Regulated cascades pop up all the time in biology–particularly in complex organisms. Whether coagulation, development or differentiation of specific cell types, they all follow the same general pattern. To keep things simple, I’m generalizing this whole idea into the task of making a birthday cake:

Creationists–particularly those under the guise of intelligent design–love these. How could something as complex as the coagulation cascade simply appear, intact and whole, due to random chance? There are no intermediate steps where small refinements can be selected for! It’s all or nothing, baby! You know what that means….

Proof of GOD! Praise Jesus!

Really? Some new work, in the field of skeletal muscle differentiation, points to another way beyond the divine to generate a cascade.

If we’re going to figure out a way something could happen by evolution, it’s important to show each step has something can can be refined by selection for the most successful versions.

Let’s say the master regulator here (“Let’s make a birthday cake”) starts off as quite promiscuous, willing to turn on a whole set of tasks all at the same time–some things you’d want in cake, some things you wouldn’t.

The cake is going to be pretty bad. Things that do not belong in cake are going to end up in the finished product. Likewise, the order is all off. It would serve as a cake-like substance, but there’s much room for incremental improvement. For example, “apply ketchup” would be strongly selected against.

We also have something wrong here. The sub-regulators (“place candle” and others) also must be turning on a whole bunch of other smaller tasks. Why couldn’t they be turning each other on as well?

We get a cake again, and a little better better one at that. Now, over time these in-order connections between the sub-regulators will get selected for until they’re nearly as strong as the master regulator. (The out of order connections between the sub-tasks would be selected against; they’re in dashed green to indicate this.) If both the master regulator and the previous sub-regulator must be turned on in order to activate the next step, the result would be a pretty damn good cake. Therefore, natural selection will pick out those cascades with that trait.

The better ordered these steps, the better the cake turns out. So, the strength of the connections between steps, relative to that of the master regulator, will increase.

Eventually, the master regulator will only be able to turn on the first step. The result? A neat cascade:

This isn’t just speculation on my part. Scientists studying the differentiation of skeletal muscle cells have found direct evidence for this sort of evolutionary process. Pretty cool.