Trends in Ecology & Evolution
Opinion
The problems with hybrids: setting conservation guidelines
Section snippets
Detection and analysis of hybridization
The detection of hybrid individuals relied upon morphological characteristics until the mid-1960s. However, not all morphological variation has a genetic basis, and the amount of morphological variation within and among populations is often greater than is actually recognized 10. The detection of hybrids using morphological characters generally assumes that hybrid individuals will be phenotypically intermediate to parental individuals 11. This is often not the case, because hybrids sometimes
Hybrid policies
An early series of interpretations of the US Endangered Species Act (ESA) 17 concluded that hybrids should not receive protection under the ESA because protection of hybrids would not help recover a listed species and could jeopardize continued existence of that species. However, this ‘Hybrid Policy’ was withdrawn in December 1990 because ‘New scientific information concerning genetic introgression has convinced us that the rigid standards set out in those previous opinions should be revisited’
Categorization of hybridization
Here, we identify six types of hybridization and discuss what conservation measures should be taken for each (Box 2). Although the situations described here refer primarily to interspecific hybridization, the principles are general and also apply to intraspecific hybridization (Box 3).
Dowling and Secor 5 presented compelling evidence that genetic mixing has played an important role in evolution of some taxa. We classify hybrid taxa that have arisen by natural genetic admixture as resulting from
Management questions
There are several controversial questions that often arise when developing conservation plans in situations where hybridization is a concern.
Conclusions
Hybridization is a natural part of evolution. Taxa that have arisen through natural hybridization should be eligible for protection. Nevertheless, increased anthropogenic hybridization is causing extinction of many taxa (species, subspecies and locally adapted populations) by both replacement and genetic mixing. Policies should be designed to reduce anthropogenic hybridization. Hybrid taxa resulting from anthropogenic causes should be protected only in exceptional circumstances. They could
Acknowledgements
We thank D. Campton, C. Daugherty, S. Forbes, J. Gardner, S. Grant, M. Johnson, R. McDowall, M. Nammack, O. Ryder, F. Utter, R. Vrijenhoek, L. Waits, R. Waples and three anonymous reviewers for their helpful comments. This article was written while FWA was supported by a New Zealand Fulbright Senior Scholar Award. Sequence of authors is by date of birth, and does not necessarily reflect relative contributions to this article.
Glossary
- Admixture
- the production of new genetic combinations in hybrid populations through recombination.
- Diagnostic locus
- a locus that is fixed or nearly fixed for different alleles in two hybridizing populations.
- Extrinsic outbreeding depression
- outbreeding depression that results from reduced adaptation to environmental conditions.
- Genetic mixing
- the loss of a formerly distinct population through hybridization.
- Hybridization
- interbreeding of individuals from genetically distinct populations, regardless of
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