However, work in the last 20 years has shown some other causes. Analysing the DNA sequence of living things has shown that there is often some hybridisation between related species. That means genes have been transferred by these crosses. In turn, that means reproductive isolation is not the only definition of a species, and speciation does not always need allopatry (species to be reproductively separated). The sections below illustrate the idea that physical separation was of prime importance in the formation of new species.