urban primroses

A characteristic spring sight in towns and cities, a group of normal-looking primroses with a sprinkle of magenta-coloured plants.

Does the urban primrose have its own taxonomic status, distinct from the country primrose, Primula vulgaris?  

Or should the population be regarded as a hybrid swarm, like what happens when Red Campion meets White, or Herb Bennet meets Water Avens?

Presumably the magenta is due to introgression from garden Polyanthus species.

But what intrigues me is the uniformity of these populations. E.g. there always seems to be the same proportion of magenta plants. For example, you never see a group which is 95% magenta and 5% yellow.

What controls this proportion and makes this a stable population type?