Quaternary phylogeography: the roots of hybrid zones

Abstract

The older history of hybrid zones is explored through consideration of recent advances in climatology, paleontology and phylogeography in the Late Cenozoic, particularly the Quaternary Period with its major climatic cycles. The fossil record shows that these ice ages and their nested millennial oscillations caused substantial changes in species distributions and with genetic evidence allows deduction of refugia and colonization routes in arctic, temperate, desert and tropical regions. The age of divergence between hybridizing lineages varies from the Late Pleistocene to the Late Miocene, implying much range change and varying selection on sister lineages. Hybridizing lineages in the Tropical and Temperate regions range in age from young to old, but those studied in the Arctic are no more than a few ice ages old and their refugial roots are not clear. Mid to low latitude regions often show parapatric patchworks of lineages and multiple refugia stable through many climatic oscillations. Particular hybrid zones may have formed more than once; while some expansions were not the same, producing reticulation and introgression in previous glacial cycles. Hybrid-zone roots are complex and deep, and considerations of their complexity can reveal evolutionary pathways of species. They are indeed windows on evolution.

Subterranean termite phylogeography reveals multiple postglacial colonization events in southwestern Europe

“…2015). Such repeated cycles of isolation and dispersal may have driven speciation, through either allopatric adaptation to new environments or the occupation of new niches after sympatric recolonization of deserted areas (Orr and Smith 1998; Hewitt 2001, 2011). Thus, to understand the influence of the glaciation cycles on speciation, studies of subspecies, subpopulations, and recently diverged sister taxa rather than well‐differentiated species may provide greater insight into the processes that led to population differentiation and, ultimately, speciation.…”

Assessing patterns of hybridization between North Atlantic eels using diagnostic single-nucleotide polymorphisms

“…The study of hybridization between individuals from genetically distinct populations or species is of considerable interest to understand the dynamics of speciation and adaptive divergence (Mallet, 1995;Grant and Grant, 2002;Rieseberg and Willis, 2007;Hewitt, 2011;Stolting et al, 2013). In particular, hybrid zones are widely recognized as 'windows' or 'natural laboratories' for evolutionary studies concerning models of speciation, selective forces involved in speciation, gene flow between species and the maintenance of species boundaries (Hewitt, 1988(Hewitt, , 2011Harrison, 1990;Mallet, 1995;Petit and Excoffier, 2009).…”

“…In particular, hybrid zones are widely recognized as 'windows' or 'natural laboratories' for evolutionary studies concerning models of speciation, selective forces involved in speciation, gene flow between species and the maintenance of species boundaries (Hewitt, 1988(Hewitt, , 2011Harrison, 1990;Mallet, 1995;Petit and Excoffier, 2009). A key question in many instances of hybridization concerns whether hybridization is restricted to F1 hybrids and first-generation backcrosses after which genetic incompatibilities and/ or natural selection eliminate hybrid offspring, or if hybridization proceeds to the extent that introgression occurs (Allendorf et al, 2001).…”

Phylogeographic pattern of range expansion provides evidence for cryptic species lineages in Silene nutans in Western Europe

“…Ultimately, these historical refugia may have promoted allopatric genetic divergence leading to different ESUs or to sibling species that may not be morphologically distinguishable, that is, cryptic species (Hewitt, 2000;Gavin et al, 2014). Subsequently, postglacial migration and species range expansion from these different refugia have allowed divergent lineages to come into secondary contact in suture zones (for example, Taberlet et al, 1998;Hewitt, 2011). Contemporary processes, such as gene flow, genetic drift and spatially heterogeneous selective pressures, in combination with species life-history traits, in particular the breeding system and seed and pollen dispersal capabilities, can also contribute to shape species' distribution, genetic diversity and population differentiation (Hamrick and Godt, 1996;Duminil et al, 2007).…”