Any heritable trait that is of advantage will spread through a population. To Darwin, even though the genetics was not understood, it was clear that this could occur, as demonstrated by artificial selection in many contexts, especially breeding of fancy pigeons, fish, cats, dogs, and farm animals. At any one time though, the vast majority of selection going on is merely eliminating deleterious mutants. Indeed this is the overwhelming force in evolution, witness the almost uniform genetic code , the sequence stability of rRNAs, histones, and most other proteins. On a more phenotypic level this is termed stabilizing selection .
Directional selection is the spread of an advantageous allele through a population.
It is hard for us to watch it happening because the time scales are too long. However, we ourselves have imposed lots of novel selection pressures on organisms. Famous examples are Biston bettularia moths in Britain and resistance to insecticides in insects and antibiotics in bacteria. The volume of examples is overwhelming, although most are inferences about process, not actual measurements of its progress. A special kind of selection is sexual selection , and provides some of the most conspicuous examples because it causes divergence within a species.