Hybridization and extinction

Abstract

Hybridization may drive rare taxa to extinction through genetic swamping, where the rare form is replaced by hybrids, or by demographic swamping, where population growth rates are reduced due to the wasteful production of maladaptive hybrids. Conversely, hybridization may rescue the viability of small, inbred populations. Understanding the factors that contribute to destructive versus constructive outcomes of hybridization is key to managing conservation concerns. Here, we survey the literature for studies of hybridization and extinction to identify the ecological, evolutionary, and genetic factors that critically affect extinction risk through hybridization. We find that while extinction risk is highly situation dependent, genetic swamping is much more frequent than demographic swamping. In addition, human involvement is associated with increased risk and high reproductive isolation with reduced risk. Although climate change is predicted to increase the risk of hybridization-induced extinction, we find little empirical support for this prediction. Similarly, theoretical and experimental studies imply that genetic rescue through hybridization may be equally or more probable than demographic swamping, but our literature survey failed to support this claim. We conclude that halting the introduction of hybridization-prone exotics and restoring mature and diverse habitats that are resistant to hybrid establishment should be management priorities.

Keywords: conservation; demographic swamping; gene flow; genetic swamping; hybrid fitness; introgression; invasive species; outbreeding depression.

Figure 1

When rare (red flowers) and common (yellow flowers) lineages come into contact, hybridization may result in the local (or global) extinction of the rare lineage through (A) demographic swamping, in which unfit hybrid individuals (light and dark orange flowers) are entirely removed and with them all rare lineage alleles or (B) genetic swamping, in which hybrids are at least partially fertile and viable and replace pure parental genotypes. Note that demographic swamping results in population or lineage extinction, whereas genetic swamping results in the extinction of pure parental genotypes (i.e., genome extinction), but not of the alleles themselves. Rare, common, and hybrid genotype percentages per generation are represented in the color‐coded bars on the right side of both panels.

Figure 2

Overview of results from literature survey of 143 empirical papers (Table S1). Count data are shown for the different groupings that were scored for each category (Table 1). Black segments correspond to cases that did not fit well into defined groupings (other). Gray segments correspond to missing data, that is, cases in which the research article did not provide the information needed to classify the study. Articles that described the absence of a given phenomenon (e.g., asymmetric introgression) are noted as not reported (N.R.). For human involvement, groupings are as follows: species introduction (I, red), habitat disturbance (D, yellow), husbandry (H, purple), various combinations of these scenarios, or no human involvement. For asymmetry cases, introgression toward threatened species (Threat. or T) is noted. Further information on the species involved in hybridization and on the nature of species introductions can be found in Table S1.

Figure 3

Number of relevant publications published between January 1975 and May 2015 that were detected by our Web of Science (Thomson Reuters) search for the keywords ‘hybridi*ation’ and ‘extinction’.