selective pressure on cells - (Mar/05/2014 )

I recently read an article stating something such as this:

"Gene X protects the cells against apoptosis, otherwise cells that lack gene X have a higher probability of undergoing apoptosis. This places less selective pressure on the survival of the cells that lack gene X to contain oncogenic variants"

I don't understand this term selective pressure used in this sentence. Does it mean that the cells lacking gene X are less likely to proliferate and thus less likely to expand any of their oncogenic mutations into the evolution? Please provide clarity. thanks in advance.

Induction of apoptosis is one of the major mechanisms whereby proliferation is controlled, most of the major oncogenes or tumour associated genes have roles in apoptosis induction somewhere in their pathways.  So, if a cell lacking gene X has a higher rate of apoptosis than cells that contain X, the cells that lack X are less likely to survive any change, and thereby less likely to contain an oncogenic change.

thanks bob. now i understand