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Colliard et al. BMC Evolutionary Biology 2010, 10:232 Page 10 of 16
http://www.biomedcentral.com/1471-2148/10/232

Figure 6 Backcrosses of F 1 (B. balearicus × B. siculus, Fig. 6a) to parental species of Sicilian green toads.a-d:Wild-caught animals
involved in backcrosses. a: B. balearicus female; b: B. balearicus male; c: B. siculus female; d: B. siculus male. e-h: Backcrosses. e: female F 1 (B.
balearicus × B. siculus) × male B. balearicus; f: female B. balearicus × male F 1 (B. balearicus × B. siculus); g: female F 1 (B. balearicus × B. siculus)×
male B. siculus; female B. siculus × male F 1 (B. balearicus × B. siculus). Photographs: a-d: G.F. Turrisi; e-h: M. Stöck.

between Mount Etna and the Ionian Sea creates a Drift might also partly account for the marked con-
peninsular situation, largely isolating populations at the trast between mitochondrial and nuclear introgression.
contact zone from conspecifics. This induces a strong Mitochondrial markers have low effective sizes (about
differentiation over a small geographic scale, which one quarter of nuclear markers), and are therefore more
somewhat blurs the overall clear pattern of isolation by prone to introgression. In small hybridizing populations,
distance observed in both species. mtDNA might sometimes get fixed into foreign taxa via