they proposed that when Asian cockroaches were transported along the trade routes from south and southwest Asia into Europe, this species could not establish persistent populations. Without local success, they were hardly noted by anybody. Only a few survivors that found ways to live within structures could make it through the cold European winters, and over time these survivors evolved into the German cockroach. The species was therefore truly European.
went to work testing the hypothesis by sequencing genetic data from German cockroaches in many parts of the world, as well as Asian cockroaches and other relatives. Last month they published the results of this work. The new data showed something a little different from the urban Europe hypothesis.
B. germanica didn't originate in Germany or elsewhere in Europe as a new species and later spread throughout other parts of the world. Island southeast Asia, Ethiopia, and China all had diversity of B. germanica that was as old as in Europe. Every one of these populations came from common ancestors that shared a history of drift and adaptation around 2100 years long. Before this, those ancestors diverged from the Asian cockroach, B. asahinai.
The German cockroach deserves to be widely known as an example of rapid evolution of a new species. Not only does it inhabit environments where few insects survive, its environments didn't even exist before a few thousand years ago.
the selection pressure which produced the starting population - these were the ones who survived long travels along the trade routes while hiding in the human cargo and after that survived in the new environment (new climate, vegetation, etc) of the destination.