[Boogieman]

“Can different Zebras with different chromosomes interbreed?
Answer: no, yes, maybe if.”

The real answer is, “if they can interbreed, they only produce sterile hybrid offspring, which are indeed an evolutionary dead end”.

“So science sticks to natural explanations, and there the fossil record shows many, many such “dead ends”, which we cannot know how or why they died off.”

The question isn’t how extinct species died off, but how a creature that is an entirely new species, unable to produce fertile offspring with its parent species, could ever pass on its genes to continue this process that Darwin dreamed up. Nobody has ever offered a satisfactory answer to that question. The only real conceivable solution is if by a miracle two creature, male and female, living in close enough proximity to each other, received exactly the same type of mutations at the same time, so that they could form a breeding pair of this new species. Of course that just multiplies by a large degree the unlikelihood of every significant step that would be required for evolution to work, making something that is already statistically impossible even more impossible.

“I don’t know if anybody fully understands how or why it happens, but it clearly does happen and not so infrequently as you might suppose.”

You haven’t demonstrated that. Just showing us animals that have different chromosome counts does not tell us how one can gain or lose chromosomes in “baby steps” so that the new species can somehow breed with the parent species and produce fertile, non-hybrid offspring of the child species.

“Fossil species are predicted by evolution theory and many, many thousands have been found over the past 150+ years.”

That fossils have been found is not evidence that other fossils which have not been found actually exist. That is just poor logic. One cannot assume facts not in evidence.

“Further, the scientific term “species” is a matter of definition — when are two populations of, say, Zebras the same “species” and when are they different?”

I agree, and the current definition of species if far too loose to be of much use in these types of discussions, but that is by design. Darwin himself argued in “Origin” for more fluid definition of the term, perhaps because he realized that such a definition muddies the waters and helps to hide some of the obvious flaws in his hypothesis.

“Using that same rate of speciation for other known families & genera produces estimates of staggering numbers of species which evolved but left no fossil records.
And how many Zebra species left no fossils?

So, based on what we know, there must have been huge numbers of species in the distant past whose fossils have not been found.”

There are an awful lot of assumptions in that line of reasoning that we have no good evidence to support. You assume you can deduce a “rate of speciation” base on how many species might have existed in the past versus now, based on what you admit is probably an incomplete fossil record. You assume those species are all correctly classified, without some who are members of the same species having been misidentified, despite the fact that the definition of species is imprecise and those types of errors happen frequently. You assume, of course, that any of those species even have a common ancestry in the first place, which has still never been demonstrated. You assume that the rate of speciation you deduce would be uniform over long periods of time, even though you are simply deriving it through a backdoor method rather than understanding the mechanisms that might cause it, in order to understand how those mechanisms might be affecting by changing conditions that would alter the rate. You assume that the “new species” of zebras we see after your starting point are actually new and didn’t simply exist all the time but were not preserved, or have not been found yet, in the fossil record.

I could go on, but that should suffice to show that this type of methodology is composed mostly of speculation rather than anything reliable.

“But there’s no reason to suppose that each fossil does not represent species that preceded and ultimately produced some of those who came after.”

There’s “no reason to suppose” if you have already taken common ancestry as a fundamental assumption of your hypothesis, as Darwin did, and have never attempted to critically challenge that assumption that the entire theory is built on. However, if you are doing real science, there is always a reason to look critically at fundamental, undemonstrable assumptions that a whole house of cards of other assumptions have been heaped upon.

“This chart is an example of current thinking on ancient pre-humans.”

Yes, and notice that chart has a huge gulf between the left side, which clearly exhibit human physical characteristics and distinctly human habits, some fossils of which can be found in close proximity to each other and even in the same layers in the same location, and the right side, which are clearly ape-like animals that at best exhibit the kind of tool use that we already witness with modern apes. That hypothetical dotted line between australopithecus and the homos is the graphical representation of the lack of actual evidence that any of these apelike creatures ever “evolved” into humans.

“Those skulls are all listed as the same species, Home Erectus.”

Of course they are classed in the same species, since being found in the same place, in the same layers, that can’t be denied. The point is that they are so morphologically different, if they had not been found in such proximity, they probably would have been classified differently. That is why the discoverer commented that this find calls into question the classification of previous finds.

“It’s true that all Homo Erectus individuals found share many common features, but they also share features which distinguish them from all other pre-human species.”

Of course, because Erectus were human and your “pre-human species” are simply extinct apes.

[BroJoeK]

Boogieman: "The real answer is, “if they can interbreed, they only produce sterile hybrid offspring, which are indeed an evolutionary dead end”."

