Many religious and ‘spiritual’ people believe in a concept coined “irreducible complexity” that they mistakenly think applies to organisms here on Earth. The concept alleges that the structure and functionality of some organisms could not have possibly evolved since – proponents claim – the organism could not function or survive if any parts were missing. In other words, they claim that some organisms could not have evolved from simpler ones by a process of developing new parts.
They then take that belief as strong evidence – or proof, for some people – that life on Earth must have had an intelligent designer.
This argument reveals a serious ignorance on the part of the creationist regarding how evolution operates.
The “irreducible complexity” argument mistakenly believes that if creatures and organic structures were not built by a magical power, then the only alternative is that they grew piece by piece into the form we see it in today. But that is not what happens. The reality is that species simply adapt to their environmental conditions. This results in species developing and losing features as conditions change. Each generation builds or subtracts from what its parents were, and uses whatever traits it has for whatever purpose benefits it.
This means that many parts originally developed for different functions than what they do generations later. Parts change, parts are gained, and others are lost — all depending on what helps that generation survive and reproduce at the time.
This also means that some new parts end up developing which were only able to develop since certain other, older parts existed. This new part is sometimes so useful that it enables the organism or structure to survive and reproduce without needing those particular older parts. The result is that the older parts are not necessary for survival and as newer organisms are born with different mutations, they survive just fine with those older parts reduced or dysfunctional until those older parts eventually disappear.
This leaves you with organisms that creationists think are “irreducibly complex” since obviously you cannot just take away the current parts without adding back in the older ones.
To learn more about the irreducible complexity argument and see it debunked, please have a look at these videos:
The Bacterial Flagellum (Clip from NOVA): The most commonly cited example of irreducible complexity is the Bacterial Flagellum. This example was proven to not be an example of irreducible complexity at the Dover trial that resulted in rejecting the ability to teach “intelligent design” in public schools. In fact, the concept of intelligent design was proven – and even admitted by the “irreducible complexity proponents – to be scientifically on par with astrology. You can watch the full documentary here: Judgment Day: Intelligent Design on Trial
Irreducible Complexity (bacterial flagellum) debunked: Ken Miller, a biologist – and believer in God for what that counts – who took part in the case against intelligent design in the Dover trial, explains what the Irreducible Complexity claim is, and why it is false.
Irreducible complexity cut down to size: This video by youtuber QualiaSoup is a clear and well explained video on the flawed assumptions of “irreducible complexity”. The vidoe is quite professional, informed, and provides good examples to demosntrate its points.
Creationist misunderstanding of “irreducible complexity” and how organisms can evolve is probably due to their religious belief that everything has a single “purpose”, so they cannot comprehend change or adaptation. They can only comprehend the idea of a single end product built for one purpose, which is not how evolution works.