Hybridisation is the interbreeding of individuals from genetically distinct populations,
regardless of the taxonomic status of the populations.

Hybridisation & Conservation

Natural hybridisation can create genetic diversity, e.g. plant species of hybrid origin.
But hybridisation resulting from human disturbances (particularly introduced species, but
also habitat modification) can compromise the genetic integrity of existing species to the
point of causing extinctions. For example, the New Zealand grey duck (Anas superciliosa)
hybridises with introduced mallards (Anas platyrynchos).

• Mallards are common but greys are rare;
• So NZ greys tend to mate with mallards, simply because more readily available;
• So pure NZ greys are disappearing.
• The domestic cat (Felis catus) is swamping the European wild cat (F sylvestris) and
  African wild cat (F libyca). 
   

Three General Outcomes Of Hybridisation

1. Hybrid zone: an area of contact between genetically distinct populations where hybridisation occurs. Geographically localised, does not affect the genetic integrity of two parent populations.

2. Hybrid swarm: a population of individuals that are all hybrids by varying numbers of generations of backcrossing with parental types, and by mating among hybrids. Less localised, blurs genetic integrity of parental populations. This blurring of genetic integrity is called genetic introgression - gene flow between populations that hybridise. Introgression is not necessarily symmetrical - so one of the parent taxa might be genetically swamped by the hybrid swarm, while the other is not.

3. Hybrid taxon: an independently evolving stable population or group of populations with a unique set of heritable traits, distinct from the two or more parent taxa from which it arose.
New genetic boundaries are established when the hybrid species forms. 
 

Hybrid Index

This is used to assess how far a population has progressed from early hybridisation (F1 only) stages to well established hybrid swarms. It is important to determine if hybridisation is natural or influenced by man's activities (anthropogenic). If we can't distinguish, then we must either fail to protect natural hybrids, or protect anthropogenic hybrids, perhaps to the detriment of parental taxa (which may become genetically swamped) [9].
 

Orchid Hybrid Swarms?

Orchid hybrid swarms have been observed in Britain where common spotted orchids and early
marsh orchids grow together, so individual plants can be impossible to identify with certainty in
the field. This situation can be made even more confusing if one of the parents dies out, perhaps
as a consequence of environmental change, leaving the hybrid swarm with the other parent [10].

Tony Clarke reported a study of the hybrid swarm Dockrillia pugioniformis x Dockrillia striolata
in the Watagan Mountains [11].

Bob Bates wrote on the observation of pollen vectors on a putative hybrid swarm of Microtis in South Australia, noting Mm. parviflora, unifolia and rara, with great numbers of apparent intermediate forms among the three species [12]. Two species of wasp each visited each of the three species of Microtis.

Phillip Cribb of RBG, Kew, has reported a well intentioned but misguided attempt at conservation of a rare orchid in Britain, in which the rare and a common species were accidentally cross-pollinated, resulting in a hybrid swarm which replaced the rare species in the wild.