Since my days as a college student, I have been fascinated by molecular machines and biochemistry and in particular by the irreducible complexity of biological systems. The argument from irreducible complexity against evolution and for design has always held strong intuitive appeal for me, and it has hence become my argument of choice in discussions about the scientific merits of evolution versus design. Here, I will address an objection to irreducible complexity I’ve encountered that attempts to handwave the argument away as though I were missing something obvious. This is, in my judgment, one of the weakest objections to irreducible complexity, though it persists as a popular one — even among experts. It is therefore worth commenting on. I will use as case examples two responses to recent articles of mine.
Recently, I published an essay on the irreducible complexity of male erectile function and the ejaculatory reflex. Nick Matzke, a well-known and long-time critic of intelligent design, was quick to dismiss my argument with the following post on X (formerly Twitter):
Clearly there is no way sending sperm outside the body could ever work unless animals were surrounded by liqui-OhWaitHadn’tThoughOfThat https://t.co/U6QMJ7x5Tz
— Nick Matzke (@NickJMatzke) September 26, 2023
Matzke’s objection is that, in most species of bony fish, reproduction involves external fertilization, where eggs and sperm are released into the water. The male fish releases seminal fluid containing sperm, called milt, into the water, and the female fish releases roe (a mass of eggs). This process is called spawning.  Fertilization takes place externally when the eggs and sperm come into contact. Fish, therefore, lack an external erectile organ like the mammalian penis. Therefore, so argues Matzke, since the male erectile function and ejaculatory reflex are not required for fertilization in all organisms, they are not irreducibly complex. But the mere fact that a system found in a majority of sexually reproducing species is not found in others tells you nothing about how such a system could have come about in organisms that do possess it.
Non-Flagellated Sperm Cells
I also recently published an article in which I discussed human sperm cells, and made the case that certain features of sperm and the seminal fluid exhibit irreducible complexity and therefore challenge evolutionary explanations of their origins. I was challenged on this claim by Jackson Wheat who pointed out, correctly, that some organisms possess non-flagellated sperm cells.
It’s curious that in your sperm IC article you say that sperm can’t fertilize an egg without their flagellum and yet that happens in various groups of organisms. Guess they didn’t get the memo: https://t.co/b74wgGQVTa pic.twitter.com/FFIbwHRl7P
— Jackson Wheat (@JacksonWheat1) July 3, 2023
Wheat is of course entirely correct. Though flagellated sperm cells are found in almost all sexually reproducing animals, there are exceptions.  For example, some crustaceans (such as barnacles) produce non-flagellated sperm. During reproduction, a barnacle extends its penis out of its protective shell and reaches toward a neighboring barnacle (see this video on barnacle reproduction). Interestingly, since barnacles are permanently attached to a surface, there is a strong selection pressure for penis size (to reach neighboring barnacles), and they have the largest penis size relative to body length among animals.  The penis contains the sperm, and it is inserted directly into the nearby barnacle’s reproductive opening. This transfer takes place through direct contact, so that sperm cells do not need to swim through water. The proximity of the eggs to the sperm, within the reproductive tract, ensures fertilization. Non-flagellated sperm are also found in some red algae, such as Polysiphonia, which, upon release, are dispersed by water currents. The roundworm Caenorhabditis elegans also produces non-flagellated sperm, though these sperm cells are amoeboid, meaning that they move by extending and retracting protrusions (similar to how amoebae move by pseudopod formation).  This type of movement is often referred to as “amoeboid crawling.” Despite lacking a flagellum, these sperm cells are able to use their amoeboid movement to travel through the hermaphrodite reproductive tract and locate and fertilize the egg cells. In the case of C. elegans, in spite of being non-flagellated, they nonetheless possess an alternative system that serves the same role as flagella (in conferring motility). The regulation of sperm activity in C. elegans is itself a fascinating subject, and I refer interested readers to this review paper for further detail.
