I'd need to know the specific spiders you are referring to in order to assess the exact reason, but there are a variety of reasons that are possible, and I'll list a few:
1. Though the offspring between the spiders, as well as between the birds, are fertile, the sexual selection common to birds renders their hybrids genetic dead ends (they don't get to mate, even though they physically could), and this doesn't apply to the spiders (these particular spiders aren't choosy, so the hybrids of spiders aren't genetic dead ends).
Are you just making things up that sound good to your ears? The DNA of those offspring of finches was more robust than the finches they came from? Did you even bother to read the Grants paper? I didnt think so.
2. The spiders hybridize a lot more frequently than the birds do. If hybridization events are infrequent and rare, they won't have much impact on genetic drift, and thus the two populations genetically are practically indistinguishable from populations that never hybridize.
Another misinformned statement off the top of your head.
Those finches are mating so frequently, two of them are what they term merging into a third.
3. The bird hybrids have noticeably reduced fertility, and the spider hybrids don't. That is, only the females of the cross are fertile, etc.
They observed no loss of fertility, even declaring the offspring more fit for survival than the parents.
4. If it's more than 2 finch species we are talking about, the "species A, species B, and species C" situation may apply. That is, even though hybridization between various species is common, they can't all interbreed with each other, so they can't all be considered the same species. A species population can't have a portion of it incapable of breeding with another portion.
Its 15 plus two on the mainland and the DNA tests showed every one of them were interbreeding with every other one. So "messy" was the word they used was their genetic strains, they were indistinguishable.
5. The first generation of bird hybrids are fertile, but inevitably give rise to infertile lineages. This is why all the species in the genus Nepenthes aren't considered to be the same species, even though they all are capable of interbreeding, most produce natural hybrids, and nearly all first generation crosses are fertile (the only first generation cross I know of that isn't fertile is N. ventricosa X N. gracilis). Basically, once there are 4 or more different species in the cross, the hybrid plant gets a huge drop in fertility, and I don't know of any crosses with more than 6 consecutive different species that are fertile at all (back-crossing with the one of the original species parents can increase how many generations remain fertile).
Every single one is fertile.
These are just some of the possible reasons why the finches aren't considered to be the same species, yet the spiders are.
And every single one of them was wrong. How are you even arguing those finches are the same species or not when you havent even read the material?
-_- I literally gave the "species A, B, and C" example straight out of a college biology textbook. That is, officially, if species A and C don't breed, even if both interbreed with species B frequently, they ARE considered 3 separate species. That is, if in nature, A and C never breed with each other, they MUST be considered separate species. This is because part of the definition of species IS that the population be of freely interbreeding individuals producing fertile offspring... which is also why there is a different definition of species entirely for bacteria.
So if A, B and C are freely interbreeding???
Yet you admit that is the definition, enough if A and C arent mating you'd consider them separate species. So if ALL are interbreeding and producing fertile offspring? Whats your college textbook say about that?
The category "species" is just another category we made to organize the world around us. There isn't even a standard degree of genetic difference between species, so this idea that evolution needs to "demonstrate speciation" is rather funny to me. After all, the genetic change within a population over time that remains the same species can match the degree of genetic change that causing speciation in another. It's also funny because speciation is one of the founding observations of the theory; that is, it was observed before the theory on why it happens was written.
It has never been observed - observation of speciation requires a definition of species. If you cant see that then there is indeed no hope. The entire theory rests upon the deffinition of species, which is why there is such a huge argument about it going on in the biological community. If those finches are the same species, then speciation never occurred. Since they were never reproductively isolated - the reason Darwin called them separate species, they never underwent speciaition.
Nah, there is a standard by which species are defined. It's just not a standard real life organisms like to fit.
I mean, I can clearly point to a part of this that is indisputably red, and another that is indisputably orange, but I couldn't tell you the precise point red ends and orange begins. Taxonomy is like trying to categorize every shade of these colors as falling within just 6 categories; obviously, the ones at the borders are disputable. The fact that organisms don't actually fit so easily into our taxonomy system drives people nuts.
And if you dont observe similar creatures mating, you then look for other defining characteristics to define if they are a species. But a species has always been the largest gene pool potentially capable of interbreeding. So if two birds mate, those wo birds belong to the same species.
So why dont we call the Afro-Asian a different species than the Asian and African? They are clearly all different. Clearly occupied different ecological and geographical niches at one point. I mean even the American indian was reproductively isolated from the rest of the human population going on 10,000 years which beats those finches never by a long shot. Want to call finches that interbreed a separeate species? Fine, I will accept that if you agree Asians and Africans should be classified as a separate species. You have no more reason to call one a separate species than you do the other.
I certainly expected someone to claim mutation to the ALX1 gene by now, the only distinct mutation they could find.
Pfft, taxonomy is not a part of the theory of evolution. Evolution is currently taken into consideration when determining taxonomy, sure, but they aren't a part of each other any more than physics theories about light have anything to do with what names we give colors.
That's a riot since it is only taxonomists that can declare a new species or subspecies or not.
Taxonomy (biology) - Wikipedia
Without taxonomy you have no theory.....
I haven't been following whatever discussion about spiders you've been having, so I'd need a species name to confirm if you claim about the only difference being appearance is true or not.
And yet you argued against finches being the same species without following that too, didnt stop you.
"For example, these happy face spiders
look different, but since they can interbreed, they are considered the same species:
Theridion grallator."
It's especially difficult to categorize organisms that rely on us to even exist. There's at least 1 dog breed that can't even reproduce without human intervention anymore. What do you define as "natural reproduction" in a species that is the product of unnatural selective pressures?
Whats unatural about it, are we not part of natural evolution? Ahh, so if famine forces a Mastiff and Husky together thats natural and the Chinook is a product of evolution. But that same Chinook produced because man brought them together who is of himself part of natural evolution, then its not evolution?
Yet you see no problem using man-made experiments to make your case for evolution. So we can discard all those experiments as unnatural, and dismiss them out of hand, correct? Would this not be in line with your statement? Ahh, but its different if it supports your viewpoint, right? Then its acceptable as proving natural selection even if it is just as unnatural. I am glad we settled this so I know its ok for me to dismiss every experiment by man as an unnatural thing. Man that sure makes it easier when you can dismiss whatever you like and accept only what you like.
Assuming you are talking about the Galapagos finches, it's due to breeding behavior and songs. That is, they don't all interbreed, and interbreeding between them is generally infrequent.
It occurred so often they used the term "messy" to describe their genetic heritage. Two are breeding so intensely they are merging into a third.
-_- why would I even care? I'm well aware of the fact that taxonomy is not perfect, and my personal opinions don't even always match the mainstream when it comes to this. I guess I see a shade everyone puts in the "red" category as more "orange" than most people do, whatever. Sometimes imperfect systems are better than nothing at all, though I personally would prefer a system that functions on the basis of genetic similarity rather than anything to do with breeding. But, uh, that would open up the potential for populations to contain individuals that can't interbreed, which is its own can of worms.
Wait, I thought taxonomy had nothing to do with evolution, so why would it matter if it was imperfect or not?
Then start with what can be determined by by breeding. Then if animals are not observed to breed by similarities and DNA testing. You work down the list. You dont throw out step one just because it is inconvenient and would cause you to have to correct mistaken classifications because someone named Darwin classified them incorrectly because he honestly (tho mistakenly) thought birds that were interbreeding were reproductively isolated. I can list other birds if you would like that were classified as separate species based upon song patterns and plumage and the belief they did not interbreed. Then when found to be interbreeding were reclassified as the same species, just subspecies. Why is Darwin's finches different just because they contain his name?