Biodivers Conserv
DOI 10.1007/s10531-008-9550-x
ORIGINAL PAPER
Natural hybridization and conservation
Meritxell Genovart
Received: 16 May 2008 / Accepted: 21 November 2008
Ó Springer Science+Business Media B.V. 2008
Abstract This review deals with natural hybridization, an important subject in conservation biology. Natural hybridization is defined as the secondary contact between two
populations that have evolved separately over a long period of time. This process is
uncommon in terms of the total number of individuals involved, but is much less unusual if
we consider the number of species that hybridize. Thus, natural hybridization may be an
important process in the shaping of the evolutionary trajectories of many plant and animal
species. The possible consequences of natural hybridization, which can either promote or
prevent evolutionary divergence between taxa and will involve many ecological factors,
are analysed here. I question whether natural hybridization poses always a problem in
conservation and try to answer when conservation biologists and managers do have a
responsibility to take decisions. Several examples of hybridization related to management
strategies are also discussed. In conclusion, I believe that it is impossible to provide
conservation managers with a simple handbook explaining how to proceed in cases of
hybridization––each case is unique and should be analyzed individually. The only advice is
that the more we know about hybridization and the factors involved, the better we will be
able to assess each situation, to establish the possible consequences and even to estimate
the probability of success of any particular conservation strategy.
Keywords Conservation Dispersal Evolutionary process Hybridization Management Selection Speciation
Populations that have been isolated for a long time may eventually come into contact,
giving rise to a mixed population and thus the incorporation of genes from one genetically
distinct population into another (Futuyama 1998). This secondary contact between taxa in
nature is what is known as natural hybridization. Historically, biologists (essentially
M. Genovart (&)
Institut Mediterrani d’Estudis Avançats IMEDEA (CSIC-UIB), Miquel Marquès 21, Esporles,
Mallorca 07190, Spain
e-mail: [email protected]
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zoologists and botanists) have held contrasting points of view regarding the evolutionary
role of natural hybridization. Whereas zoologists have tended to consider hybridization as
an isolated process or, rather, a taxonomical problem, botanists have regarded hybridization as just another evolutionary process. However, more serious dilemmas appear when
analysing hybridization processes between species since, according to the most widely
used concept of species (the ‘species concept’; Mayr 1942), species are reproductively
isolated from one other and so hybridization processes should never in fact occur. The root
of the problem probably lies in the acceptance of species as a static concept and not as a
dynamic process, given that speciation is a long and complex process. Speciation involves
the establishment of many reproductive barriers and as long as they are incomplete,
hybridization processes may occur. Thus, the establishment of the boundaries delimiting
taxonomic species will be in some cases an arbitrary decision with no evolutionary relevance. However (and to avoid over-complicating this already thorny issue), henceforth I
am going to consider natural hybridization as the gene flow between clearly differentiated
taxa, leaving aside the question as to whether they should be treated as species or
subspecies.
Natural hybridization is a rare process in terms of the total number of individuals that
hybridize, but is not so rare when we look at the number of species that actually hybridize
(Mallet 2005). Hybridization influences the evolutionary trajectory of around 25% of plant
species and 10% of animal species, above all young, recently diverged species (Arnold
1997). Thus, natural hybridization may be an important process in shaping the evolutionary
trajectories of many plants and animals (Eckenwalder 1984; Grant and Grant 1992;
Dowling and DeMarais 1993; Arnold 1997; Rieseberg et al. 2003; Seehausen 2004).
Interestingly, among vertebrates, these processes seem to be most frequent in birds (Grant
and Grant 1992), where speciation usually involves low genetic differentiation and where
post-mating barriers seem to evolve more slowly than in other vertebrates (Prager and
Wilson 1975; Fitzpatrick 2004).
The consequences of hybridization
Natural hybridization can either promote evolutionary divergence between taxa, for
example by reinforcement (Servedio 2004; Hoskin et al. 2005; Urbanelli and Porretta 2008),
or prevent it (Mayr 1963; Coyne and Orr 2004). Species’ integrity will depend on rates of
dispersal and gene flow between taxa, and natural selection in the species involved and in
the resulting hybrids. Thus many different ecological factors, may be involved in the
process. Hybridization between different taxa may potentially lead to a number of different
situations: the establishment of a stable and localized hybrid zone that does not cause the
disappearance of the original species; the disappearance of one of the two original species;
or even the appearance of a new species, resulting from hybrid speciation (Mallet 2007). For
example, if dispersal were high and there were no natural selection forces at work, the gene
exchange between the two diverging lineages would be high and would reduce genetic
differentiation between lineages, thereby turning them into a single species (Arnold 1997).
