This book is a collection of the works submitted at the twelfth meeting of the Italian evolutionary biology group, held in Florence in February 2004. The evolutionary theories are treated in a balanced manner, from various points of view (those of the historian, the zoologist, the palaeontologist, the anthropologist and the molecular biologist) in the light of various recent discoveries of both an experimental and theoretical nature. The debate and the diversity of the approaches demonstrate the validity and topicality of the study of biological evolution.