Madeira House Mouse

House mice on the Portuguese island of Madeira demonstrate gene flow in action. In 1978, biologists discovered that mice in the highlands carried a Robertsonian chromosomal fusion at 95% frequency, while mice in the lowlands just 15 kilometers away showed 0% frequency. The mutation was introduced from mainland populations via cargo ships and spread through gene flow - migration and interbreeding - not natural selection. Each generation, migrating mice carried alleles from source to destination populations, creating a moving wave of genetic change across the landscape.