High sexual display trait diversity without measured genetic divergence in a montane hybrid zone involving young species (Habronattus americanus subgroup, Araneae, Salticidae)

Genetic introgression, allele exchange across species boundaries, is a
commonly recognized feature of animal evolution. Under such a paradigm
contemporary contact zones provide first-hand and complementary insight
into the geographic, phenotypic, and genetic details of introgression.
Also, when mate choice phenotypes are conspicuous and variable in hybrids,
contact zones provide potential insight into how sexual selection
interacts with species boundary maintenance, particularly when postzygotic
reproductive isolation is weak. The Habronattus americanus subgroup
includes several recently evolved jumping spider species, with an
estimated age of about 200,000 years, and substantial evidence for
hybridization and introgression. We explored a contact zone involving H.
americanus (Keyserling, 1885) and H. kubai (Griswold, 1979) on Mount
Shasta, California, in alpine habitats that would have been unavailable
(under ice) at the Last Glacial Maximum. We characterized morphological
diversity within the contact zone, including the fine-scale geographic
distribution of hybrid and parental individuals, and assessed genetic
variation using ddRADseq data. Combined results indicate a lack of
measured genomic differentiation between specimens with distinct
morphologies, including individuals with phenotypes of the parental
species. We identified a diverse array of hybrid morphologies, with
phenotypic evidence for backcrossing, essentially forming a phenotypic
bridge between parental taxa. The study area is characterized by more
hybrid than parental individuals, with a significantly larger number of
red-palped morphologies than white and/or yellow-palped morphologies; the
novel, white-palped phenotype is perhaps transgressive. Overall, these
results contribute to a better understanding of the expected ebb and flow
of lineage interactions during the early stages of speciation.