New species arise abruptly because of mutations in the genes that control the development of embryos. If such mutations occur in several individuals that then breed among themselves, they can lead to a new species. Jeffrey Schwartz, palaeontologist at the University of Pittsburgh, USA, has proposed this new theory of the origin of species. Charles Darwin's theory of natural selection is accepted by biologists as the explanation for how species change and adapt. Some scientists have disputed the argument that the slow accumulation of small changes leads to major structural differences between species, such as whether they have a backbone or not ( New Scientist , Vol 161, No 2173).