Pfennig Karin S, Simovich Marie A
Section of Integrative Biology, University of Texas, Austin 78712, USA.
Evolution. 2002 Sep;56(9):1840-8. doi: 10.1111/j.0014-3820.2002.tb00198.x.
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.
杂交的适应性后果对物种形成过程有着至关重要的影响。当杂交成本高昂时，自然选择会促进合子前隔离机制（例如交配行为）的进化，这些机制会减少异种交配，从而增强物种间的生殖隔离（这一过程称为强化）。然而，如果不同物种间避免杂交的选择存在差异，强化过程可能会受到阻碍。在此，我们研究了平原锄足蟾（Spea bombifrons）和新墨西哥锄足蟾（S. multiplicata）之间杂交的频率及其适应性影响。杂交在较小的、往往是临时性的繁殖池塘中最为频繁，异种配对几乎完全由平原锄足蟾雌蟾和新墨西哥锄足蟾雄蟾组成。此外，在受控的实验杂交中，以新墨西哥锄足蟾为母本的杂交后代的存活率显著低于纯种新墨西哥锄足蟾后代，且发育时间更长。相比之下，以平原锄足蟾为母本的杂交后代比纯种平原锄足蟾后代变态发育得更快。这些数据表明，尽管新墨西哥锄足蟾雌蟾受到避免杂交的选择，但如果它们的繁殖池塘临时性很强，自然选择可能会青睐那些与新墨西哥锄足蟾杂交的平原锄足蟾雌蟾。一般来说，杂交物种间避免杂交的选择强度可能不同，这可能会阻碍强化过程。
