International Painted Ladies

This morning, a painted lady settled on one of the many clover flowers on our lawn. These lovely butterflies arrive in Britain after a long migration from Morocco and, at the end of the year, migrate back again. However, since an individual lives less than a month, each butterfly arriving here is the result of several cycles of reproduction upon the way; and then they do the same on the way back.

It seems a very complicated and precarious way of life but, if they get it right, it's highly successful: thousands can appear at their most northerly point.

Imagine my surprise when some of the butterflies which came to sip nectar from the lilac flowers at the Buffalo Ranch in British Columbia were also painted ladies! They have a similar migratory system to the Eur-African ones, travelling from Mexico to the northern United States and Canada and back on an annual journey of up to 15,000 km (9,320 miles).

It happens that 2019 is a bumper year for the American version, prompted by good rainfall near the U.S.-Mexico border where the painted ladies feed on thistle and stinging nettle and lay their first batch of eggs. 

I simply do not understand how the same species exists on two sides of the Atlantic, following identical ways of life. Did they evolve and colonise both continents before they drifted apart as the Atlantic opened? And do they maintain their species by regular interbreeding across the Atlantic?