Differential Selection to Avoid Hybridization in Two Toad Species

Abstract

Abstract. The fitness consequences of hybridization critically affect the speciation process. When hybridization is costly, selection favors the evolution of prezygotic isolating mechanisms (e.g., mating behaviors) that reduce heterospecific matings and, consequently, enhance reproductive isolation between species (a process termed reinforcement). If, however, selection to avoid hybridization differs between species, reinforcement may be impeded. Here, we examined both the frequency and fitness effects of hybridization between plains spadefoot toads (Spea bombifrons) and New Mexico spadefoot toads (S. multiplicata). Hybridization was most frequent in smaller breeding ponds that tend to be ephemeral, and heterospecific pairs consisted almost entirely of S. bombifrons females and S. multiplicata males. Moreover, in controlled experimental crosses, hybrid offspring from crosses in which S. multiplicata was maternal had significantly lower survival and longer development time than pure S. multiplicata offspring. By contrast, hybrid offspring from crosses in which S. bombifrons was maternal outperformed pure S. bombifrons offspring by reaching metamorphosis faster. These data suggest that, although S. multiplicata females are under selection to avoid hybridization, selection might favor those S. bombifrons females that hybridize with S. multiplicata if their breeding pond is highly ephemeral. Generally, the strength of selection to avoid hybridization may differ for hybridizing species, possibly impeding reinforcement. Speciation by reinforcement is a potentially important means by which populations become reproductively isolated. When hybridization by two sympatric species produces offspring that have lower fitness than pure-species types, selection can lead to the evolution of premating isolating barriers (e.g., mating behaviors) that reduce hybridization

Geographic variation in hybridization across a reinforcement contact zone of chorus frogs (Pseudacris)

“…These estimates are also outside the range of frequencies estimated in another well‐studied frog reinforcement contact zone (i.e., 0.3%–6% F1 hybrids; Pfennig & Simovich, 2002; Pfennig, 2003). One question is whether these estimates reflect historical or present‐day hybridization, or more specifically, was there a high rate of hybridization upon initial secondary contact followed by a decline in rate through time?…”

“…2001; Dobzhansky, 1940; Jones, 1973). Longitudinal studies of reinforcement contact zones through time are consistent with this prediction (Pfennig, 2003; Pfennig & Simovich, 2002). An additional approach for testing this prediction is to compare levels of hybridization in older versus more recent contact zones.…”

Why Do Species Co-Occur? A Test of Alternative Hypotheses Describing Abiotic Differences in Sympatry versus Allopatry Using Spadefoot Toads

“…Consequently, the abiotic environment can also indirectly affect the distribution of these species by governing the distribution of food resources for which they compete. Moreover, as we also describe below, the two species interbreed, and hybrid fitness is determined in part by the abiotic environment [27], [28]. Yet, whether such interactions scale up to affect regional patterns of sympatry and allopatry, as opposed to only affecting local distributions of the two species, is unclear.…”

“…However, hybrids appear competitively equivalent or even superior to pure species for both resources [37]. Nevertheless, the fitness consequences of hybridization are environmentally dependent and differ for the two species [27], [28]. In particular, in situations where ponds dry rapidly, hybridization is advantageous for S. bombifrons females, but not for S. multiplicata females, because hybrid offspring develop faster than pure species S. bombifrons offspring but slower than pure S. multiplicata offspring [27].…”

“…Nevertheless, the fitness consequences of hybridization are environmentally dependent and differ for the two species [27], [28]. In particular, in situations where ponds dry rapidly, hybridization is advantageous for S. bombifrons females, but not for S. multiplicata females, because hybrid offspring develop faster than pure species S. bombifrons offspring but slower than pure S. multiplicata offspring [27]. Because ponds frequently dry before tadpoles complete metamorphosis, fast development time is critical in fast drying ponds [38].…”