Physical and Conceptual Precursors
That there exists some simpler way of achieving the same objective does not mean that the system under present consideration is reducible. This point was brought out by Michael Behe in Darwin’s Black Box. Here’s what he wrote :
To feel the full force of the conclusion that a system is irreducibly complex and therefore has no functional precursors, we need to distinguish between a physical precursor and a conceptual precursor. The trap described above is not the only system that can immobilize a mouse. On other occasions my family has used a glue trap. In theory, at least, one can use a box propped open with a stick that could be tripped. Or one can simply shoot the mouse with a BB gun. These are not physical precursors to the standard mousetrap, however, since they cannot be transformed, step by Darwinian step, into a trap with a base, hammer, spring, catch, and holding bar.
To take another example, a bicycle requires two wheels in order to perform its function. However, a simpler system than a bicycle can be designed that depends on only a single wheel — that is, a unicycle. But a unicycle cannot be converted into a bicycle simply by adding an extra wheel. Rather, multiple co-dependent changes are required to transform a unicycle into a bicycle.
Likewise, the mammalian reproductive system is not the only possible approach to fertilization. Simpler systems are possible — such as external fertilization, described previously. But this does not mean that external fertilization may be converted into the mammalian system in a Darwinian step-wise manner, making one minor adjustment at a time. Indeed, going from an aquatic to a terrestrial environment requires a lot of co-dependent changes, including reproductively. To use Behe’s terminology, it is a conceptual precursor, but not a physical precursor.
Similarly, the reproductive systems with non-flagellated sperm cells are not physical precursors to those with flagellated sperm cells (such as the human reproductive system). In humans, flagellated sperm are critical to natural (unassisted) reproduction and in fact immotile sperm are a well-known cause of male infertility. Perhaps the essentiality of flagella in human sperm is a weaker example than the other instances discussed in my articles on the design of sperm and the seminal fluid, since one might envision acquisition of flagella as conferring an advantage even while being dispensable, prior to the emergence of the more complex reproductive system that renders them essential.
Nonetheless, I remain skeptical that a structure of the sophistication and design of the sperm flagellum (which you can see animated here) could have arisen in a step-wise Darwinian manner. Sperm flagella require the coordinated action of many dynein motor proteins. Regulatory signals lead to the inhibition of dynein motors on one side of the flagellum while, on the other side, the dynein motor proteins walk along the microtubules as they hydrolyze ATP generated from the mitochondria in the sperm’s middle piece.  The flagellum bends in one direction due to molecular linkers that resist this sliding. The flagellar bending alternates by repeatedly switching the side of dynein inhibition. To add one dynein protein at a time would not lead to a series of intermediate stages all of which confer a selective advantage. Moreover, the sperm flagellum is of little utility until the emergence of the alternating regulatory signals that inhibit the dynein motors on one side of the flagellum, and then the other, in a coordinated manner. Sperm possessing flagella are also significantly costlier to produce in terms of energy and time, and this fitness cost must be offset by a strong advantage, which is only realized after the flagellum is fully operational. A proposed intermediate stage is therefore likely to be eliminated by purifying selection rather than preserved.
Don’t Be So Hasty
In summary, critics of intelligent design should not be so hasty to dismiss the argument from irreducible complexity on the basis of the fact that simpler methods of achieving the same outcome exist. Rather, what needs to be shown is either that these complex biological systems can in fact plausibly be arrived at one small Darwinian step at a time, or that multiple co-dependent modifications are not as improbable as critics of evolution believe them to be.
1. Fitzpatrick JL. Sperm competition and fertilization mode in fishes. Philos Trans R Soc Lond B Biol Sci. 2020 Dec 7;375(1813):20200074.
2. Morrow EH. How the sperm lost its tail: the evolution of aflagellate sperm. Biol Rev Camb Philos Soc. 2004 Nov;79(4):795-814.
3. Hoch JM, Schneck DT, Neufeld CJ. Ecology and Evolution of Phenotypic Plasticity in the Penis and Cirri of Barnacles. Integr Comp Biol. 2016 Oct;56(4):728-40.
4. Ma X, Zhao Y, Sun W, Shimabukuro K, Miao L. Transformation: how do nematode sperm become activated and crawl? Protein Cell. 2012 Oct;3(10):755-61.
5. Behe, Michael J. Darwin’s Black Box: The Biochemical Challenge to Evolution. Free Press 1996, kindle.
6. Lin J, Nicastro D. Asymmetric distribution and spatial switching of dynein activity generates ciliary motility. Science. 2018 Apr 27;360(6387):eaar1968.