"Swans that avoid cities have a genetic difference from the ones that are human-tolerant," she says. And she points to the difference between UK-migrating blackcaps and birds that still migrate to Iberia as being "very clearly genetic". "The young carry this difference," she says. Changes like this are the first steps to the emergence of a new species. "The London Underground mosquitoes are an example where we might be forming a new niche and creating new opportunities for speciation," Otto adds.