o.k. to explain what ToE says about how we get new species. (apologies to those of you who have heard this before.)
Let's use lizards, since we had an example of that. Let's say there's a specific species of lizard, Skitteri arendi. (pronounced "RND", get it?) It lives near the sea. It's around 3" long, brown with orange spots, lays around 15 eggs at a time, and grows to adulthood in around 90 days. It eats tiny bugs. With me so far?
Now, as you know, some of the arendi babies will be 2.9", and some will be 3.1". Some will have more orange spots, some lighter spots, etc. Some will be a little quicker, and some will have better resistance to different diseases. That's not controversial; we all know that. That's called "descent with modification." That means that offspring resemble their parents and each other, but not exactly. There's some variation.
Over time, the whole species may change a little, like say increase resistance to arendi fungus, but since a species is a reproductive population, any changes get stirred around in the gene pool and the whole population remains a single species.
With me so far?
But imagine that a few arendis crawl onto a boat, which sails to an island, and they climb off. So you have a group of arendis that are isolated from the main group. That group is also going to continue to change a little over time. On the island, there are different bugs, different diseases, different ecology, different predators. So the environment is going to lead to different traits surviving and getting the opportunity to reproduce. Say, for example, there's no natural predators, so there's no need for the fastest to survive. But there's a scarcity of food, so the one's that can survive on other bugs get to live, whatever.
Over time, the population of arendi2 is going to be a little different from arendi1. After say 1000 years, these creatures average 4" long, are brown with orange and yellow spots, lay around 40 eggs into holes in the sand, grow to adulthood in 4 months, and so forth. At the point when they can no longer breed with the main population, biologists call that a new species. We'll call them Skitteri yecii.
According to ToE, that's how we get new species.
That's the core of the theory.
Please let me know that you have read and understood this much, before I go on.