To rephrase my question more articulately:
Speciation begins when two groups within a species starts to become reproductively isolated, and is complete when the two groups can't interbreed (for the purposes of this discussion). But then how is this possible when any time a reproductively-isolating mutation occurs in a member of that group, the member carrying that mutation could no longer breed with the other members? Therefore, speciation is theoretically a negative feedback loop. In this context, how does speciation occur?
One of the mechanisms documented by James Patton is chromosome evolution, where changes in chromosome number and shape occurred in pocket mouse and reproductively isolated the different species-complex (https://www.jstor.org/stable/2406859). But then the question still remains that the first mouse to have a different number of chromosomes would not be able to have offsprings because there are no other members with the same number of chromosomes as it does.
unless the genetics of reproductive isolation is continuous i.e. a gentically reproductively-isolated member of a species could
A. mate and have offspring with the general population but with greater cost, for instance, hybrid offspring could be at higher risk of dying in utero or be at higher risk of becoming sterile.
B. hybrid offspring, will go on to produce more offspring, still at higher cost, unless they do so with each other AND with the general population. creating a subpopulation that mates more successfully (i.e. at lower cost) with each other than with the general population.
But I have not found any articles saying reproductive-isolation is continuous rather than binary.