I was typing on a phone a 4 in the morning with a lunatic 2 year old on the loose, so apologies for not getting completely across the answer.
Like I stated initially, and I mean this sincerely, get the eBook for The Greatest Show on Earth: The Evidence for Evolution. It is a very clear and easy to read explanation of all your questions and others. The examples used are things that you could go out and measure for yourself, which is why it is such an excellent introduction to the subject.
Once you have devoured that, the answers will be as clear as day. However, I'll try again for you now that I have some quiet.
Your major issue is that your assuming the child, via a single mutation in a single gene [or it seems many genes by your assumption that child is a new species or genus] is suddenly a new species, it just does not work that way. Look up some of the recent news articles on the rise of blue eyes over the last 6000 years. We blue eyed all descended from one individual on the Black Sea who had a mutation on one of the eye color expressing genes. By no means are blue eyed folks a different species, however, continue with such small changes over hundreds of thousands, or millions of years and you get something new.
Evolution only works if two brunettes can spontaneously create a blonde, by way of some unknown, outside catalyst.
Evolution works via a KNOWN internal catalyst that does create spontaneous changes. It is called Random Mutation. There is no rhyme or rule to it, it just happens and is measurable across species. You are seeking for an invisible hand guiding the process and it is not there. Once you let that preconception go, evolution becomes far easier to understand.
In order for a group of "apes" to become humans, the "apes" must give birth to a population of children incapable of reproducing with their parents' genus. They have a different set of chromosome and are now incompatible. This is the same, observable condition as modern man attempting to breed with his closest, simian ancestor. It is a required step in the creation of an entirely new genus, as opposed to a branch of species which may/may not be capable of inter-species breeding.This is an incredibly rare occurrence (and has never been observed once, in recorded history), but would need to happen for macro-evolution to hold true.
You are assuming here the change is huge and sudden. Trace us and Chimps back about 4 million years ago and you find our most recent common ancestor. This was not a chimp or a human [Nakalipithecus is one of the candidates]. From this or a similar species, at the most basic level, two populations developed slowly over time, due likely to geographic separation from natural movement,or by some climatic or tectonic event. These two populations, each developing different random mutations and responding to the environment developed different forms and eventually were unable to produce offspring if they mated, a critical break and one of the good lines in the sand for defining a species. Roll forward a couple of more million years and you have two very distinct species, which still share many traits and have a common ancestor.
Not only does this rare and strange occurrence need to happen once, it must happen simultaneously within a colony. Several females in a population of apes must give birth to a new genus at exactly the same period of time (within a single generation), else the new creature has no mate with which to breed. It would live out its life, never be capable of offspring and die out. So it needs enough partners with the same genus and species to continue existing. This must also occur for macro-evolution to work, since the new species would die out at the end of its own lifetime.
This is incorrect.
A single individual, who can still breed within the group, gains a small change and passes it on to its offspring. Other individuals do this over the course of time as well, adding to the initial small change and it is the cumulative changes over a long period of time that allow a new species to arise, especially when a parent population is broken up for the reasons set out above.
As humans we short circuited the breakup of our gene pool via technology, so the human population is now sharing genes across the entire earth. No population was isolated for long enough for change to take hold in enough genes to prevent continued interbreeding.
As we all know, genetic diversity plays a part in how well a species can breed. Too much in-breeding causes genetic defect until a species is simply incapable of producing viable offspring. This now means that this rare occurrence, spread across many different females within a group, must now travel OUTSIDE of the group, geographically. A separate group of "apes" must give birth to modern humans (or at least homo-erectus) at the exact same time as the first group.
Now how does the second group experience the same catalyst for a chromosomal change at exactly the same time as the first group? What outside stimuli would need to be present and what COULD be present over such a distance? This would be akin to a baby suddenly being born with three eyes, then realizing the entire hospital has three-eyed babies. THEN realizing the hospital 25 miles away all have three-eyed babies as well.
There is no arising of the same random set of mutations in a second group as outlined above. An ancestor population is split up by whatever means and then random mutations passed on generation to generation create differences in the two populations. Check in on them in a million years time and 2 different species now exist, but both can trace their lineage back to the original population.
