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Introduction : extent, processes and evolutionary impact of interspecific hybridization in animals
(2008)
Since the time of Charles Darwin, studies of interspecific hybridization have been a major focus for evolutionary biologists. Although this phenomenon has often been viewed as problematic in the fields of ecology, taxonomy and systematics, it has become a primary source of data for studies on speciation and adaptation. Effects from genetic/evolutionary processes, such as recombination and natural selection, usually develop over extended periods of time; however, they are accelerated in cases of hybridization. Interspecific hybrids exhibit novel genomes that are exposed to natural selection, thus providing a key to unravel the ultimate causes of adaptation and speciation. Here we provide firstly a historic perspective of hybridization research, secondly a novel attempt to assess the extent of hybridization among animals and thirdly an overview of the reviews and case studies presented in this theme issue.
Hurd (1952), in revising the Nearctic species of Pepsis Fabricius, separated P. cerberus Lucas from P. elegans Lepeletier based on external morphology and geography. Vardy (2005), in his Western Hemisphere Pepsis revision, combined these taxa and several Neotropical color and structural variants in a broad definition of P. menechma Lepeletier extending across ~11,250 km and two continents. Vardy (2005) synonymized the familiar and well-documented, 160-year-old P. elegans under P. menechma probably because it appeared several pages later in Lepeletier’s (1845) Histoire Naturelle des Insectes. Hyménoptères. Vardy’s (2005) interpretation of Pepsis menechma as a viable species presents a taxonomic and nomenclatural problem. He violated the principle of nomenclatural stability in synonymizing the widely and established species names P. elegans and P. cerberus under P. menechma, a name that had not been used for 160 years. Recent discoveries warrant a re-evaluation of the problematic taxonomy of this species complex. Morphological and ecological divergence of P. elegans and its sister taxon, P. cerberus, combined with their narrow sympatric distribution justifies species recognition. Hurd’s (1952) two species concept for P. elegans and P. cerberus is more practicable, useful, and nomenclaturally acceptable than Vardy’s (2005) P. menechma. Pepsis cerberus Lucas and P. elegans Lepeletier should be reinstated as species and removed from the synonymy of Pepsis menechma Lepeletier.
ZooBank registration. urn:lsid:zoobank.org:pub:F59B3131-74DE-4704-9936-337E380BF3E0
Background: In the speciation continuum the strength of reproductive isolation varies, and species boundaries are blurred by gene flow. Interbreeding among giraffe (Giraffa spp.) in captivity is known and anecdotal reports of natural hybrids exist. In Kenya, Nubian (G. camelopardalis camelopardalis), reticulated (G. reticulata), and Masai giraffe sensu stricto (G. tippelskirchi tippelskirchi) are parapatric, and thus the country might be a melting pot for these taxa. We analyzed 128 genomes of wild giraffe, 113 newly sequenced, representing these three taxa.
Results: We found varying levels of Nubian ancestry in 13 reticulated giraffe sampled across the Laikipia Plateau most likely reflecting historical gene flow between these two lineages. Although comparatively weaker signs of ancestral gene flow and potential mitochondrial introgression from reticulated into Masai giraffe were also detected, estimated admixture levels between these two lineages are minimal. Importantly, contemporary gene flow between East African giraffe lineages was not statistically significant. Effective population sizes have declined since the Late Pleistocene, more severely for Nubian and reticulated giraffe.
Conclusions: Despite historically hybridizing, these three giraffe lineages have maintained their overall genomic integrity suggesting effective reproductive isolation, consistent with the previous classification of giraffe into four species.