Biological devolution: a hidden engine of biodiversity

Illustration of biological devolution, showing the simplification of marine organisms — Evolution does not always lead to increasing complexity; it can also involve the loss of traits that have become superfluous in the face of new ecological constraints. Top left: snakes and the regression of their limbs. Top right: the olm, an underground amphibian that has regressed in its eyes. Bottom left: the kakapo, a ground parrot that has lost the ability to fly. Bottom right: the sacculina, a parasitic crustacean that has undergone extreme simplification of its body.
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Scientific Abstract

This article examines the concept of devolution, describing the evolutionary processes of simplification or functional loss, as opposed to the idea of linear progress toward complexity. Evolution has no predefined direction; natural selection favors adaptation, even when it involves morphological regression (e.g., loss of eyes in cave fish, loss of limbs in snakes). The increasing complexity observed in certain organisms is not a universal trend but a collateral effect of self-organization in open systems, far from thermodynamic equilibrium, as described by Prigogine's theory of dissipative structures.

Is biological evolution a continuous progress toward greater complexity?

No, evolution is not linear progress. Devolution refers to cases where adaptation involves simplification or the loss of complex characters (such as the loss of sight in cave fish). Evolution has neither direction nor purpose; it selects what works in a given environment. Complexity is not a goal, but sometimes a side effect of the self-organization of living systems. In reality, nature does not "progress", it adapts, and simplification is often a winning strategy for saving energy.

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Devolution: the other vision of species evolution

An evolution without direction or purpose

Since Charles Darwin (1809-1882), the theory of evolution rests on the principle of natural selection (natural selection, the process by which organisms adapt to their environment). It favors the transmission of characters best adapted to the environment. In the collective imagination, this evolution is often perceived as linear progress toward increasing complexity. Yet nature shows that evolution has no privileged direction: it can also lead to simplified forms, loss of organs, or even a true devolution.

Evolution has neither direction nor purpose. It explores the possibilities offered by the environment and preserves what works, even if this means apparent simplification. As emphasized by Stephen Jay Gould (1941-2002), complexity is merely an accidental consequence of life, not its destination. Devolution reminds us that nature does not "progress", it adapts.

N.B.:
The term devolution is not recognized as a formal concept by modern biology. It is a metaphorical description used to designate processes of functional loss or evolutionary simplification.

"On the Origin of Species by Means of Natural Selection"

Darwin and the word "evolution"

In his theory "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life" (1859), Charles Darwin never uses the word "evolution". The term appears only once, and only in the last edition (the 6th of 1872), in the final sentence: "There is grandeur in this view of life… that from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

Darwin was wary of the word "evolution", because before him it mainly designated a preprogrammed development, an unfolding of an internal plan (notably in Lamarck). Yet his theory was precisely based on the absence of plan or direction.

Devolution, or the art of losing to win better

When simplification becomes an advantage

The continuous ascent toward more intelligence, size and perfection is a vision tinged with anthropocentrism, a misinterpretation of Charles Darwin's theory. Sometimes the best adaptation consists in regressing.

Devolution is not a "regression" in the pejorative sense of the term. It rather describes an evolutionary phenomenon in which an organism loses complex characters in favor of a simpler form. It is not a return to an ancestor, but a new adaptation by subtraction. The driving force is not "regression" but a selection pressure that favors simplicity when complexity becomes a burden.

The illusion of progress: when evolution regresses

Every complex structure has a cost. Each organ, each cellular network or expressed gene consumes energy, requires genetic control and maintenance. When the environment no longer requires certain functions, the selective pressure that maintained them disappears. The species gains in energy economy by simplifying its biological architecture.

The table below gathers some representative examples of biological devolution.

Examples of biological devolution
Organism	Lost or simplified trait	Cause or adaptive context	Commentary
Cave fish Astyanax mexicanus	Loss of eyes and pigmentation	Life in total darkness, energy saving	Losing sight is not an "error" of evolution; it is a remarkable adaptation that allowed these species to conquer an extreme ecological niche.
Tapeworm (Taenia solium)	Disappearance of the digestive tract	Direct absorption of nutrients from the host	Extreme reduction of metabolism and loss of its digestive system, leading to complete parasitic specialization where the organism survives by directly absorbing nutrients from its host.
Snakes (descendants of lizards)	Forelimbs Hind limbs	Adaptation to burrowing Locomotion by undulation	The loss of limbs is correlated with burrowing lifestyles or locomotion by body undulation, more effective for slipping into burrows or pursuing prey.
Whale (Balaenoptera musculus)	Loss of hind limbs	Complete adaptation to the aquatic environment	Hind limbs would be a source of drag and would make swimming much less efficient.
Penguin (Aptenodytes forsteri)	Loss of flight	Transformation of wings into flippers	Aerodynamic conversion to hydrodynamics for efficient propulsion underwater.
Ostrich (Struthio camelus)	Inability to fly	Adaptation to fast terrestrial running	Energy redirected toward running: the residual wings serve for balance and courtship display.
Leafcutter ant Atta cephalotes	Ability to digest cellulose	Symbiosis with fungus Division of labor	Having externalized their cellulose digestion to symbiotic fungi, they have lost this physiological capacity, thus favoring a collective specialization where each member of the colony contributes to a mutualistic food system.
Bird Apteryx australis (kiwi)	Reduction of wings and eyes	Nocturnal and terrestrial life in New Zealand forests	Its plumage has regressed to a downy texture similar to hair, while its terminal nostrils and hyper-developed sense of smell compensate for this simplification, making it a specialized nocturnal predator.
Amphibian Proteus anguinus	Loss of functional eyes	Underground life in limestone caves	Atrophied visual organs replaced by cutaneous sensitivity to light.

