Creationists often like to claim that there are two fundamentally different types of evolution: microevolution and macroevolution. They argue that microevolution does actually occur, but only produces small changes within a species or “kind” of animal. For example, most creationists are generally okay with the concept that all finches evolved from a common ancestor, all crows evolved from a common ancestor, all ducks evolved from a common ancestor, etc. However, they draw the line roughly at the taxonomic level of family (e.g., ducks are in the Anatidae family), and they argue that evolution beyond that level (what they call macroevolution) is impossible and has never and can never happen. Thus, they dismiss the notion that finches, crows, and ducks all share a common ancestor. I have written about this before and explained that this distinction is completely arbitrary and meaningless because the exact same evolutionary mechanisms that caused the evolution of finch species could (and indeed did) cause the evolution of all birds. In other words, macroevolution is simply the accumulation of microevolutionary steps, and one inherently leads to the other.
Since I have written a detailed explanation of the problems with creationists’ reasoning, I decided to take a different approach for this post and provide a visual explanation. The image below shows a hypothetical pathway through which turtles could have evolved from their lizard-like ancestors (several of these images are renderings of actual fossils: B6 = Milleretta, A15 = Eunotosaurus, C22 = Odontochelys, B30 = Proganochelys, D37 = Chelydra [modern turtles]; these are just screen shots from Dr. Tyler Lyson’s excellent video). This full progression is, of course, what creationists would consider to be macroevolution, and creationists are adamant that today’s turtle families were uniquely created and did not evolve from a lizard-like ancestor. However, because they accept microevolution, most creationists would have no problem with any particular pair of images, and they would accept that A1 could evolve into B1, B1 could evolve into C1, etc. In other words, each pair of images shows “microevolution” (which creationists almost universally accept), but when we string all of those steps together, we get “macroevolution” (which creationists say is impossible).
You can probably see where I am going with this, but just to be sure, I will state it explicitly. If you are going to say that macroevolution is impossible and turtles could not have evolved from lizard-like ancestors, then which step do you think is impossible? Please show me which step could not have occurred, and justify that claim. Additionally, please explain the obvious transitional fossils. Remember, B6, A15, C22, B30, and D37 are actual fossils, and they perfectly match the expectations for what a transitional fossil should look like (details here). So, if turtles and their lizard like ancestors were uniquely created kinds, then at what point in this progression do lizard-like reptiles end and turtles begin?
At this point, some people will likely be inclined to ignore my questions and harp instead on the fact that this pathway is hypothetical, but that argument completely misses the point in several ways. First, this pathway is only partially hypothetical because B6, A15, C22, B30, and D37 are actual fossils that we have found. Additionally, of course the pathway is partially hypothetical. We will never find every single one of these steps, and we don’t need to in order to defeat creationism. Evolution is very much like the visible light spectrum. Each color gradually fades into the next color without a clear breaking point. In other words, there is a point along the spectrum that is clearly red, and there is a point that is clearly blue, and there is a point that is clearly violet, but there is a spectrum of change in between those points and it is not possible to pick an exact point where the blue ends and violet begins, just as you cannot pinpoint the exact step at which the reptile becomes a turtle as we know it.
The problem is that the fossil record is extremely incomplete. The conditions that are required for a fossil to from very rarely occur, and the vast majority of dead animals are eaten rather than fossilized. So only a remarkably small subset of animals ever become fossilized. Additionally, even if a fossil is formed, it has to survive for millions of years without being eroded away, and we ultimately have to find. As a result, the fossil record will always be incomplete and it is totally unreasonable to expect that we would find every single one of the steps illustrated above. However, what we have been able to find is plenty to refute creationism, because creationism claims that animals were created as distinct kinds. In other words, it does not predict a spectrum. Rather, it predicts that there should be blues and there should be violets, but there should not be intermediate steps. What we actually find in the fossil record is, of course, an incomplete spectrum with lots of intermediates, which is exactly what we expect from evolution. Look at A15 (Eunotosaurus), for example. It’s certainly not a modern turtle, but it’s not a lizard either. It has half the traits of both groups. It is precisely the type of intermediate that creationism says shouldn’t exist.
Finally, the argument that this pathway is meaningless because it is partially hypothetical misses the point because it is absolutely fine to use hypotheticals to defeat absolute claims. Creationists claim that macroevolution cannot happen, and this pathway shows that it can happen. In other words, to defeat the claim that macroevolution is impossible, I don’t need to prove that this pathway actually occurred; rather, I simply have to show that a pathway is possible, which it clearly is. We can of course do this for tons of examples of macroevolution. For example, scientists have known the steps involved in the evolution of an eye for a very long time, and a close examination of the structure of bacterial flagella has shown that it is entirely possible for flagella to have evolved by evolution.
In short, if you are going to insist that macroevolution is impossible, then I want you to look at the evolution of the turtle and tell me which step is impossible (and justify that claim). I also want you to explain the existence of the known intermediate fossils (without committing an ad hoc fallacy), and I want tell me the exact point at which modern turtles first appear. If you are going to comment in defense of creationism, then I expect an actual answer to those questions.
Note: Please read this post before bringing up the fundamentally flawed “irreducible complexity argument” (spoiler alert: it ignores the fact that evolution is blind and each step simply needs to be useful for something, rather than being useful for a particular end product).
Note: Before anyone responds by saying that “Darwin himself said that the notion that an eye could evolve was ‘absurd,’” please realize that this argument misquotes Darwin. What he actually said was that saying that an eye could form naturally seems absurd…until you understand evolution. He went on to explain how the eye could have evolved via natural selection.