Abstract
Natural hybridization may result in the exchange of genetic material between divergent lineages and even the formation of new taxa. Many of the Neo-Darwinian architects argued that, particularly for animal clades, natural hybridization was maladaptive. Recent evidence, however, has falsified this hypothesis, instead indicating that this process may lead to increased biodiversity through the formation of new species. Although such cases of hybrid speciation have been described in plants, fish and insects, they are considered exceptionally rare in mammals. Here we present evidence for a marine mammal, Stenella clymene, arising through natural hybridization. We found phylogenetic discordance between mitochondrial and nuclear markers, which, coupled with a pattern of transgressive segregation seen in the morphometric variation of some characters, support a case of hybrid speciation. S. clymene is currently genetically differentiated from its putative parental species, Stenella coerueloalba and Stenella longisrostris, although low levels of introgressive hybridization may be occurring. Although non-reticulate forms of evolution, such as incomplete lineage sorting, could explain our genetic results, we consider that the genetic and morphological evidence taken together argue more convincingly towards a case of hybrid speciation. We anticipate that our study will bring attention to this important aspect of reticulate evolution in non-model mammal species. The study of speciation through hybridization is an excellent opportunity to understand the mechanisms leading to speciation in the context of gene flow.
Highlights
The consequences of the exchange of genetic material between individuals belonging to different species have long been a matter of debate among biologists [1,2]
Levels of genetic diversity found in the mitochondrial DNA (mtDNA) were high for all species, with S. clymene showing the highest nucleotide diversity, but the lowest haplotype diversity (Table 2)
Our results suggest that the evolution of Stenella clymene does not follow a simple bifurcating tree, but more likely the result of reticulation through the admixture between two other closely related species, S. coeuruleolba and S. longisrostris
Summary
The consequences of the exchange of genetic material between individuals belonging to different species have long been a matter of debate among biologists [1,2]. Hybridization leading to speciation is a phenomenon that has attracted the attention of scientists for some time it is still poorly understood [7,8] It has been reported in plants, fishes, insects and birds, but very rarely in mammals and never in marine mammals [9,10]. Because morphological variation may not always have a genetic basis, hybrid individuals can have the exact same morphotype as one of the parental species [4], leading to instances in which hybrids are cryptic. Their identification is sometimes only possible with the use of molecular tools
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