for several genes and for plants and animals.
