Actually, I think you should reconsider that sentence.
If we use the definition first formulated by Dr. Ernst Mayr (see Wiki below), successful reproductive compatibility really is the Gold Standard of speciation.
Horses and Donkeys can produce offspring, but the offspring are infertile.
For the use of the discussion you had in mind, a better example would have been Lions and Tigers, since they are able to produce fertile offspring (Wiki has an article on Ligers too!).
But conundrum posed by Lions mating with Tigers should not be dismissed. As we know, one of the easiest ways to prepare a population for speciation is if there is a physical barrier that comes between two groups. Over time, the genetic variation becomes sufficient that successful mating is no longer possible.
But sometimes the "physical barrier" is really just a "virtual barrier" - - as when one branch of the feline genus gravitated into the open plains of Africa (the lions) while another branch gravitated into the jungles of lower Asia (the tigers). In this peculiar case, the behaviors and appearance of the two cat populations changed more than there DNA did!
Lions became social, and females took the lead in hunting in the presence of a distinctive and larger Alpha male. Tigers became solitary (I wonder whether the common ancestral population was social or solitary?), and they developed those distinctive stripes.
But if we were to discover Lions and Tigers today, in a post-Ernst-Mayr world, would we be so quick to call them distinct species? I think not. For they are still reproductively compatible!
I think the two increasingly "famous" verses in Genesis go a long way to resolving the issue of Biblical "kind" vs. biological "species":
"And God said, Let the earth bring forth the living creature
after his kind, cattle, and creeping thing,
and beast of the earth
after his kind: and it was so. "
"And God made the beast of the earth
after his kind, and cattle
after their kind, and
every thing that creepeth upon the earth
after his kind:
and God saw that it was good."
In Biblical terminology, Lions and Tigers (while looking very different, say, like
very different breeds of dogs), are able to bring forth new generations,
"after their kind", that are able to continue to bring forth more generations.
Compare this to the even more complex situation of the Rabbit "ring species"
that exists in North America: Florida rabbits can breed with Minnesota rabbits,
but not with Alaska rabbits.
Taken as a whole, then, these 3 populations represent a single kind. But should God,
in his terrible wrath smite the Minnesota rabbits for their profligate ways (those dang
Fargo Rabbits!) - - that would be yet another way God helped create two kinds out
of one: for the Alaska and Florida rabbits would never be able to bring forth anything.
If you haven't watched this video on this topic, including footage from a rather
clever Creationist instructor, you really should take a moment to do so now.
I think it's a great video, less than 20 minutes!
**- YEC ALERT - YEC ALERT - YEC ALERT - YEC ALERT - **
This is Post 159 in the thread currently titled: "My Theory About the Flood"
DISAVOWAL - - DISAVOWAL - - DISAVOWAL - - DISAVOWAL
I, George Brooks, have been compelled to conclude that all this analysis about Alaska or "Northern" Rabbits vs. Florida Rabbits (in reference to them being 1 "Kind" or 2 "Kinds" is all spurious. I am publishing a separate thread today (June 3, 2017) discussing the matter, but as far as I can tell, with the crucial help and observations of another BioLogos writer, Evangelical , Kent Hovind,
was told something that either wasn't true or that he misunderstood, regarding the breeding of rabbits in North America. It triggered the further analysis of anti-Creationist blogger "PotHoler" which is presented in detail in the video below. It's only real value is that it treats a fictional case study with some good Evolutionary principles, and would be, on the whole, helpful - - if in fact Hovind's original exposition of the facts had been based in reality.
The irony is, even if his Rabbit study had been based on factual material, it still would not have supported Hovind's analysis of the Biblical view of "kinds".
DISAVOWAL - - DISAVOWAL - - DISAVOWAL - - DISAVOWAL
Post # 84, from the thread: "The mathematical probability of Evolution?"
. . . watch the video below, about Alaska and Florida Rabbits you will learn about one of the examples of common descent that even many Evangelicals accept.
"As introduced by Socratic Fanatic: "Youtube's Potholer54 . . . ['Potholer54' is the name of the YouTube user/channel who has a strong interest in debating with YEC's] . . . has a great video were he explains how even Kent Hovind had come to agree with the idea of evolution---as long the word is avoided . . . . I sometimes use that video as an example of directly observing macroevolution today, because a North American rabbit species has diversified to where Florida rabbits of that species can't/won't breed with [Alaska] rabbits . . . . but rabbits living in the Midwest can breed with both. "
[This video was introduced by @Socratic.Fanatic
at this post:https://discourse.biologos.org/t/what-is-the-evidence-for-evolution/35086/11?u=gbrooks9
The leading cause for common descent leading to 2 populations that cannot breed with each other is through "independent mutation" of the populations (usually when separated by significant barriers or distance). In Birds, such failures to breed can be triggered by "innovations in song", making mates from the other population treat the candidate like a completely different animal.
But I don't think too many people have heard rabbits sing...."
[End of Excerpt from a second thread]
Everything points to God-Guided Evolution. Don't be misled by untrained amateur scientists!