Oscillation of an Anuran Hybrid Zone: Morphological Evidence Spanning 50 Years

Abstract

BackgroundThe hybrid zone between the primarily forest-dwelling American toad, Anaxyrus americanus, and the prairie-adapted Canadian toad, A. hemiophrys, in southeastern Manitoba is known to have shifted its position during the past 50 years. Hybrid zones are areas of interbreeding between species and their movement across a landscape should reflect their underlying dynamics and environmental change. However, empirical demonstrations of hybrid zone movements over long periods of time are rare. This hybrid zone is dominated by individuals of intermediate morphology and genetic composition. We sought to determine if it had continued to move and if that movement was associated with shifts in habitat, as predicted.Methodology/Principle FindingsWe used variation in the toads’ most diagnostic morphological feature, the separation between their interorbital cranial crests, to determine the geographic position of the hybrid zone center at four times between 1960 and 2009 using maximum likelihood methods. The hybrid zone center moved west by 38 km over 19 years and then east again by 10 km over the succeeding 29 years. The position of the hybrid zone did not track either the direction or the magnitude of movement of the forest-prairie habitat transition over the same time period.Conclusions/SignificanceThis is the first reported evidence of oscillation in the position of a hybrid zone. The back and forth movement indicates that neither species maintains a selective advantage over the other in the long term. However, the movement of the hybrid zone was not bounded by the breadth of the habitat transition. Its oscillation suggests that the hybrid zone is better described as being elastically tethered to the habitat transition.

Stabilization of a salamander moving hybrid zone

“…The argument for movement is mostly inferential (Buggs, ; see also Carling & Zuckerberg, ; Charpentier et al., ; Gay, Crochet, Bell, & Lenormand, ; Krosby & Rohwer, ; Leafloor, Moore, & Scribner, ). Examples of direct observation on hybrid zone dynamics are rare, because of the time frame involved (Buggs, ; see also Engebretsen, Barrow, Rittmeyer, Brown, & Moriarty Lemmon, ; Ouanes, Bahri‐Sfar, Ben Hassine, & Bonhomme, ; Roy, O'Connor, & Green, ; Smith et al., ; Taylor, Curry, White, Ferretti, & Lovette, ). We here document by direct observation a unique case of a hybrid zone that first moved and then stabilized in historical times.…”

Climate-mediated hybrid zone movement revealed with genomics, museum collection, and simulation modeling

“…Similar to this scenario would be a ratcheting of movement northward (46), with northward expansion in warm years and persistence achieved in cooler years through diapause (i.e., individuals do not attempt a second generation). Evidence of oscillating hybrid zones has only been documented in a single case (47). Unfortunately, as is true with most historical datasets, we lack the fine-scale temporal resolution to test these alternative hypotheses.…”

Movement of aHeliconiushybrid zone over 30 years: A Bayesian approach

“…For some hybrid zones, this balance is relatively stable and clines remain unchanged for decades (e.g., Mettler & Spellman, ; Rosser, Dasmahapatra, & Mallet, ). Many hybrid zones, however, have moved or changed over relatively short time spans (reviewed in Buggs, ; more recent examples include Hunter, Matocq, Murphy, & Shoemaker, ; Leaché, Grummer, Harris, & Breckheimer, ; Roy, O'Connor, & Green, ). Studies of hybrid zone dynamics are thus an excellent way to watch evolution in action and understand how and whether natural selection and gene flow vary through time.…”