^^^ CB:OK The "struggle" between individuals can give rise to new traits. However , most new traits don't give rise to a new species. The only new trait that gives rise to a new species is one tht makes the new trait bearer ( and offspring) sexually incompatible with the old trait bearers and their offspring. This is based the definition of species developed in the 1940's , I think. So , that might explain some of Darwin's concepts being superseded in the New Synthesis.
Darwin got his idea based on selection by humans of their domesticated animals. So, "fitness" of one domestic animal over another created breeds of the same species. But I see what you mean about Darwin operating at this level of individuals.