Definition of species and other Evolution questions

First of all, this is NOT an evolution vs creationism vs intelligent design thread. Assume for the purposes of this thread that the current theory is correct. There are questions that are brought up by the theory of evolution and the modern synthesis of evolution and genetics.

My question has to do with what makes species distinct from near "relatives". The most often cited definition of species says that 2 different species cannot produce viable offspring, but often, in taxonomy, we classify species based on habitat and appearance. So how do we tell where individual variation ends and speciation begins? Are all dogs the same species? or are there some breeds that cannot have viable offspring because they are so different? A chihuahua/St. Bernard mix anyone? We classify wolves as a different species from dogs, but I've heard that wolfdogs can actually be viable (capable of reproducing).

In the case of evolution, can a line be drawn between one species and its succeeding specy? If we look at human evolution, It seems fairly obvious that Homo Erectus and Homo Sapiens are different species, but what about the individuals who would have come in between? At what point do we stop classifying fossils as one species and start classifying them as the next?

If anyone has any thoughts on this I'd love to hear them. It would also be appropriate to post other related questions.

Difference in fossil classification comes from changes in physical characteristics. I'm unclear as to whether or not there have been any conclusive genetic studies. But as far as I know two different species of homanids likely had the same chance of interbreeding that most mammals from different groups have today.

As for different animal species being able to mate, I know there are several species that can not reproduce after being interbred. However, I would assume that variant species that exist in teh same ecotone interbreed all the time.

Awhile back, I watched a television series that showed very good evidence of Neanderthal and Cro-Magnon not only living in close proximity to each other but also signs of trading among the peoples and at the tail end of the show, they showed remnants of an adolescent skeleton which showed both Neanderthal and Cro-Magnon characteristics, suggesting that both species interbreed.

Originally posted by Robtard
Awhile back, I watched a television series that showed very good evidence of Neanderthal and Cro-Magnon not only living in close proximity to each other but also signs of trading among the peoples and at the tail end of the show, they showed remnants of an adolescent skeleton which showed both Neanderthal and Cro-Magnon characteristics, suggesting that both species interbreed.

It was a show somewhat like that that got me thinking about it. I know that some interspecies breeding happens, but like Capt_Fantastic said, most hybrids are sterile. There are exceptions however. The wolfdog I mentioned being one, I've also heard that Polar Bears and Grizzlies could be close enough genetic matches to mate if their territories overlapped (I don't remember the reference, so take that one with a grain of salt).

My point I suppose is that individuals vary within species, and some species are very similar in almost all ways but habitat - in other words, the variation between 2 species is sometimes less than the variation within a single species. So the current scientific definition of species must be inadequate.

In its most basic "sense"... a species is defind as a group of organisms that can interbreed with eachother and produce fertile offspring.

A turkey and a rabbit cant breed...hence different species.

Horses and Donkeys can breed...but can't have fertile offspring...hence different species.

This has some holes though:

1. Fossils don't breed. All dogs (C. domesticus) look very different from eachother, but can all interbreed...amazingly. Breeding is not always able to be determined from fossil evidence.

2. It assumes things breed sexually. Bacteria bud...there is no "breeding." How do you define species there?

These holes are often covered up by DNA evidence (which is exceedingly convinient) and other morphological distinctions.

Tradition and the opinon of the scientific community also play MAJOR roles.

No defintion of species has been created that accurately applies to all life.

there are some examples of species which are genetically compatible yet do not breed only because of some minor physiological difference

for example, 2 species of north european gulls that do not breed together simply because they have different eye ring colour...and experiments were conducted whereby they had lenses placed on one species to match the other and they breeded successfully

yet they are considered different species in nature because naturally they would never interbreed

a tiger and a lion are perfectly genetically compatible to produce offspring (although the "liger" is also incapable of breeding) yet would never interbreed in the wild because they are geologically seperated...one being in Africa and the other in Asia....they are seperate species

aint ligers just the cutest

Originally posted by docb77
My point I suppose is that individuals vary within species, and some species are very similar in almost all ways but habitat - in other words, the variation between 2 species is sometimes less than the variation within a single species. So the current scientific definition of species must be inadequate.

Well, some characterists develop because of a shared ancestor, while others similarities arise due to living in the same environment ofr so many generations. So I'm sure that cro-magnon and neanderthal may have had a far distant relative in common, but the geological relation at the rise of both species would have had nothing to do with their similarities.

Originally posted by jaden101
aint ligers just the cutest

and unreasonably huge.

just a note, most hybrid species have bred. The majority of hybrids are not viable, however, many have been. Mules have been bred to both donkeys and horses. Ligers and Tigons have both been bred to both Lions and Tigers. They classify those offsprings as Ti-Liger, Ti-Tigon, Li-Ligers or Ti-Ligers.

all this talk about "incapable of breeding" and "impossible" to breed is unwarranted. A quick google search will not only give you the specific information on any of these hybrid species procreating but will also give you pictures of the offspring.

as for wolves.........wolves can be bred to just about any breed of dog.

Originally posted by Evil Dead
just a note, most hybrid species have bred. The majority of hybrids are not viable, however, many have been. Mules have been bred to both donkeys and horses. Ligers and Tigons have both been bred to both Lions and Tigers. They classify those offsprings as Ti-Liger, Ti-Tigon, Li-Ligers or Ti-Ligers.

all this talk about "incapable of breeding" and "impossible" to breed is unwarranted. A quick google search will not only give you the specific information on any of these hybrid species procreating but will also give you pictures of the offspring.

female ligers can be rebred...but males cannot