To start with, I do not have a sound knowledge in biology or any formal education in the area.

I was told that one of the definition of a species is a reproductive barrier, which means that if two animals can't reproduce, they are of different species. The barrier can be either the inability of the sperm to fertilize an egg, or a physical trait that inhibits reproduction, e.g. a cricket species that has a different mating song than another species or two species of flies, one that mates on yellow flowers, and the other on red flowers.

But what happens when, while two species can't reproduce, but there is a "chain" of "intermediate" sexual partners that can produce reproductive connection step by step. Like 6 degrees to Kavin Bacon, but with animal sex.

I'll try to explain with an example:

A Great Dane and a Miniature Pinscher dogs can't mate due to obvious size differences. But The Pincher can mate with a German Pincher (a slightly bigger breed of Pincher), which can mate with a Doberman Pincher. And the Doberman can mate with a German Shepard which can mate with a Great Dane.

I've also heard that such things happen with birds and crickets, where there is an original species, from which evolved several other, and while the original species (which still exists) can mate with all the new species. Some new species can't mate with some, or all of the other new species.

How are such species are defined, and at what point dogs stop being dogs anymore?

  • 3
    $\begingroup$ All dogs belong to the Canis lupus familiaris species (which is a subspecies of Canis lupus, that is gray wolf). Great Dane and Miniature Pinscher are not two separate species, but rather two different breeds. A better example would be horse and donkey, different species that can mate to give a mule. $\endgroup$
    – nico
    Commented Mar 29, 2012 at 18:13
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    $\begingroup$ How come they are not of different species if they can't reproduce, while crickets that develop a different mating song are different species? $\endgroup$
    – SIMEL
    Commented Mar 29, 2012 at 18:19
  • $\begingroup$ Are you sure a Great Dane cannot reproduce with a Miniature Pinscher (e.g. through in vitro fertilization)? Anywat, crickets are not domesticated so they don't have breeds! Have a look at this: en.wikipedia.org/wiki/Species_Problem $\endgroup$
    – nico
    Commented Mar 29, 2012 at 19:41
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    $\begingroup$ As an aside, I'm fairly sure that a Miniature Pinscher can mate with a Great Dane and produce viable offspring. While I haven't seen that particular crossbreed myself, a friend of mine used to have a dog who was half Siberian Husky and half Papillon. (And before you ask, yes, the Husky was the mother. The moral of the story being either "love conquers all" or "make sure to keep female dogs in heat away from all males if you don't want puppies", depending on how you look at it.) $\endgroup$ Commented Apr 6, 2012 at 16:47
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    $\begingroup$ You may want to have a look to Ring species and to this post. $\endgroup$
    – Remi.b
    Commented Apr 29, 2014 at 15:54

3 Answers 3


There are many definitions of a species, which may or may not include the concept of reproductive barrier. The Biological Species Concept (BSC) is quite popular and involves a reproductive barrier, but other concepts such as the Phylogenetic Species Concept do not include a reproductive barrier.

Disagreements and confusion also happen over just what the best criteria are for identifying new species. In 1942 the famous biologist Ernst Mayr wrote that because biologists have different ways of identifying species, they actually have different species concepts. Mayr proceeded to list five different species concepts, and since then many more have been added. The question of which species concept is best has occupied many printed pages and many hours of discussion.

Some debates are philosophical in nature. One common disagreement is over whether a species is defined by the characteristics that biologists use to identify the species, or whether a species is an evolving entity in nature. Every named species has been formally described as a type of organism with particular defining characteristics. These defining traits are used to identify which species organisms belong to. But for many species, all of the individuals that fit the defining criteria also make up a single evolving unit. These two different ways of thinking about species, as a category and as an evolving population, are quite different from each other. Wikipedia Species Problem

The Biological Species Concept does have it's problems:

It is also true that there are many cases where members of different species will hybridize and produce fertile offspring when they are under confined conditions, such as in zoos. One fairly extreme example is that lions and tigers will hybridize in captivity, and at least some of the offspring have been reported to be fertile. (see also Lions and Tigers)

Mayr's response to cases like these is that the reproductive barriers that are important for species are the ones that occur in the wild. But even so it is also the case that there are many cases of different species that are known to hybridize and produce fertile offspring in nature. Wikipedia Species Problem

The Phylogenetic Species Concept also has its problems:

it permits successive species to be defined even if they have evolved in an unbroken line of descent, with continuity of sexual fertility. However, because slight differences can be found among virtually any group of organisms, the concept tends to encourage extreme division of species into ever-smaller groups. Phylogenetic Species Concept

When does a dog stop being a dog? Technically all dogs are in the same species: See the definition of Dog, which is a subspecies of the species, Canis lupus, or Grey Wolf. The line between dog and wolf is poorly-defined as there may be hybrid wolf-dogs, since they are both from the same species. Up one level is the family Canidae, which includes wolves, foxes, jackals, and coyote species.


How are such species are defined, and at what point dogs stop being dogs anymore?

This is a bit like the is-Pluto-a-planet-discussion. A group of scientists have to come together and hold a big conference. You have a few principles that you want to adhere to and then it's big groups of people making decisions.

  • $\begingroup$ Please cite a source and/or big conference and name a few principles. $\endgroup$
    – Dale
    Commented Apr 5, 2012 at 20:20
  • $\begingroup$ @JoeHobbit : If you want to pretend that decisions like this are purely made by following a few principles go ahead and write your own answer. I don't think that there's much peer reviewed sociology literature about biology taxonomy but if it exists I would doubt that the result would be that the biological taxonomists follow straightforward principles. $\endgroup$
    – Christian
    Commented Apr 11, 2012 at 0:10

The Biological Species Concept is largely useful in looking at the process of evolution. Since speciation isn't typically (though this isn't always true either!) an instantaneous process, it is useful to observe it in action. The BSC is the best way of approaching this (at least for sexually reproducing organisms). Back to your original question. It might be useful for you to search "ring species" as this is what you are talking about. The most famous example is the Ensantina salamander complex. They are essentially several species/one species depending on your perspective. It is more-or-less a spatial version of the hypothetical dog analogy, but it is also a natural phenomenon that occurs in many other species....


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