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How Are Sea Slugs a Failed Prediction of the Evolutionary Paradigm?

Test them all; hold on to what is good.

–1 Thessalonians 5:21

What is your definition of success?

The answer to this question most likely depends on the person you ask. People view success differently.

However, subjectivity is not the case when it comes to scientific theories. Success in science is based on a singular criterion: how well does the theory perform at predicting future scientific outcomes?

Scientific predictions arise as the logical entailments of the theory at hand. In turn, scientists use these predictions to assess the theory’s validity. If experimental results and observations fulfill the theory’s predictions, then scientists consider it sound. If observations and results don’t match the predictions, then scientists are forced to revise and even discard, the theory under evaluation. In short, successful scientific theories have explanatory and predictive power.

It is for this reason many biologists view the theory of evolution as a valid paradigm for interpreting the origin, history, and design of life. And it is for this reason many biologists regard the theory of evolution as biology’s grand unifying theory.

However, the evolutionary paradigm has yet to adequately explain key events in life’s history, such as (1) the origin of life, (2) the origin of body plans, (3) the origin of sexual reproduction, (4) the trigger for the sociocultural big bang and human exceptionalism, and (5) the origin of consciousness. The evolutionary paradigm also suffers from failed predictions, as recent work by a team of neuroscientists from Georgia State University attests.1

Swimming Sea Slugs

The Georgia State University researchers characterized the neural circuits involved in the swimming behavior of a group of sea slugs called the nudibranchs. These creatures serve as an ideal model system to study neural circuits because relatively large neurons make up their neural systems. The sea slugs’ neural circuits are simple and straightforward to map. On top of that, the sea slugs’ neural circuits regulate simple behaviors. These properties make it easy to characterize and, then, manipulate the neural circuitry of these creatures.

Biologists have identified about 2,000 species of nudibranchs. Of this number, about 50 swim with a characteristic left-right motion.

The Georgia State scientists investigated the neural mechanism associated with the left-right swimming behavior of two sea slug species: the giant nudibranch and the hooded nudibranch. From an evolutionary perspective, these two sea slugs share an evolutionary ancestor. In fact, all 50 left-right swimming sea slugs belong to the same branch of the evolutionary tree. (In technical terms, they are monophyletic.)

Predictions of the Evolutionary Model

Given that the left-right swimming nudibranchs are monophyletic, the evolutionary model predicts that the morphology, genetics, and behavior originated in the common ancestor of this group. And, given that the swimming behavior of this group is shared among all members (homologous), the expectation is that the neurons and neural circuitry that control this behavior should also be shared among all members.

The Georgia State scientists say, “. . . Behavioral morphology is often assumed to involve similarity in underlying neuronal mechanisms. . . . Behaviors that are homologous and similar in form would naturally be assumed to be produced by similar neural mechanisms.”2

Sea Slug Neural Circuitry

Consistent with the predictions of the evolutionary paradigm, the researchers discovered that the neurons of the giant and hooded nudibranchs were homologous. But, to their surprise, they discovered that the underlying neural mechanisms that controlled the swimming behavior of the two sea slugs were distinct.

In fact, using a technique called dynamic clamping, the Georgia State scientists could modify the neural circuitry of one sea slug to be the same as the other, all the while inducing the same swimming behavior.

Masking the Failure of the Evolutionary Paradigm

The unexpected discovery of distinct neural circuitry in the giant and hooded nudibranchs stands as a failed prediction of the evolutionary paradigm. So how do the Georgia State scientists respond to this discovery?

First, they point out that their findings support the notion of neural plasticity, with the same neurons supporting multiple neural circuits and varying neural circuits producing the same behavior. But, neural plasticity doesn’t fully account for this finding. If the two sea slugs weren’t part of the same branch on the evolutionary tree, one could argue that the difference in neural circuits represents an example of convergence.

The researchers suggest that perhaps the divergence of the neural circuitry from the neural mechanism displayed by the shared ancestor of the nudibranch is due to a phenomenon they dub neural drift. This doesn’t seem plausible given the importance of the swimming behavior for sea slug survival. Altering the neural circuitry would alter this behavior, compromising the sea slug’s fitness.

In fact, there is no independent evidence whatsoever for neural drift. It is a made-up, ad hoc phenomenon that creates a diversion, masking the fact that the results from this study represent a failed prediction of the evolutionary paradigm.

While this failed prediction is not sufficient to overthrow the evolutionary paradigm, it does justify skepticism about the capacity of evolutionary theory—as currently conceived—to fully explain life’s design and diversity.



  1. Akira Sakurai and Paul S. Katz, “Artificial Synaptic Rewiring Demonstrates that Distinct Neural Circuit Configuration Underlie Homologous Behaviors,” Current Biology 27 (June 19, 2017): 1–14, doi:10.1016/j.cub.2017.05.016.
  2. Ibid.