For instance, simple copy errors across SIX BILLION nucleotides (the size of the nuclear DNA in a single human cell) are very very very rare, such that only about fifty mistakes usually occur. This also means that NO TWO cells in anyone’s body will ever have identical DNA. (Go ahead and argue statistics with me – it’s possible, but the odds against are mathematically enormous.)
Mutation dusts tiny bits of fuzz across your DNA, and the bits of fuzz that land on your sperm/egg cell producing organs, which make your germ cells, pass on to your children. Evolution is ab so lute ly un stop pa ble.
When some event separates populations, this inevitable drift, over tens to hundreds of thousands of generations, takes that species from Version One to Versions Two and Three, thus resulting in three distinct species. The same factors that take the two populations past the point of interbreeding successfully have the same effect on either one relative to Version One. (Halfway stations: horses-zebra doesn’t work well, lion-tiger works so-so, and horse-doney makes mules – sturdy, and sterile.)
An opponent of evolution has proposed that, since cows losing their legs can’t make whales, the whole idea is bogus. What’s bogus in such a diminutized dataset is that cows are Version HumtyDumptySeven while whales are many different versions in the series HiggledyPiggledy, of whatever it was that both have derived from. And, by the way, a whale’s skeleton really does have teeny leg bones, and its flippers are basically front legs that grew out flat. That tail the cow swishes? It’s the real motor that drives the whale through the water. Both cows and whales do have tails, y’know.
Genetic mutation is constant. Over long periods of time it’s the engine that makes adaptation and speciation possible.
Forgive the long post – a detail relative to Darwin’s finches on the Galapagos Islands:
They are all the same species; they can all interbreed. Whenever it was that the first breeding pair of finches produced young there, the food sources were numerous. Over time populations specialized. Don’t ask how, the facts on the ground make that outcome obvious.
Each one drifted to do a better job of exploiting a specific food source, and along with functional changes to beaks (etc.) to optimize for that food source, their plumage and song drifted too. The plumage and song changes helped each population stay separate, i.e. to form artificially isolated breeding populations.
Cross-breeding still succeeds, though it is naturally rare. Crossbred progeny don’t survive as well because their beaks (etc.) aren’t well specialized for either parent’s food source.
Why are they still one species? Because they can still interbreed. Why is that? Because there hasn’t been the requisite hundreds of thousands of generations. Someone with the time and interest should be able to compare their DNA and work backwards to how long ago that first pair started things off.
Adam, for instance, lived 70 to 75 thousand years ago; Eve lived 200 to 250 thousand years ago. Implication: Eve was in the same situation as that first mother finch, i.e. of all homo sapiens females, only her daughters survived to carry the line forward. The narrowing-down may have finalized fifty generations after she lived – – – but ONLY HER maternal line went forward. Ditto Adam; only his Y chromosome carried forward. (datapoint: there are a few Y’s in isolated areas of Africa with a 400K year pedigree.)
Novelists in the crowd, make up all the stories you can to dramatize such hard times. Did that Adam carry some Super-Mental gene that organized his gray matter in some devastatingly new manner? Or was he just lucky? Whatever, go for it! Start a new ONE FEMALE SURVIVING / ONE MALE SURVIVING fiction genre. (grins)