The enigma of increasing complexity: Chance and necessity

How does nature, without intention or direction, produce an increasing organization of living matter, from the primitive cell to complex multicellular organisms?

The answer lies in the thermodynamics of open systems and in the logic of self-organization.

The hidden force: Blind processes, organized results

Biological evolution is not progress, but an exploration of possibilities. Evolution has no finality (no goal of moving toward complexity). Each biological transformation is simply the result of local constraints: random mutations, physical and chemical interactions, and natural selection in a given environment. Yet some of these constraints favor the emergence of stable structures, that is, more organized ones.

Thus, the increasing complexity we observe in the biosphere is not a universal trend, but a collateral effect of the physics of dissipative systems.

Living systems as dissipative structures

A living system is an open system, far from thermodynamic equilibrium. It continuously exchanges matter and energy with its environment. According to the theory of Ilya Prigogine (1917-2003), these systems can self-organize when the energy flow exceeds a certain critical threshold.

N.B.:
An ordered structure (an organism, for example) maintains itself only if it receives energy continuously and rejects outward the disorder it produces. Without this exchange, it eventually becomes disorganized. Physically, a constant energy flow can maintain an ordered structure, as long as entropy is dissipated outward.

From the simple cell to the nucleated cell (eukaryote)

The transition to the eukaryotic cell is not a "progress", but the result of a stabilized symbiosis. A primitive cell (archaeon) integrated an aerobic bacterium (a unicellular microorganism using oxygen for its respiration and energy production), which became a mitochondrion (a cell organelle responsible for respiration and ATP production). This process of endosymbiosis (a durable symbiotic association between two distinct living organisms) allowed more efficient use of energy, thus increasing the capacity for self-organization. It is an energy transition before being a hierarchical transition.

The magic of blind processes: Emergence and selection

When similar cells cooperate to better manage energy and nutrient flows, a functional differentiation emerges naturally. Some cells specialize for structure, others for reproduction, others for communication.

Each level of organization (cell → tissue → organ → organism) is not the product of a "plan" but of a progressive stabilization of interactions. The more a system exchanges energy and maintains a memory (genetic, epigenetic, or chemical information), the more it can structure itself without losing its dynamic equilibrium. From a physical point of view, maintaining an ordered and complex structure requires a constant flow of energy.

\( \text{Complexity} \approx \text{Stability} + \text{Energy flow} + \text{Conserved information} \)

References

FAQ: Everything you need to know about biological devolution

What exactly is devolution, and is it a real scientific concept?

Devolution is a metaphorical term, not formally recognized in biology, used to describe evolutionary paths that lead to simplification or loss of functions. It does not designate a "return to the past", but an adaptation by subtraction, where the loss of a complex character (e.g., an organ) becomes a selective advantage when that organ is no longer useful or costs too much energy to maintain.

Why is simplification sometimes an evolutionary advantage?

Every complex structure (organ, gene) has an energy and maintenance cost. When an environment changes and a function is no longer necessary, natural selection no longer maintains it. Individuals that simplify their architecture (e.g., losing a costly organ) can save energy and reproduce better. Simplification is therefore a winning adaptive strategy in stable or extreme environments (caves, parasitism).

How can complexity increase despite the absence of plan or direction?

Complexity is not a programmed progress, but an emergent effect of self-organization. According to Prigogine's theory, living beings are open systems exchanging energy and matter. A constant energy flow can maintain ordered structures (local decrease in entropy). Processes such as endosymbiosis (e.g., mitochondrion) increase energy efficiency, allowing greater complexity. This is a physical consequence and not an evolutionary intention.

Is devolution irreversible?

Devolution is not a simple return to the past. It corresponds to a loss of characters that has become advantageous in a new environment. A return to the ancestral form is theoretically possible, but rare, because genetic constraints and the evolutionary paths already taken limit the possibilities of an exact reversal.

What are the most famous examples of devolution?

The cave fish of the genus Astyanax, which lose their eyes, and parasites like the tapeworm, which lose their digestive tract, are classic cases. We can also cite snakes that have lost their limbs, whales their hind legs, and running birds like the ostrich their ability to fly.

Does devolution challenge the theory of evolution?

No, it confirms it. Devolution illustrates that natural selection does not aim for complexity, but for adaptation to a given environment. It shows that evolution is a process without finality, where simplification can be as effective as complexification.