It would be the organism's inability to survive. If genetic change push is too far the organism can not survive. Most genetic mutations are fatal.
For example: An iguana can evolve to be able swim in salt water ( that would be micro evolution) but it would be unlikely to evolve to be a bird. At some point it would neither be well adapted for igunahood or for birdhood. :-)
But...Hey!...You are really pushing the envelop of what I learned in the 20 minutes we spent studying the topic on the undergrad level. :-)
When my daughter was 14 she took a biology major's course and ( because of her young age and inadequate background) she and I read every assigned page of her college text together. In a college text that was easily 2 to 3 inches thick, the author devoted 4 or 5 pages to macro-evolution.
Even in biology macro-evolution is really a tiny part of the study of biology.
For example: An iguana can evolve to be able swim in salt water ( that would be micro evolution) but it would be unlikely to evolve to be a bird. At some point it would neither be well adapted for igunahood or for birdhood. :-)
You are assuming that the end point of the iguana´s evolution is ´known´, and is evidence that you don´t really understand the topic.
At any stage of a given species evolution, whether or not it is well adapted for it´s environment is entirely dependent on how it is vis a vis it´s environment at that point and not at any predicted future point.
Logic and mathemathics tell us that no mater how large a given system is or how small any changes we make to it are, if we make enough changes then, eventually, the system will change beyond all recognition to it´s starting point.
You have already agreed that small changes can occur to any animal. So, the question still stands - what machanism limits the ´micro-evolutionary´ changes and prevents them radically altering it in the longer term?
Both a human and a chimp are perfectly viable, despite the 2% difference in their genes and the 6% difference in their genome.
So where does this inability to survive come in? What is nonviable or nonfunctional about changing a working protein by 2% into a nearly identical working protein?
How does a 6% change in mostly noncoding DNA lead to non-viability?
If both organisms bridging this divide are viable, what gives you the impression that an organism that “split the difference” would be nonviable?
Your comments throughout this thread are very interesting. Okay, you and your accomplished husband and college aged daughter took lots of biology courses, and amazingly, what you define as “macroevolution” was barely touched upon.
What’s interesting is that a) you seem blithely unaware of how you certainly must have moved the goal posts wrt your arbitrary distinction between micro and macro.
Of COURSE the courses/texts spent the most time on what you call “micro.” That’s how evolution works! Teeny tiny changes in the alleles over (usually) vast amounts of time that would pretty much never look “evolutionary” or even revolutionary at the time of the chages!
It’s the creationist in you that still expects to see that iguana turn into a bird in one generation. Go back to your texts and class notes that surely have more than 20 minutes worth of the reptilia to avian evolution. you picked a great example with plenty of fossil and DNA evidence to support the facts.
To me, this is sort of a major issue. You admit to tiny changes (ie, salt-water swimming iguanas) but refuse to accept it can go beyond that. Using your example, One iguana population adapts to salt water. Many generations pass. Salt water requires less bouyancy and certain adaptations result in speedier swimmers but less bouyant ones. Many generations pass. Blood proteins change and diets change. Many generations pass. Teeth change in response to the diet change, jaws change as a result and their ears change with the jaws. On and on.
Now the two populations are too genetically distinct to mate successfully. Voila, 2 species. Oh I know... they’re still iguanas. True. Just wait a million years as that salt water lagoon dries up more and more each dry season.
but tha’ts where your brain stops working, unfortunately. That’s when God steps in and either kills off yoru salty population or... I don’t know. He makes them evolve over several million years to adapt?
Hmm.
But I think what you said was in line with what I have been convinced of lately. I think when you say macro evolution, that equates to Darwinism.
In an ealrier thread I wrote...even Darwin himself admitted that without fossil records to bridge the evolution gaps(which dont exist), his theory was bunk. And Darwin never imagined the nano-tech like machinery of the DNA contained in cellular structure. The cell was just a black box to him from which life could spring from nothing. If Darwin knew what we know today about the intelligent design going on within DNA and the complex machinery required for something as simple as blood clotting, he would not come to the same conclusion he did.
But some scientists will never admit the answer lies in God. They would rather suppose it was placed here by some other worldly species...just pushing out the question they dont want to actually find the right answer to.
It sounds like this is more your area of expertise than mine. But I thought my post was supportive of your position. I would be interested in your thoughts.