Dear Ben Carson:
Here is how evolution works. When we define a species, what we are really doing is describing the most common traits that appear in that species. However, not all of most common traits appear in all of the members of the species. So, while it sound like we are describing an average member of the species, what we are really describing is a combination of the most common traits in a population, not a particular individual.
For example, do a image search for wild rabbits. Most of the pictures will have long ears, black eyes, a similar shaped head and body with very powerful hind legs. They will have tan, grey, white, brown, and even blackish fur in some sort of combination. But when you look closer, some will have longer ears than others, some will have larger or smaller bodies, and the color schemes will vary pretty widely. (And that is just a superficial features.) It is pretty clear that there is significant variation in the traits of a rabbit, even before humans started selectively breeding them for specific traits.
Let us assume we have a colony of rabbits living on the edge of a grassy meadow with some woody brush. The rabbits with tan/grey/white mottled color scheme may have an advantage since they blend in with prevailing colors. The rabbits with darker browns, obvious white tails, or bluish greys will not do as well. However, that does not mean they will disappear, just that they will be less common. Other traits will have similar distributions, larger ears may help the rabbits to hear danger, but too large ears will make them more visible. Even behavioral traits will be distributed throughout the colony. A rabbit with the combination of speed and size may thrive even if doesn’t stay under cover as much, while the smaller or slower rabbit may need to blend into it’s surroundings more thoroughly.
But, for some reason the colony has move into denser woods, or the woods overgrow their meadow. Now the rabbits with blueish greys will blend into the twilight, or darker browns may blend into the trees and gain an advantage. So over a few generations there will be more babies born to the parents who have this sort of coloration and less to the lighter tan and white colors. Or perhaps the longer ears will now be more helpful as the predators will have to move through trees instead of lie in wait in tall grass. So there may be more rabbits with longer ears, but once again it there will be a distribution of ear sizes, color schemes, and other factors. The way we would describe the average rabbit has changed, and yet the traits found in the colony are still essentially the same. Just the number of rabbits showing each trait has changed.
Scientists refer to this as micro-evolution and all but the most ardent anti-evolutionists agree this is common, and that all species drift in this way.
So now we’ll take the next step. Our original of colony of meadow rabbits has done well, and is getting to be too large to support itself. So several of the stronger members of the colony will take part of the colony and leave. One group of the rabbits go into the denser woods as we described before, and some of the rabbits continue on towards the snow line. The most common traits will be found in all three smaller colonies, but the less common traits may or may not be present in each of the groups. The population of rabbits which moves toward the snow line will find itself in an environment more similar to the original meadow for part of the year, but with more snow and less food the rest of the year. The rabbits with lighter tans and white splotches will be harder to see, while the darker rabbits have a harder time foraging for food safely. Due to the cold, rabbits with smaller ears and feet may fare better as they loose less body heat, and animals with smaller bodies may have an easier time burrowing deeper and surviving the leaner, colder winters. These rabbits who are less active or sleep most of the winter may also become more common. The females who only come into heat in the late winter early spring may help their offspring to survive more readily. In short, they are still rabbits, but are beginning to look and behave differently than the rabbits who remained in the original meadow, or stopped in the denser forest.
Meanwhile, the rabbits of the forest colony have continued to get darker and larger. They have plenty of food year round, are able to dig smaller warrens in the tree roots and continue breeding even in the winter months. The rabbits that eat different plants and are able to blend into the forest become the most common members of their colony. In short, they are still rabbits but look and behave differently than the rabbits near the snow line. The gene pools of all three colonies are still pretty similar but the frequencies of the genes have changed. And now that they are no longer interbreeding each population will have its own mutations and the frequency of certain traits is low enough to go nearly unnoticed. By this time, a person classifying the rabbits would likely list each as a different species, based on the behavioral and physical differences and the fact they do not interbreed in nature. The original species of meadow rabbits has split into three species, each of which may flourish or go extinct on its own.
So that idea of one species evolving into another species is not exactly what is happening in nature. One species, for whatever reason splits into several populations, and each population continues to micro-evolve to better fit into its environment. If the populations become isolated enough to stop interbreeding, each population may become a new species as it changes to better fit into its environment. From a distance, (especially if the original species goes extinct) it may appear like one species evolved into another, but the real process is just simply micro-evolution of several populations, each in its own direction.
Thus, if you accept the evidence and acknowledge that micro-evolution exists, you are acknowledging that macro-evolution (a.k.a. speciation) exists since it is the same process. And amazingly enough, there is no real need for an intelligent designer or magical creator. It doesn’t rule out the existence of one, but it does rule out the need for one.