Definition of Species

I thought the definition of a species was that all members could interbreed with each other… yet I also heard that all dogs are of the same species yet a Great Dane could never interbreed with a Toy Poodle… at least it doesn’t appear that it would be physically possible. Is my definition of a species outdated or just wrong?

If toy poodle egg met Great Dane sperm, it could be a viable match, and reproduction could occur. If chicken sperm met with dolphin egg, nothing would happen. The ACT of “breeding” doesn’t matter.

Okay… then how do we get Tigons or Ligers? I understand that Lions and Tigers, which are different species, have been interbreed successfully. Could a dog egg and a cat sperm create a hybrid? How close do the different species have to be?

This introduction to zoology page has several definitions of species, including:

Note the concept “gene flow” in that second definition. Your Great Dane may not be able to interbreed with a Toy Poodle, but the Great Dane can interbreed with a German Shepherd, say, which could interbreed with some sort of hound, which could interbreed with a terrier or something, and eventually you could get Great Dane genes into a Chihuahua. Note that by this definition, wolves and perhaps coyotes aren’t really separate species from dogs, since genes can and sometimes do flow between these populations. This sort of blurriness isn’t all that uncommon, and is to be expected since species are formed by splitting off from other species.

There is much confusion between “species” and “variations.” When does a variation evolve into a new species? I wish I had some links to give you. Darwin even mentioned this problem. A geographical barrier usually provides for speciation as interbreeding is not possible and different species evolve on each side. However, that is not always the case.

Animals of different species can reproduce, but the offspring is usually sterile; eg., horse and donkey.

What belongs to one species and what belongs to a different one is, IMHO, a man-made distinction. No really hard and fast rules or definitions can be laid down.

Ok, slight correction: two animals are of the same species if they can produce offspring that can also reproduce viably-- ruling out mules, etc.

This is of course one of the strongest pieces of evidence for evolution: namely, that there is no clear boundary between similar species, as we would expect in an evolutionary universe.

It’s possible that dogs are in the process of speciating, due to artificial selection, and that 5,000 years from now we will have two different species of dogs, just like we now have dogs and wolves, where we once just had wolves.

As it now stands, a Great Dane bitch could carry a male poodle’s offspring, but the reverse could of course never happen. The litter would either spontaneously abort, or kill the mother as they grew too big for her.

Cats and dogs are too far removed evolutionarily to interbreed. Tigers and lions are close enough that they still can, although their offspring are not fertile.

Like barbitu8 said, there are a lot of gray areas. The “no interbreeding” definition is more of a rule of thumb, not an absolute.

An interesting sidebar…
I recently heard a report on NPR about genetic studies comparing a harmful strain of E. coli to a beneficial one. It turned out that their genes were only 75% similar (compare that to the 98%-or-whatever similarity of human and chimp genes). So, there is an example of different types of one species that suddenly is forcing microbiologists to evaluate whether they are actually different species or not.

I’m no biologist, but it seems like, historically, species were defined through anatomy/morphology, behavior, breeding, etc. but nowadays genetics is playing a bigger part in the definition.