[atmanthedog]

That is true at the entire-species level assuming a uniform selection pressure against the entire species. However, evolution also occurs when a species adapts specific traits to fill an environment niche. Traits can still be selected for when the trait involved allows the organism to compete for a different set of resources. Humans also do not reproduce randomly. Currently, it is widely accepted in the Western world that people get to choose who they have children with, and that choice can select for similar traits. Marriage customs in other cultures are different, but still can have an effect on trait selection. For example, the royal families of Europe positively selected for some fairly harmful traits (like hemophilia), because other traits (already belonging to the family) were also being positively selected for filling the niche of royalty.

Consider something like a "Highlander" allele, that has a chance to randomly appear in the general population, granting them the same stipulations as in the movie. Given that there can be only one, this particular allele has terrible reproductive success in the sense that practically no offspring will ever survive. However, the gene will always tend to exist in the population. Now, if there is some event that occurs that increases the maximum of number of Highlanders, this event will put positive selection pressure on the allele, even though the general population can reproduce much faster.