What prevents evolution?

[Borodog]

This is a question for NotReady and anyone else who would care to answer. In another thread NotReady claimed that "atheistic" evolution wasn't science (I'm not sure what "atheistic" has to do with it--is that like atheistic gravity or atheistic plate tectonics?).

So let me pose this question. If you have:

a) Self-replicators organisms whose phenotype (i.e. their internal and external structures, organs, behaviors, etc) depends on their genotype (a genetic code that contains the "recipe" for growing the organism), and

b) The fidelity of their genetic replication is good but not perfect (i.e. errors are made), and

c) Small difference in the genetic codes of two similar organisms can lead to small differences in phenotype (not that all small difference in genetic code must necessarily lead to small difference in phenotype; some small difference in genetic code lead to huge differences in phenotype, and some small, and even large, differences in genetic code do not lead to any phenotypic difference at all), and

d) The differential reproductive success of individuals replicators within the population depends to any extent on phenotype, then

Evolution is inevitable.

So, what prevents evolution from occuring? If if it does occur, how can you claim that it "isn't science" ?