For one thing, all of the answers to your understandable questions are not yet fully known.We do know though,that these changes in species happened over millions of years.To address one of your examples,the long necks of giraffes--this is how the process works: at first,in what became the giraffes, for instance the animals lived longer if they had longer necks. At first, they ate what was available and over centuries and centuries, there were probably some animals who died out completely, some who became more like sheep and goats and some who became what we now know as llamas. And some became very long necked and were only eventually giraffes.
Splitting of cells even happened very slowly,so none of this was something one can count in a generation or two. And as you know, many mutations were by chance, or seemingly so because of changes in the earth, water and atmosphere.
An animal didn't suddenly start reproducing or switching suddenly from a cold blooded to warm blooded creature.Because survival depended on some changes--some happened slowly, some didn't.
Just as humans are taller and broader now than ever--it did not take a year or two of change.An example that is interesting to follow though is one that is of shoter generation,that of certain moths,that were one color ,and changed after several years to another--that was the new color of their environment.
This moth only lives for a very short time, so many generations can be observed in a few years time. As you say, no animal would sudenly be able to reproduce without another of its kind, nor would an animal suddenly be able to change it's DNA just "overnight".You're correct--it *doesn't* happen like that.there are fish and insects today that can reproduce and there is only one sex or--th--it can reproduce if there are 2 sexes, but one of the 2 are not available.That surely could have been millions of years ago as well.But--I'm really not arguing that most had that abiity.
What I understand is that eventually there were 2 of the same kind that had evolved at the same time, and eventually mated.Before then ther were many differnt kinds that lived, still live and some that died out,some long ago some not so long ago.(as in Dodo birds just for example).
I personally have no trouble beleiivng in God and evolution.I think God started it all,and is still all part of it.And we, humans, are all part of it all,as well.
I imagine that for God, time is not measured as it is by we humans anyway.:-)