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For example, take Canis lupus, the species of dog and wolf. Within their species, can all dog and wolf types crossbreed? We can forget the logistics and assume this is all done through artificial insemination or in vitro methods (let's assume that the in vitro methods would be feasible).

If yes to the main question, does cross-breeding stop at the species class, or does cross-breeding extend across other taxonomic classes? From what I gathered, the taxonomic classes seem somewhat loose in their definitions. Perhaps I should ask, is there a rule "in general" to cross-breeding?

Let me know if this question ought to be refined in any way.

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  • $\begingroup$ To clarify; you are mainly asking if all individuals within the same species can interbreed? $\endgroup$ – fileunderwater Jun 17 '15 at 8:37
  • $\begingroup$ Whether dogs (Canis familliaris or Canis lupus familliaris) and wolves (Canis lupus) are the same species is debatable. I believe the modern consensus tends towards considering them different subspecies of the same species but as far as I know, the details are not entirely clear. Remember that the line between species is blurry at best. $\endgroup$ – terdon Jun 17 '15 at 9:03
  • $\begingroup$ @fileunderwater Yes, for the most part. The choice of species was a bit arbitrary. I'm wondering if there is a general rule as to what individuals can interbreed with others. Within a species just makes the question a bit more specific. $\endgroup$ – zahbaz Jun 17 '15 at 15:03
  • $\begingroup$ Can and do is a big difference a lot of things we consider different species can breed if we force them. $\endgroup$ – John Mar 27 '18 at 13:05
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Species definitions are a somewhat contentious part of biology. There are no hard boundaries in nature that mean "this group here is one species, this group here is another species". Some people don't even believe that species truly exist and there are only gradations of relatedness. That being said, the Biological Species Concept is one of the more popular species definitions.

From the linked website: "The biological species concept defines a species as members of populations that actually or potentially interbreed in nature". Thus, by definition, all animals of the same species can and do interbreed in nature. If two individuals breed in nature, they are considered the same species (under this definition).

Of course, you can already see some problems with this definition. Under the BSC, are a dog and a wolf the same species? Do dogs and wolves breed "in nature"? Of course, fertile half-dog half-wolf animals exist, but does it even make sense to ask whether a domestic species breeds "in nature". I suppose you can say they can "potentially" breed in nature. But how do you define "potentially" breeding? Asking questions about species can lead to many exciting debates.

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  • $\begingroup$ breed and give rise to fertile offspring will be more precise definition? $\endgroup$ – aaaaaa Jun 17 '15 at 0:54
  • $\begingroup$ @aaaaaa Yes, that is correct $\endgroup$ – C_Z_ Jun 17 '15 at 14:16
  • $\begingroup$ I'd say it's "in nature" if the male wolf or coyote comes from the wild to your domestic bitch who happens to be in heat. $\endgroup$ – jamesqf Jun 17 '15 at 17:21
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    $\begingroup$ even this definition runs into problems things like ring species A can breed with B and B with C but A can't breed with C,, it is also completely useless for asexual organisms or extinct organisms. that is why there are more than a dozen working definitions of species. $\endgroup$ – John Mar 27 '18 at 13:01
  • $\begingroup$ Even under the Biological Species Concept, there is no requirement for universality of interbreeding. It is a population level definition, not an individual ones. Genetic incompatibilities that prevent successful reproduction are extremely common. $\endgroup$ – Jack Aidley Mar 27 '18 at 16:05
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No, definitely not. Even under the Biological Species Concept (BSC) the requirement for interbreeding applies at the population level not at the individual level.

The most common example of how the mechanism breaks down are Ring Species. In the simplest case there are three populations - call them A, B, and C - where A can breed with B, B can breed with C, but A and C cannot directly breed. Because there continues to be genetic flow across the whole collective they are considered to be a single species under the BSC. Two real world examples of ring species are Larus gulls and the Greenish Warbler.

However, the biggest example is simply the majority of species which are asexual and thus cannot cross-breed at all. This, and a host of other problems, are why many biologists reject the BSC as a functional definition of the species concept.

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  • $\begingroup$ Try breeding a Chihuahua female with a Great Dane male. Both are dogs. $\endgroup$ – Mindwin Oct 23 '18 at 16:35

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