The international study showed that around 60 per cent of Eurasian grey wolf genomes carried small blocks of the DNA of domestic dogs, suggesting that wolves cross-bred with dogs in past generations.
The results suggest that wolf-dog hybridization has been geographically widespread in Europe and Asia and has been occurring for centuries. The phenomenon is seen less frequently in wild wolf populations of North America.
Researchers led the study from the University of Lincoln, UK, the Italian National Institute for Environmental Protection and Research and the University of California, Los Angeles.
Dr Malgorzata Pilot, from the School of Life Sciences at the University of Lincoln, said: "The fact that wild wolves can cross breed with dogs is well-documented, but little was previously known about how widespread this phenomenon has been and how it has affected the genetic composition of wild wolf populations.
"We found that while hybridization has not compromised the genetic distinctiveness of wolf populations, a large number of wild wolves in Eurasia carry a small proportion of gene variants derived from dogs, leading to the ambiguity of how we define genetically 'pure wolves'.
"Our research highlighted that some individual wolves which had been identified as 'pure wolves' according to their physical characteristics were actually shown to be of mixed ancestry. On the other hand, two Italian wolves with an unusual, black coat color did not show any genetic signatures of hybridization, except for carrying a dog derived variant of a gene linked to dark coloration. This suggests that the definition of genetically 'pure' wolves can be ambiguous and identifying admixed individuals can be difficult, implying that management strategies based on removal of suspected hybrids from wolf populations may be inefficient.