Urban infrastructure as an evolutionary force
The common brown rat, Rattus norvegicus, is one of the most successful urban mammals on the planet. In the city of Colombo, these rats are participating in a rapid, city-scale evolutionary experiment. Genetic studies reveal that rat populations separated by just a few kilometers within the city are surprisingly distinct from one another. This field of study, known as urban genetics, examines how city features influence the genetic makeup of wildlife.
Major roads, canals, and dense commercial districts in Colombo are formidable barriers to rat movement. While a rat may live its entire life within a few city blocks, these large structures prevent it from breeding with rats on the other side. This creates isolated genetic pockets. The effect is so pronounced that the urban grid effectively is a natural barrier, such as a river or mountain range, stopping the flow of genes between populations. Similar phenomena appear in other cities globally. In New Orleans, the Inner Harbor Navigation Canal separates genetically distinct rat populations in the Lower Ninth Ward from the rest of the city. In Manhattan, the dense commercial zone of Midtown prevents rats from Uptown and Downtown from mixing.