I don’t see why this should be so complicated.
Your reasoning implies that you believe that species diverging means you go from an onion to a potato literally in one giant leap.
Thats obviously not the case. Speciation occurs over a vast time scale. Changes are very gradual, to the point of being imperceptible.
This isn’t like Windows 7 not being compatible with Windows XP, its Windows 7 being incompatible with DOS from 1986. Generation x that you speak of has been separated from and evolving separately from their parent species by a million or more years.
The DNA on the Gametes carry each species’ individual DNA. Key changes in nucleic acids, up to differences in Chromosome numbers make species incapable of procreating. Chalk these changes up to general mutations across countless generations, until finally you have a complete divergence.
Two species, separated from one another, undergoing mutation, selection etc, are going to diverge over time, thats inevitable. Simple analogy, if we take your post, and randomly remove one letter ever year for the next 1000 years, for the first one hundred years your post would still make some sense and people would understand its your post. But by the end of it, it will be complete gibberish with no relation to your original post. Similarly, species will also diverge if at a steady rate you introduce new mutations, that alter their Genome to the point where its incompletely different from the original.
All Variation are due to small differences within the DNA of a given species. These variations can be small and insignificant, and may well be diluted within the population. So blonde hair for example. Some variations are selected for by nature, skin color, height, being the more obvious. Other variations in the genome that are selected for but are less obvious over the short term, Sickle Cell anemia among Africans, or Cystic Fibrosis among Europeans.
Humans were separated from each other for many millennia. To the extent that we evolved separately in very different environments, subject to different selective forces. Hence we became distinct races. Had we been separated for long enough, the variation among our different races would have become so great, that we would no longer have been able to mate since our original genome would have been altered to such an extent as to not be complimentary.
So variations due to mutation, and the process of selection, which promotes certain variation over others, given enough time, given enough change to the genome, and a separation between a species so as to allow only minor if any admixture, must eventually lead to speciation. Hope thats clearer.