So, to use an example, let's pick say some little mice. We'll call them Mousius sandius. They live in a large desert valley in New Mexico. They're around 2" long, not counting their long, hairless tail, light sandy brown, bearing litters of 4-8 offspring around 4 times a year, and they eat the seeds of certain plants that grow there, O.K? And, as we know, some of the offspring will be a little bigger, little smaller, slightly different antibodies and disease resistance, little faster, little slower, etc. So the entire species is changing over time, but as long as they all interbreed, they stay the same species, more or less.
Meanwhile, the land is changing over geologic time. Eventually, a river grows wide enough to divide their habitat into two parts that the mice cannot cross. The northern part has a different distribution of plants, more of one kind of snake than another, some different mouse transmitted bacteria, and is around a degree colder and slightly damper than the southern half. The mice on the northern part continue to change over time. In particular, those resistant to a certain strain of Micitis persist, while those that are not die out. And it happens that slightly smaller mice do a better job of evading the snakes they have there. In the south, there are different diseases, and it turns out that slightly bigger mice with a lightly spotted coat do better. They continue to change too, in a different direction.
After thousands of years, the two races of mice have grown significantly different. The southern race is around 2.5" long, with darker brown spots, lives primarily on seeds of the Whosits Whatsis plant, and bears an average of 5 offspring every 11 weeks, while those in the north are around 1.8" long, no spots, eats mostly seeds of the Thingummy plant, and bears an average of 7 offspring every 14 weeks.
At this point, even if you put the mice back together again, they would no longer interbreed naturally. At that point, Biologists say, somewhat arbitrarily, that they have grown different enough that we should consider them two different species. They call the Southern Race Micius whitlingeri. Behold, a new species, via descent with modification plus natural selection. According to ToE, that is how we get new species.
So, Sandy, do you disagree with anything I've said so far?