The not so singular process of hybridization

What, if anything, are hybrids?

© G. Horatiu

© G. Horatiu

Zach Gompert and Alex Buerkle ask this question in a special issue in Evolutionary Applications.
Hybrids occur when unrelated individuals mate, but how distant do the taxa need to be to constitute a cross? The varied definitions of hybridization downplay the continuous nature of genetic and phenotypic differentiation. Anywhere you are along this continuum, gene flow could generate similar consequences.
Gompert and Buerkle take an interesting approach in reviewing and synthesizing existing literature on hybridization while also adding new simulations. The issues they address

have a relatively long history, some of which is underappreciated, and clarifying these ideas should have practical consequences for managing hybridization and gene flow in plants,

and likely other taxonomic groups as well.
They argue that it is the evolutionary and ecological consequences of gene flow that should be considered when we define hybridization. This is not unlike what Pante and colleague argued awhile back in building “species hypotheses” to test with estimates of gene flow. For species, a careful exploration of connectivity can inform on species delimitation. For hybridization, we should worry less about a taxon and more about the differences between two groups.
Gompert and Buerkle define hybridization as

cases where outcrossing and gene flow occur between populations that differ, at least quantitatively, at multiple heritable characters or genetic loci that affect fitness.

Using simulations, Gompert and Buerkle explored cases in which management decisions could be misled while studying hybridization. They simulated genetic data under conditions of primary divergence or secondary contact as well as quantitative traits along an environmental gradient or reduced hybrid fitness.
They found that it will often be difficult to distinguish different histories of selection and gene flow from genetic data, but they did find that recent primary divergence and secondary contact generate different variation. Managers should, therefore, treat recent primary divergence and secondary contact as distinct processes. It is only after greater periods of time that the patterns of variation from these two processes begin to look similar.
The variability in outcomes makes it difficult to describe categorical statements about

the composition, importance and … threats of hybrids.

The make up of parental taxa and hybrids at site A may be completely uninformative about other locations where these taxonomic groups co-occur.
The challenges facing hybridization studies arise due to the complexity and uncertainty of the process of hybridization itself. In order to overcome these difficulties, it will be necessary to perform detailed studies that include sampling multiple geographic locations and contexts, characterizing the demography of parents and hybrids and estimate the multiple dimensions of ancestry.
Gompert Z and C. A. Buerkle (2016) What, if anything, are hybrids: enduring truths and challenges associated with population structure and gene flow. Evolutionary Applications doi:10.1111/eva.12380.

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