An amphibian species pushed out of Britain by a moving hybrid zone

Abstract

Classical theory states that hybrid zones will be stable in troughs of low population density where dispersal is hampered. Yet, evidence for moving hybrid zones is mounting. One possible reason that moving zones have been underappreciated is that they may drive themselves into oblivion and with just the superseding species remaining, morphological and genetic signals of past species replacement may be difficult to appreciate. Using genetic data (32 diagnostic single nucleotide polymorphisms) from a clinal hybrid zone of the common toad (Bufo bufo) and the spined toad (Bufo spinosus) in France for comparison, alleles of the latter species were documented in common toads in the south of Great Britain, at frequencies in excess of 10%. Because long distance dispersal across the Channel is unlikely, the conclusion reached was that the continental toad hybrid zone which previously extended into Britain, moved southwards and extirpated B. spinosus. Species distribution models for the mid‐Holocene and the present support that climate has locally changed in favour of B. bufo. The system bears resemblance with the demise of Homo neanderthalensis and the rise of Homo sapiens and provides an example that some paleoanthropologists demanded in support of a hominin “leaky replacement” scenario. The toad example is informative just because surviving pure B. spinosus and an extant slowly moving interspecific hybrid zone are available for comparison.

Unexpected post‐glacial colonisation route explains the white colour of barn owls (Tyto alba) from the British Isles

“…This early history of colonisation of the British Isles inferred here from whole‐genome sequences and supported by SDM projections on past climatic features is apparently unique among terrestrial vertebrates thus far, but it is far from the first to deviate from the most common colonisation route directly over Doggerland (e.g., Boston et al, 2015 ; Kelly et al, 2010 ; Snell et al, 2005 ; Stewart & Lister, 2001 ; Teacher et al, 2009 ) or to indicate an earlier colonisation than generally assumed (Martínková et al, 2007 ; McDevitt et al, 2020 ). Actually, it has often been conjectured that some organisms of the British Isles had an Iberian origin though this remained unconfirmed (e.g., Arntzen, 2019 ; Corbet, 1961 ; McDevitt et al, 2011 ). The case of the stoat ( Mustela erminea ) is particularly interesting as it was found to have had an isolated glacial refugium also in now submerged land southwest of today's French coastline on the Bay of Biscay (Figure 4c – LGM; Martínková et al, 2007 ).…”

Genetic traces of hybrid zone movement across a fragmented habitat

“…Although hybrid zone movement has not previously been considered to be common (Buggs, 2007), the spatial and genetic signatures of moving hybrid zones are by now been well documented in a wide range of vertebrate taxa including fishes (Souissi et al, 2018), salamanders (Arntzen & Wallis, 1991; Wielstra, Burke, Butlin, Arntzen, et al, 2017), toads (Arntzen, 1978, 2019; Arntzen et al, 2017), birds (Carling & Zuckerberg, 2011) and mammals (Lado et al, 2018; Macholán et al, 2011). Two important reservations apply to our interpretations.…”

Environmental correlates of the European common toad hybrid zone

“…The total of missing data was 2.0%. A subset of 2524 individuals from 185 localities was studied for another 27 diagnostic SNP markers (as in van Riemsdijk et al, 2019b;Arntzen, 2019) to determine the center of the two species contact zone more accurately. These latter localities were arranged in eight transects with hybrid zone centers located in France or Italy.…”