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Figure 7 Survival and development of backcrosses. a, c to e, g: tadpoles, one week after spawning. a: female B. siculus × male F 1 (B. balearicus ×
B. siculus) - note dead embryos and malformations; b: postmetamorphic toadlets exhibited size differences among siblings and low survival; c:F 2
from among hybrid crosses, female F 1 (B. balearicus × B. siculus) × male F 1 (B. balearicus × B. siculus) - all tadpoles malformed; d: female F 1
(B. balearicus × B. siculus) × male B. balearicus - most tadpoles malformed; e, f: female B. balearicus × male F 1 (B. balearicus × B. siculus) - note
enormous size differences among siblings in the age of one month after spawning (f); g: female F 1 (B. balearicus × B. siculus) × male B. siculus -
note most tadpoles show malformations; h: F 1 -hybrid from crossing female B. siculus × male B. balearicus (as shown in Figure 6b) in the age of one
year. Photographs: M. Stöck.

drift or founder effects, even with little or no gene flow bufonids; [32-36]]. Effective population sizes at the front
at nuclear loci [31]. This contrast should be further wave of expansions (or in any poorly connected popula-
amplified if females display lower effective size than tion) largely depend on gene flow from incoming immi-
males, which occurs when dispersal is male biased [as grants, and thus on the mode of inheritance of markers
expected in polygynous mating system such as found in when dispersal is sex biased [37,38].