Trends in Ecology & Evolution
Volume 16, Issue 11, 1 November 2001, Pages 613-622
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Opinion
The problems with hybrids: setting conservation guidelines

https://doi.org/10.1016/S0169-5347(01)02290-X Get rights and content

Abstract

Rates of hybridization and introgression are increasing dramatically worldwide because of translocations of organisms and habitat modifications by humans. Hybridization has contributed to the extinction of many species through direct and indirect means. However, recent studies have found that natural hybridization has played an important role in the evolution of many plant and animal taxa. Determining whether hybridization is natural or anthropogenic is crucial for conservation, but is often difficult to achieve. Controversy has surrounded the setting of appropriate conservation policies to deal with hybridization and introgression. Any policy that deals with hybrids must be flexible and must recognize that nearly every situation involving hybridization is different enough that general rules are not likely to be effective. We provide a categorization of hybridization to help guide management decisions

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 However, this ‘Hybrid Policy’ was withdrawn in December 1990 because ‘New scientific information concerning genetic introgression has convinced us that the rigid standards set out in those previous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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