At least one member of the genus Capra will be familiar to most people - the domestic goat Capra hircus. The goats are a mostly Palaearctic genus of alpine bovid, easily recognised by their distinctive large, ornamented horns. Different sources disagree on just how many species there are, but species are divisible into five distinct 'horn groups' (theres a good picture of all five in Pidancier et al., 2006, if you have access).
The bezoar (Capra aegagrus), the ancestor of domestic goats, has long, laterally-compressed, scimitar-shaped horns. The ibex group (Capra ibex and others) also has scimitar-shaped horns, but oval or subtriangular in cross-section with transverse ridges on the front - it is this group that causes problems taxonomically, with some authors recognising a single species and others recognising a number of geographically isolated species. The Pyrenean ibex (Capra pyrenaica) has lyre-shaped triangular horns. The eastern tur (Capra cylindricornis) has subtriangular horns curving in an open spiral. The most spectacular horns of all probably belong to the markhor (Capra falconeri) which has massive corkscrewing horns - go to Tetrapod Zoology and scroll down the page to see a photo showing just how unbelievable these appendages are. There are variations within these groups, of course - the western tur (Capra caucasica) has less well-developed transverse ridges on its horns than other members of the ibex group, while the extinct Portuguese ibex (Capra pyrenaica lusitanica) had horns with less of a backwards sweep than the other subspecies of Capra pyrenaica (see The Extinction Website).
Hybridisation between Capra species is apparently not uncommon. The webpage for a 2000 IUCN Workshop on Caprinae Taxonomy refers to hybrids between domestic goats and wild species such as Nubian ibex (Capra nubiana), "Asiatic ibex" (I'm not sure if this is supposed to refer to Siberian ibex [Capra sibirica] or one of the two tur species) and markhor being bred in order to improve stock. Pidancier et al. (2006) suggest that hybridisation may explain why relationships in Capra inferred from mitochondrial cytochrome b (which is maternally inherited) contradict those inferred from Y-chromosome sequences (which are paternally inherited) and morphology. In the former case, the Siberian ibex is sister to all other species, while in the latter, the division is between a clade formed of the markhor, bezoar and domestic goat and a clade of the ibexes and turs. The Pyrenean ibex is very close related to the Alpine ibex (Capra ibex) - most individuals were even identical in Y-chromosome data - and despite its autapomorphic horn form appears to be a fairly recent derivation from the ibex (I'd also say that the nested position of the Pyrenean ibex and the eastern tur among the more typical ibex forms makes it all the more logical to recognise the isolated ibex populations as separate species). One individual of the eastern tur actually clustered among bezoar sequences from the same location rather than with other tur sequences, providing another possible case of hybridisation (though personally, I'd want to have this checked against the possibility of misidentification or mix-up).
Pidancier, N., S. Jordan, G. Luikart & P. Taberlet. 2006. Evolutionary history of the genus Capra (Mammalia, Artiodactyla): discordance between mitochondrial DNA and Y-chromosome phylogenies. Molecular Phylogenetics and Evolution 40 (3): 739-749.
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