Per the college textbook Principles of Genetics "Compared to other species, the humans species is genetically rather uniform. At the nucleotide level, humans have about one-fourth the genetic variation of chimpanzees and about one-tenth that of Drosophila [a genus of fruit flies]. Furthermore, most of the genetic variation in the human species--perhaps 85 to 95 percent of it--is within rather than between populations." Likewise, the teaching guide "Human Genetic Variation" explains that "research results consistently demonstrate that about 85 percent of all human genetic variation exists within human populations, whereas about only 15 percent of variation exists between populations. ... This finding has caused some biologists to call the validity of race as a biological construct into serious question."