Whether natural or anthropogenic in origin, zones of secondary contact provide a powerful test of the compatibility of previously allopatric species, and lend insight into the mechanisms responsible for reproductive isolation and speciation. When reproductive isolation is incomplete and secondary contact results in the formation of a hybrid zone, the resulting dynamics also give us a glimpse into the genetics underlying phenotypic divergence and the evolution of reproductive isolating mechanisms. On those rare occasions when we observe the early generations of contact, we can also examine ecological and genetic changes as they unfold, providing a view of short-term dynamics that is not available in most well-established, natural hybrid zones.
Hybridization between two species has many potential evolutionary consequences [e.g., ]. At one extreme, hybridization may end in the fusion of two lineages in which speciation has failed to occur fully [2, 3]. In this case, the homogenizing process of gene flow may erase the genetic signature of earlier lineage diversification and subsequent reticulate evolution. Alternatively, two lineages in secondary contact may create a stable hybrid zone in which parental taxa maintain separate evolutionary trajectories outside of the hybrid zone but continue to produce dysfunctional hybrids in the contact zone [4, 5]. Finally, hybridization can lead to increased genetic variation and evolutionary novelty in the form of recombinant genotypes [6–8]. In this situation, even when average fitness is quite low, hybrid populations may produce a few highly successful recombinant genotypes across all or a subset of ecological backgrounds, which can breed true and increase in frequency. When hybridization occurs as the result of introductions of an exotic species, the evolutionary outcomes are somewhat modified because ongoing gene flow from the introduction source is limited or absent [9, 10]. Homogenization of genotypes can still occur with the introgression of introduced alleles and loss of native variants, but the effects on parental populations are asymmetrical since gene flow from "pure" individuals is unidirectional (assuming that there are not frequent subsequent introductions of non-native individuals).
In either natural or human-mediated hybrid zones, exceptionally fit individuals have the potential to establish new evolutionary lineages, either displacing one or both parental lineages or diverging into a new ecological zone [7, 11–13]. The realization of this potential depends on the strength of selection, the number of genes involved in fitness variation, and the mode of action of those genes (additive, dominance, or epistasis). For example, a strong initial barrier to gene flow in the first generation (F1) of secondary contact will slow the establishment and spread of fit recombinant genotypes that may arise in subsequent generations, whereas F1 heterosis may speed up this process. Further, if the beneficial fitness effects of an allele in one genetic background do not outweigh deleterious effects or correlated selection in another genetic background, polymorphism may be eliminated before fit recombinant genotypes can reach a high enough frequency to be maintained by selection. Thus, the fitness consequences of the initial stages of hybridization, which are virtually always unknown in natural hybrid zones, can have profound effects on the longer-term consequences of hybridization.
The mode of gene action underlying particular phenotypes also plays an important role in the potential of hybrid populations to maintain genetic diversity and respond to selection . Phenotypes with largely additive inheritance express main effects across different genetic backgrounds whereas traits experiencing strong epistatic effects vary depending on the interaction of other genetic factors [15–17], and traits affected largely by dominance typically exhibit striking phenotypic differences within cross types [18, 19]. Therefore, traits with high dominance or epistatic variance might not respond to selection, even when they have large fitness effects, and the probability and rate of adaptive evolution in hybrid populations might be constrained by the genetic basis of transgressive traits.
For decades, secondary contact between species mediated by incidental human transport or deliberate introduction has been recognized as potentially detrimental to native species [20–23]. When introductions result in hybridization with native species, new dimensions are added to conservation issues. While hybridization and subsequent introgression has been characterized as a threat simply because replacement of native by introduced alleles is philosophically undesirable [12, 24], or might compromise the legal status of protected native species , more objectively detrimental impacts are also possible. Hybridization can create novel invasive phenotypes with negative ecological impacts [26–28], or hybrid dysfunction might make admixed populations more vulnerable to extinction [12, 29, 30]. For these reasons, studies of the evolutionary and ecological changes set in motion by hybridization have both applied and theoretical significance.
One of the key issues facing empirical analyses of hybrid zones is that the initial evolutionary dynamics of secondary contact are often obscured by many generations of admixture. Our understanding of hybrid zones is dominated by examples of secondary contact that have not resulted in extinction of one or both parental species or reinforcement of reproductive isolation, since these are the situations most often identified in the wild. Analyses of these zones have provided tremendous insights into the genes and characters that remain differentiated in the face of hybridization, but they are potentially a biased subset of the genes and characters that differed prior to secondary contact .
Our research on a recently established, human-mediated hybrid zone offers the rare opportunity to observe the initial dynamics of secondary contact between gene pools formerly separated ~5 mya . In the 1940s, bait dealers introduced thousands of the barred tiger salamander (BTS; Ambystoma tigrinum mavortium) from the Great Plains of the US into the range of the native California tiger salamander (CTS; A. californiense). Riley et al.  report (based on discussions with some of the individuals responsible for the introductions) that California's emerging bass (Micropterus spp.) fishing industry and known life history variation between the salamander species motivated the introductions. Bass fishermen use larval tiger salamanders (waterdogs) for bait, and they favor large larvae to catch large bass. CTS metamorphose at small sizes and have a relatively short larval period during which time they can be harvested from wild ponds. In contrast, BTS have a highly plastic larval period and can remain aquatic and attain very large sizes if ponds are permanent, providing a source of bait that is both larger and potentially available year-round. These intentional introductions have resulted in a large number of hybrid populations within the Salinas Valley of California. While there have been many generations of admixture in the heart of the Salinas Valley hybrid swarm, there has not been enough time for any nonnative allele to become fixed throughout the range of the native species . Furthermore, the success of hybrid genotypes appears heavily influenced by local environmental conditions, with anthropogenic changes in breeding habitat supporting increasingly non-native admixed populations [35, 36].
We have two main objectives in this study. First, we provide a direct comparison of viability among first- and second-generation hybrids to determine the strength of the initial barrier to gene exchange. Given that we have observed both hybrid dysfunction and vigor in contemporary hybrid populations [37, 38], it is not clear what the fitness of F1 hybrids may have been when they first appeared 50-60 years ago. Second, we examine the extent to which natural selection has affected mean fitness in a contemporary hybrid population. If hybridization is always an important source of variation for adaptation [39–42], we expect to see the mean fitness of admixed populations increase following the initial mixture, particularly if the early hybrid generations show hybrid dysfunction.
To investigate whether there was a significant barrier to gene flow during the initial stages of hybridization, we performed breeding crosses and individually reared in the laboratory all possible combinations of first- and second-generation hybrids that could have resulted from the initial contact of CTS and BTS. Simultaneously, we examined contemporary-generation individuals that were collected from the wild as larvae, bred in the laboratory, and reared to maturity. This experimental approach allows us to describe the genetics of phenotypic variation, understand the intrinsic effects of recombination resulting from hybridization, and evaluate the result of approximately 20 generations of genomic admixture in the wild.