Sense of selfish?

Does "sense of selfish" mean "sense about selfish"?
The question here is why not has used "sense of selfishness". The word selfish is an adjective and "of selfish" sounds cool yet seems not perfect in grammar. "of selfishness" is grammatically perfect, yet sounds a bit redundant.
What do you say?

Background info:

The logic of Darwinism concludes that the unit in the hierarchy
of life which survives and passes through the filter of natural
selection will tend to be selfish. The units that survive in the world
will be the ones that succeeded in surviving at the expense of their
rivals at their own level in the hierarchy. That, precisely, is what
selfish means in this context. The question is, what is the level of
the action? The whole idea of the selfish gene, with the stress
properly applied to the last word, is that the unit of natural
selection (i.e. the unit of self-interest) is not the selfish organism,
nor the selfish group or selfish species or selfish ecosystem, but the
selfish gene. It is the gene that, in the form of information, either
survives for many generations or does not. Unlike the gene (and
arguably the meme), the organism, the group and the species are
not the right kind of entity to serve as a unit in this sense, because
they do not make exact copies of themselves, and do not compete
in a pool of such self-replicating entities. That is precisely what
genes do, and that is the - essentially logical - justification for
singling the gene out as the unit of 'selfishness' in the special
Darwinian sense of selfish.