The gaggle that continue to raid evolutionary biology blogs, patrolling for things that can be naively or intentionally misinterpreted as evidence for their theological views, specifically 'Intelligent Design' (ID), loves to concentrate on complex traits. They claim such traits cannot have evolved because the independent components won't function on their own and the whole breaks down without them. They call that Irreducible Complexity: since you can't take any components of complex traits away and still be viable, such traits could not have arisen gradually by natural selection. Therefore (the IDeologs say), Intelligent Design is true. But this is false on several grounds, not all of them even recognized by biologists, who often defend evolution by needlessly agreeing to do it on the IDeologs' turf.
First, it is IDiotic to argue that if an evolutionary claim is false, therefore creationism is true. That is simply a logical fallacy. If evolution as biologists see it were being misperceived, that in no way provides evidence for any specific counter explanation. Only an IDeolog would make such an argument. It would be just as sensible--that is, as nonsensical--to say that our misperception proved that life came to earth from a parallel universe in a spaceship made of banana peels. We get things wrong or understand them incompletely in evolutionary biology, which is why it remains an active science, but that is not evidence that evolution didn't happen.
Second, the major IDiotic argument about the need for completeness was one Darwin was aware of and even speculated on in regard to the eye, a favorite irreducible complexity example cited from that time to the present day. Darwin suggested ways that primitive light sensitivity could have evolved bit by bit. In what was really striking prescience, his basic speculations have been shown to be about right, because species alive today with 'partial' vision have been found, and genetic components of vision are shared among species with simple as well as complex light reception. Even saying 'partial' vision is a subtle misnomer, because each species uses what it has: the light sensitivity of a worm or bacterium is not partial for their uses, and to use the adjective suggests the IDiological view that humans are at an intended pinnacle, that our vision is somehow more complete or real than a clam's. That's an egocentric misperception of evolution.
Complexity is reducible! It always has been. It's a central aspect of life. Right here and now
Thirdly, and perhaps even more important than the first two reasons why the anti-evolutionary IDeology is just plain wrong is that complexity is typically reducible! The basic IDeologs' premise doesn't have to be refuted because it's not true.
What we know very well is that most traits of organisms are, in fact, the result of multiple interacting factors (gene networks, the polymeric, cooperative nature of DNA and proteins, signaling and receptors systems, gene regulation, and multipart proteins, etc.). And, eyes, too. That is a central fact, and a main point of MT (the blog and the book). We know from thousands of studies (yes, even the GWAS and other 'omics' studies whose excesses we love to point out) that complex traits really are complex at the gene level.
The same studies also show by their very nature--by the very fact that we are doing so many of them in the first place--that each person will have a different genotype, a different set of variants, involved--even if they have the 'same' trait, like stature, insulin levels, blood pressure, or behavior. That is why personalized genetic medicine is unlikely to work nearly as well as advertised. Personalized medicine almost assumes irreducible complexity: enumerate the parts and then any variation in the trait must be due to a broken part that can be identified. But that isn't how Nature works.
Reducible complexity is true even of vision: Color-blind people are people and they have vision, yet they are missing functional light-sensitive genes (e.g., genes that are used in red or green detection, or overall light sensitivity). Visual acuity varies in all sorts of ways among perfectly viable people.
This is typical of biological traits. And recent studies have clearly shown that each of us is walking around with numerous completely inactivated genes, whose 'damaged' sequence variants we have inherited--from parents who somehow had managed without them. One recent paper found that around 165 different genes were completely inactivated (both copies not working) in a typical person. And there are many others in which one of our two copies is not working normally. The combination of inactive genes would be different for each person, but the truth is that we do not normally need all the genes in our genome. That tolerance of variation is exactly the working material that biologists have known is at the basis for evolution from Darwin's own time.
Confirming this in another way, and also very clearly, is that it is routine that a gene experimentally inactivated in a laboratory animal, like a mouse, has serious effects in some strains but little or even no effect in others. A mutation causing a serious disease in humans may do nothing when the same mutation is tested in a mouse, or it may have similarly bad effects only in some strains. That's one of the notorious problems with mouse models for human traits: mice and people share many traits but we make them differently to various extents. There is more than one way to make the same trait. Complexity is reducible.
The reducibility of a trait, to put it in terms even an IDeolog could understand, depends on the combination of genes being viable, not on every gene having the most functionally efficient variants. The importance of component cooperation, a favorite MT word, is in part that various types of cooperation are viable. That aspect of redundancy and variation is one of the central reasons that complexity could evolve in the first place, exactly in the general fashion argued by Darwin and since. No biologist suggests that an eye just emerged wholesale from the primeval slime.
But there's more. Studies of the nature and evolution of genomes shows very clearly that genetic mechanisms arise largely by means that generate redundancy as well as alternative pathways to given outcomes, as cells respond to their local environment. Gene duplication occasionally leads to individuals with two copies of a gene where in their ancestors there was only one (this happens in species generally, not particular to humans in any way). That can provide redundancy, so that one of the copies can acquire mutations that alter what the gene does, while the other copy keeps plugging along with the original function. The new function can be due to mutations in the protein code of one of the copies, or the DNA sequences that regulate when and where the gene is used.
For these reasons, traits are the result of many different genetic contributions, all varying among individuals, each reaching similarly viable traits with different combinations of that variation. Those combinations that aren't functional don't survive or reproduce; those that have an advantage may do better. Over time, the mix of variation, including even the number and set of contributing genes, allow traits to evolve new or altered function.
This is how evolution works, gradually producing new or varied traits. We understand this because we are aware that complexity is often, or even typically, reducible. Although it hasn't been put this way before to our knowledge, this is nothing more than a modern understanding of classical evolutionary ideas.
The IDeologs claim that reduced complexity could not have existed in a stepwise, bit by bit, assembly of a new trait from parts that would not work on their own--that evolution couldn't get from there to here. But the deeper truth is that evolution is both there ('incomplete') and here ('complete') today and has been that way at any or even every time in the past. It isn't just that things have to be assembled over time by different steps, but that they exist at any given time in various steps or stages of 'completeness.' To a great extent, biological complexity is inherently reducible at any time as well as over time.
And one more reason: Of course, we needn't have gone through all of this to convince you that complexity was reducible, after all. That is because the IDeologs disprove their own irreducibility argument by their very existence: one can function as a human being even with a brain that allows you intentionally not to use it to recognize the realities of the world--by not using the thinking complexity they were born with! We would apply this to those who lead the movement, and do or should know better, but not those who they naively lure into adopting its know-nothing IDeology.
Finally, we may make sport of intentionally or willfully self-deluded critics of evolution. For any of those who are sincere but naive, one can only say that it's too bad, and poignant, too, that science shows the evolutionary nature of life, rather than the comforting existence of a benign divinity who graced the earth with our presence. How nice if that could be true! How hard it makes it to understand the injustices and suffering in the world. But science is about the real world, not the one we might wish for.