According to this article:

"In certain regions of Kenya, plains zebras and Grévy's zebra coexist, and fertile hybrids occur.[10] "

Plains Zebras have 44 chromosomes, Grevy's have 46.

Boogieman: "The question isn’t how extinct species died off, but how a creature that is an entirely new species, unable to produce fertile offspring with its parent species, could ever pass on its genes to continue this process that Darwin dreamed up."

Again, your problem is false premises.
Offspring born with different numbers of chromosomes are not automatically "a different species."
They still can & do interbreed, albeit with more likelihood of miscarriage or other problems.
If we remember that pre-human populations were often quite small & scattered, then it can be no surprise to imagine two identical twins, or others closely related, with different numbers of chromosomes from their ancestors, but the same as each other, bearing offspring that went on to populate the earth.

Indeed, isn't that just what the Bible tells us happened?

Boogieman: "Nobody has ever offered a satisfactory answer to that question.
The only real conceivable solution is if by a miracle two creature, male and female, living in close enough proximity to each other, received exactly the same type of mutations at the same time, so that they could form a breeding pair of this new species.
Of course that just multiplies by a large degree the unlikelihood of every significant step that would be required for evolution to work, making something that is already statistically impossible even more impossible."

Twins or others closely related (i.e., cousins) born with the same number of chromosomes.

Boogieman: "You haven’t demonstrated that.
Just showing us animals that have different chromosome counts does not tell us how one can gain or lose chromosomes in “baby steps” so that the new species can somehow breed with the parent species and produce fertile, non-hybrid offspring of the child species."

Again, your problem is false premises.
Different numbers of chromosomes do not automatically rule out interbreeding.

Boogieman: "That fossils have been found is not evidence that other fossils which have not been found actually exist.
That is just poor logic.
One cannot assume facts not in evidence."

The important fact in evidence here is that hundreds to thousands of new fossil species are discovered every year and each one fills in a "gap" in the previous fossil record.
The fact that most of these are marine animals doesn't reduce their value as representatives of transitional forms.

Boogieman: "I agree, and the current definition of species if far too loose to be of much use in these types of discussions, but that is by design. "

All such definitions are matters of practical convenience -- whether we call any two populations different breeds, or sub-species, or species, genera, families, etc. is irrelevant except as those words suggest how closely, or distantly, the two populations are related by evolution.
Outside of evolution theory, no such classifications make any sense.

Boogieman: "You assume you can deduce a “rate of speciation”..."
"You assume those species are all correctly classified..."
"You assume, of course, that any of those species even have a common ancestry in the first place..."
"You assume that the rate of speciation you deduce would be uniform over long periods of time..."
"You assume that the “new species” of zebras we see after your starting point are actually new..."

"I could go on, but that should suffice to show that this type of methodology is composed mostly of speculation rather than anything reliable."

First, just so we're clear on this: what's not observed can never be fact, but must remain at best a confirmed theory.
Second, all of those items you list as "assumptions" are based on far more than just "speculation".
Instead they are reasonable conclusions based on huge volumes of evidence.
Third, there's no known reason not to suppose (sorry for the double negative) that the numbers of species alive today is roughly equivalent to the numbers alive in previous geological eras, absent episodes of mass extinctions.

Finally, when new fossil species are found they confirm our previous evolution-based expectations -- again, I'll cite the timeline of pre-human skulls.

Boogieman: "However, if you are doing real science, there is always a reason to look critically at fundamental, undemonstrable assumptions that a whole house of cards of other assumptions have been heaped upon."

Sure, but here's your problem: no other explanation based on natural science has ever been proposed, much less tested for falsification.
Every other alleged "theory" is in fact thinly-disguised theology masquerading as "science".
And propagandists for such "theories" practice the worst sorts of denial to skirt around obvious issues.
I'll refer you to my post #316 above.

Boogieman: "That hypothetical dotted line between australopithecus and the homos is the graphical representation of the lack of actual evidence that any of these apelike creatures ever “evolved” into humans."

The chart, so far as I know, is an accurate representation of what is presently accepted, including an inventory of numbers of individuals found in each category.
The fossils themselves do not tell us how closely each group is related to the others, but importantly, there's no evidence suggesting they are not related.
Indeed, DNA for Neanderthals and other extinct pre-humans suggests they were not only closely related, they also interbred with ancient humans.

Again, your theological opinions notwithstanding, there's no other scientific theory which explains all the evidence we have.

Boogieman: "The point is that they are so morphologically different, if they had not been found in such proximity, they probably would have been classified differently.
That is why the discoverer commented that this find calls into question the classification of previous finds."

Sounds like possibly just one person's opinions, not shared by his colleagues.
I see no reason why different sup-species or breeds couldn't live in proximity -- consider the example of humans & Neanderthals.