Nevertheless, dispersal––once thought of as a fixed trait––is now considered to be flexible
(Clobert et al. 2001). Consequently, particular changes in environmental factors (e.g. in
breeding sites or food availability) may change dispersal rates over time, diminishing or
increasing the gene flow between taxa and influencing the relationship between evolutionary lines. Thus, when two diverging lineages come into contact, as in any evolutionary
process, a lot of factors come into play and each situation should be treated as unique.
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Is hybridization a conservation concern?
In light of the above rationale––that is, the acceptance of natural hybridization as natural
phenomenon––the answer to this question should be negative. However, no simple answer
is possible, as humans are commonly either directly or indirectly the main cause of secondary contacts between species. A good example is the introduction of exotic species,
which adapt to their new habitats and may hybridize with local species (Huxel 1999;
Meldgaard et al. 2007; Muñoz-Fuentes et al. 2007). Another example is of the the
hybridizations occurring due to human-mediated habitat modifications (Wayne and Jenks
1991; Haig et al. 2004). This situation becomes even trickier when one of the species is
endangered (Rhymer and Simberloff 1996; Fredrickson and Hedrick 2006). Consequently,
we should first ask if the hybridization event is due to anthropogenic reasons and, secondly,
if one of the two species involved is endangered. If the answers to these two questions are
both negative, then the course of action to be undertaken is clear: we should just observe
and learn. However, how should we proceed in any of the other cases? In my view,
conservation biologists and managers do have a responsibility to take decisions and their
commitment should depend on the severity of the consequences of the hybridization in
question and on the viability of conservation management strategies. When hybridization is
incipient or restricted (in space or time) and management is thus viable, a heuristic action
would be to cull introduced individuals and the resulting hybrids. This type of management
has been carried out in the case of the White-headed Duck Oxyura leucocephala and the
Ruddy Duck O. jamaicensis. The latter species was introduced into Great Britain by
humans in the middle of the twentieth century and has spread throughout Europe,
hybridizing with the endangered White-headed Duck (Muñoz-Fuentes et al. 2007). In this
case, even though the eradication of Ruddy Ducks is very difficult, the restricted localization of the endangered species allows (at least temporally) exotic and hybrid individuals
to be controlled. Another example is the release of foreign trout or salmon species for
recreational fishing on rivers. Although it is quite clear that this can constitute a serious
conservation problem wherever they displace the local species (Ruzycki et al. 2003;
McHugh et al. 2008), some introduced salmonid species do not cross with local species or,
where they do, only produce non-fertile descendents. However, other species are able to
interbreed with local species and produce fertile hybrids (Meldgaard et al. 2007). If
hybridization is not generalized, we can act by controlling exotic individuals and, even
more importantly, by protecting rivers that are free of introduced individuals. When
hybridization is extensive in space and time, efficient management options are very
complex, if not impossible. An example of this is the hybridization of the Pacific Black
Duck Anas superciliosa in New Zealand with the introduced Mallard Anas platyrhynchos.
Mallards are much more abundant than Pacific Black Ducks and hybridization is widespread. Management is thus very difficult and the probabilities that the Pacific Black Duck
will become extinct are very high. We should also bear in mind the fact that if management
strategies are to be useful, then the original cause of the hybridization should first be
resolved. For instance, the Spotted Owl Strix occidentalis in northwest America is clearly
in regression as a result of the logging of old forests (Haig et al. 2004). On the other hand,
the Barred Owl Strix varia seems not to be affected by this habitat modification and has
been expanding westwards. The distribution of the two species now overlaps, thereby
favouring hybridization. In this case, before any other management actions are contemplated, habitat modification should be halted and the previous situation of the habitat
should be restored as much as is possible. Only then will it make sense to eliminate Barred
Owls and hybrids within the distribution area of the Spotted Owl. Another example of a
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problematic situation in conservation is that of the Balearic Shearwater Puffinus mauretanicus. This species is critically endangered (Oro et al. 2004) and hybridization with the
Yelkouan Shearwater has been detected (Genovart et al. 2007; Genovart et al. submitted).
These authors suggest that the hybridization between these two young species is a natural
evolutionary process and, owing to the highly endangered situation of the Balearic
shearwater, immediate conservation efforts should concentrate on enhancing adult survival
rates in this species––its main conservation problem––and on protecting its breeding areas
as means to reversing the dramatic decline that it is undergoing.
In conclusion, it is impossible to provide managers with a simple handbook that will
explain how to proceed in cases of hybridization: each situation will be different and
should be analyzed on a case-by-case basis. The only advice is that the more we know
about the hybridization process and the ecological factors involved, the more able we will
be to analyse each situation as a means to establishing possible consequences and even
estimating the probability of success of any particular conservation action.
Acknowledgments I am very grateful to J.M. Igual, D. Oro, A. Sanz and G. Tavecchia for providing
helpful comments on an earlier version of the manuscript. MG was funded by an I3P postdoctoral fellowship
from the Spanish Ministry of Education and Science.
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