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I thought that the definition of species is "can interbreed"

From Wikipedia:

The wildcat (Felis silvestris) is a small cat found throughout most of Africa, Europe, and southwest and central Asia into India, China, and Mongolia. Because of its wide range, it is classed by the IUCN as Least Concern. However, crossbreeding with housecats is extensive and has occurred throughout almost the entirety of the species' range.[2]

So why does it have a different name than the house cat?

From Species at Wikipedia:

A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, the difficulty of defining species is known as the species problem.

While I understand that sometimes problems arise, what makes biologist mark the wild cat as a distinct species from the house cat? If they are genetically different, how different?

Donkeys can breed with horses. The result is sterile. Tigers can breed with lions with lots of complication. Wildcat seems to interbreed with housecat without problem whatsoever. Or does it? That's actually the essence of the question.

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There are many definitions of species - look at the wikipedia page for an overview (this is a large and tricky subject). Some hybridization is not uncommon between "proper" species (often in narrow contact zones), also with fully or partially fertile hybrids. Definitions of species in these cases often comes down to genetic similarity/dissimilarity of populations. Even though hybridization is taking place it usually does so at very low rates, so the gene pools stay relatively distinct. The biological species concept (which is what you are referring to) is also problematic when it comes to e.g. ring species.

As for wild cats and domestic cats, my feeling is that modern evidence (e.g. molecular studies and phylogenies) places domesticated cats firmly as a subspecies of wild cats (Driscoll et al. 2007, 2009), with the domesticated cat as Felis silvestris catus. In the study by Driscoll et al. (2009), domesticated cats fall in the same mtDNA clade as F. silvestris lybica. However, domesticated cats were traditionally described as a separate species (by Linnaeus), and this has stuck and some still label them as separate species (as does Mammal Species of the World). I can imagine that some who argue for species status now might use a mixture of ecological, phenetic, and biological species definitions.

From Driscoll et al. (2009):

In contrast, the world's domestic cats carried genotypes that differentiated them from all local wildcats except those from the Near East. Domestic cats show no reduction in genetic diversity compared with the wild subspecies (37), thus giving no indication for a founding genetic bottleneck. Multiple genetic analyses produced concordant results, in each case tracing the maternal origins of cat domestication to at least 5 wildcat lines (A through E, Fig. 2B) originating in the Near East. The domestic cat is referred to as a sixth subspecies, F. silvestris catus, although it is clear that domestic cats derive very recently from F. silvestris lybica (37).

... the vast majority of sampled domestic cats fall into the same mtDNA clade, which also includes F. silvestris lybica...

There is also evidence that hybridization can help adaptation by facilitating gene transfer (even though hybrid fertility/viability might be low), so you might want to check out e.g. results from the Heliconius Genome Consortium (2012).

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Am I (an Asian) within the same species of whites and blacks? I can interbreed with white or black females though I haven't really tried that. What makes the wild cat a different species? –  Jim Thio Nov 7 '13 at 9:59
    
@JimThio That is not a very useful analogy (for many reasons). For one, genetic differences between human populations are much smaller that within population variability, while wild and domesticated cats have relatively separate gene pools and hybridization is rare. However, hybridization can become a problem in small threatened wild cat populations (e.g. when they have a problem to find mates), which will dilute their gene pool. –  fileunderwater Nov 7 '13 at 10:41
    
@JimThio See the paper by Driscoll et al (2009) for molecular evidence for placing domesticated cats as a subspecies. –  fileunderwater Nov 7 '13 at 12:02
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@JimThio you are attaching way too much importance to the ability to interbreed. Donkeys can breed with horses, as can zebras, tigers can breed with lions, and various other feline combinations are also possible. The ability to interbreed is not suffiecient to describe a species. In fact, defining what makes a species is still the subject of many PhD theses. –  terdon Nov 7 '13 at 13:37
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The "species" concept is an attempt to fit a simple human conceptual category onto a reality that is not that simple. The reality is a continuum of reproductive compatibility between an individual and any other organism rather than a binary compatible/incompatible relationship which would actually be a basis for defining "species". –  mgkrebbs Nov 7 '13 at 19:58
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