Species definitions are discussed, with particular reference to biological species concepts and the conservation viewpoint.

Techniques available for examining differences between populations and species of animals are reviewed.  The most recent techniques using nuclear or mitochondrial DNA have been effectively used in hybridisation studies of mammal species.

The occurrence of hybridisation between four pairs of British mammals are examined in detail.

Hybridisation between mountain hare and brown hare is very rare under natural conditions and hybrids in Britain would appear to present only novelty value.

The genetic mix of red and sika deer in Britain is complex.  Morphometric studies have suggested that hybridisation between these two species is widespread in some areas.  Current investigations using molecular and genetic techniques hope to further evaluate the real extent of hybridisation and introgression.  Current evidence suggests that introgression of sika into the red deer populations will increase rather than decrease.

Whether wildcats and domestic cats can be considered to be subspecies or separate species is unresolved.  Past European studies, mainly skull morphometrics, suggested that hybridisation between the two types was widespread.  The proportion of hybrids within a population has yet to be objectively measured.  Current research in Scotland is using DNA techniques, sampling living and historic cats across Scotland.

British ferrets are probably domesticated directly from European polecats.  The recent spread of polecats from Wales into the English Midlands may lead to the introgression of domestic genes into wild polecats.  However, it is not known to what extent feral ferrets survive in mainland Britain, and as yet hybridisation is not perceived as a substantial threat to the species.

Examples of hybridisation in British Birds and fish are summarised.  Many bird species hybridise and the reasons for this are discussed.  Conservation issues concerning ruddy ducks, crossbills and goshawks are briefly outlined.

In conclusion, the two mammals at greatest risk from hybridisation and subsequent loss of native type, are the wildcat and red deer.  The importance of maintaining native habitat for native species is also stressed.  It is recommended that studies initiated using modern molecular techniques be followed through, and that hypotheses based on these findings, about the ecological/behavioural reasons for hybridisation be investigated.  Even though the best methods available are used, it should be recognised that descriptions of a species for legal purposes will contain a degree of subjectivity.