Demystifying Negative Connotations of Hybridization for Less Biased Conservation Policies

Abstract

Interspecific hybridization is one of the most controversial—and usually neglected—issues in conservation due to its multiple evolutionary consequences that might include the origin and transfer of adaptations, the blur of distinctive lineages or the formation of maladaptive hybrids. However, despite different outcomes, most conservation laws do not offer any possibility of hybrids being protected since they are perceived as a threat to the survival of pure species. We assessed how much hybridization has contributed to species extinction considering all IUCN Red Data assessments. However, we found that it has been scarcely reported as a threat contributing to extinction: only 11 extinct species out of 120,369 assessments mentioned hybridization. Although the causes that contribute to species extinctions should be controlled, the reasons for not conserving hybrids seem subjective rather than empirically supported. In a genomic era where hybridization is being more frequently detected, the debate involving the conservation of hybrids should be re-opened. Should we conserve hybrids despite the possibility of gene flow with parental species? Should we protect only natural hybrids? The resolution of this debate goes to the heart of what we mean to conserve and the time scale of conservation. But hybridization is part of the evolutionary process and might even increase in the future due to human-induced changes. As such, it becomes clear that we need to move beyond the causes and instead tackle the consequences of hybridization to create environmental policies for the management of hybrids, considering both positive and negative consequences.

Consequences of Hybridization in Mammals: A Systematic Review

“…On the other hand, hybridization can be also considered as a potential threat to species survival [ 22 , 23 , 24 ]. Accumulation of deleterious variation [ 25 ], outbreeding depression [ 26 , 27 , 28 ] and genetic swamping [ 29 , 30 , 31 ] are among detrimental consequences of hybridization.…”

“…High risk of extinction due to hybridization has been reported for rare or endangered species interbreeding with more common relatives [ 33 , 34 ]. However, hybridization was mentioned as a factor contributing to extinction in only 11 species out of 120,369 extinct species assessed [ 23 ]. The negative impact of hybridization should not be neglected, but the conservation policies should not be focused on the negative aspects of hybridization only [ 23 ].…”

“…However, hybridization was mentioned as a factor contributing to extinction in only 11 species out of 120,369 extinct species assessed [ 23 ]. The negative impact of hybridization should not be neglected, but the conservation policies should not be focused on the negative aspects of hybridization only [ 23 ]. From a conservation perspective, hybridization outcomes may range from considerable introgression with significant negative impacts, e.g., reduced survival or reproductive success of hybrids, through minimal introgression with negligible impact, to moderate introgression with significant positive impacts, e.g., increased fitness of admixed individuals [ 35 ].…”

“…They were focused on specific aspects of hybridization and/or particular taxonomic groups, for example the evolutionary importance of natural hybridization [ 2 ], the role of hybridization in extinction [ 32 ], hybrid fitness [ 55 ], introgression during anthropogenic hybridization [ 56 ], mammalian hybrid zones [ 57 ], taxonomic problems associated with inter-specific gene flow [ 58 ], hybridization in European ungulates [ 59 ] and hybridization in New Zealand taxa [ 60 ]. Most studies to date focused on documenting hybridization events and analyzing their causes, while relatively little is known about the consequences of hybridization and its impact on the parental species [ 23 ]. To address this knowledge gap, we conducted a systematic review of studies on hybridization in mammals and assessed the frequency of different consequences of hybridization reported.…”

Genomic sequencing confirms absence of introgression despite past hybridisation between a common and a critically endangered bird and its common congener

“…However, the potential conservation value of hybrids should not be viewed as static, especially as altered species ranges increase the prevalence of hybrids (Chunco, 2014), and the inclusion of genomic data continues to improve our understanding of the impacts of hybridisation (vonHoldt et al, 2018). Further, there are growing calls for researchers identify and avoid unscientific and biased perspectives on hybrids and hybridisation in conservation (Draper et al, 2021;Hirashiki et al, 2021).…”