On the Question of Evolutionary Stasis

Let us turn to the topic of evolutionary stasis.

Many people have probably encountered popular memes or articles claiming that certain organisms—such as horseshoe crabs or coelacanths—have remained unchanged for hundreds of millions of years. These organisms are sometimes described as examples of evolutionary stasis, a situation in which a lineage appears to undergo little or no evolutionary change for extremely long periods. Fossil records frequently reveal similar patterns: certain fossil forms persist across tens of millions, or even over one hundred million years.

Specimen of the extant coelacanth species Indonesian coelacanth (Latimeria menadoensis)（圖片來源：Claudio Martino，採用 CC BY-SA 4.0 授權）

At first glance, this seems puzzling. Evolutionary theory describes evolution as a continuous process in which genetic variation and natural selection gradually alter populations over time. If evolution proceeds continuously, why do some fossil lineages appear to remain unchanged for such long spans? This question has led to significant debate among researchers. Some scientists argue that evolutionary stasis is merely an illusion created by incomplete observations, while others have proposed theoretical frameworks to explain why it might occur.

Three Modes of Natural Selection

Most people learn early in biology that evolution is driven primarily by two mechanisms: genetic mutation and natural selection. Mutations arise randomly, but selection acts in a directional manner. Mutations that do not hinder survival may persist in a population and spread through successive generations.

To describe how natural selection influences populations, evolutionary biologists commonly distinguish three major modes of selection.

The red curve represents the original distribution of a trait, while the blue curve shows the distribution after selection. From top to bottom: directional selection, stabilizing selection, and disruptive selection.（圖片來源：Ealbert17，採用 CC BY-SA 4.0 授權）

1. Directional selection occurs when evolution toward a particular trait becomes advantageous. Over time, the average characteristics of the population shift in that direction. For example, animals living in polar environments often evolve increasingly white fur because it provides camouflage in snowy surroundings.

2. Stabilizing selection occurs when individuals that maintain an intermediate or original trait have the highest survival and reproductive success. In this case, extreme variations are removed, and the population remains relatively stable in appearance.

3. Disruptive selection occurs when individuals at both extremes of a trait distribution gain an advantage over intermediate forms. As a result, the population gradually splits toward two contrasting forms, potentially leading to the emergence of distinct species.

Among these three patterns, stabilizing selection appears capable of producing long periods during which organisms show little visible change. Because of this, some biologists have proposed that stabilizing selection might explain apparent evolutionary stasis.

Punctuated Equilibrium

The upper figure shows a phylogenetic tree based on Punctuated Equilibrium, while the lower figure shows a phylogenetic tree produced under the conventional model of gradual evolution.（圖片來源：Miguel Chavez，採用 CC BY-SA 4.0 授權）

Punctuated Equilibrium

Another influential idea is the theory of punctuated equilibrium. According to this model, once a species forms it may remain morphologically stable for long periods. Evolutionary change occurs rapidly during relatively brief intervals, often associated with major environmental shifts that fragment populations and trigger speciation.

Supporters of this theory argue that large populations tend to maintain genetic stability. Even when beneficial mutations arise in a few individuals, gene flow within the large population may dilute these changes, preventing them from spreading widely. In this view, gradual evolutionary change is rare or negligible.

Punctuated equilibrium appears to fit the fossil record quite well. Many fossil lineages show long intervals of morphological stability punctuated by sudden appearances of new forms. This pattern can also help explain events such as the Cambrian explosion, during which many new animal groups appear relatively abruptly in geological strata.

Evaluating the Competing Ideas

However, when we examine living organisms, punctuated equilibrium becomes more difficult to support as a universal explanation. A well-known example involves the peppered moth during the Industrial Revolution. As industrial pollution darkened tree bark, darker moth variants gained a survival advantage because they were less visible to predators. The population rapidly shifted toward darker coloration, demonstrating that beneficial genetic variants can spread through large populations rather than being diluted.

The dark peppered moth that gained an advantage during the Industrial Revolution（圖片來源：Chiswick Chap，採用 CC BY-SA 2.5 授權）

On the other hand, stabilizing selection also fails to explain many observed patterns in modern organisms. Numerous populations display ongoing directional change under varying environmental conditions.

If neither model fully explains evolutionary stasis, how can the fossil record be interpreted?

To answer this question, biologists approached the issue from two different directions.

The Underlying Reality

The first approach compared fossil groups that appear to show evolutionary stasis with those that show obvious morphological change. Surprisingly, the overall amount of variation through time is often similar in both groups. The difference lies in the pattern of change. In groups labeled as "stagnant," traits tend to fluctuate back and forth around a stable average, while in other groups the change proceeds consistently in one direction. When viewed over long time scales, these oscillations may create the illusion that little change occurred.

The second approach examines differences among living representatives of supposedly stagnant lineages. Horseshoe crabs provide a useful example. Although the four living species look remarkably similar externally, detailed studies reveal substantial differences in physiology and internal structure. These organisms are not evolutionarily static; instead, much of their evolutionary change occurs in features that are not immediately visible in their external morphology. Their body plan may already function effectively in their ecological niche, while finer adjustments continue to occur in internal systems.

Male brown anole (Anolis sagrei)（圖片來源：Hans Hillewaert，採用 CC BY-SA 4.0 授權）

An illuminating study published in 2023 investigated wild populations of the brown anole (Anolis sagrei). Researchers divided a two-and-a-half-year observation period into five intervals. If one compares only the beginning and end of the study, the population appears almost unchanged. Yet when the intermediate stages are examined, each interval reveals directional selection acting in different directions. The pattern resembles a mountain ridge rising and falling repeatedly while remaining near the same overall elevation. This study demonstrates how repeated shifts in selective pressures can create the appearance of stasis when viewed over longer timescales.

These findings suggest that evolutionary stasis may often represent an observational artifact created by fluctuating selective forces rather than a true absence of evolution.

Are the Earlier Theories Completely Wrong?

Despite these insights, neither stabilizing selection nor punctuated equilibrium should be dismissed entirely.

Although stabilizing selection has rarely been documented clearly in natural populations, it certainly exists in principle. Artificial breeding provides clear examples. When humans maintain particular traits in domesticated animals or plants, stabilizing selection operates to preserve those characteristics across generations.

Similarly, punctuated equilibrium highlights an important evolutionary mechanism: allopatric speciation, in which geographic isolation plays a crucial role in the formation of new species. This process is well documented in nature. A striking example occurs in the Galápagos giant tortoises, where at least fifteen distinct species inhabit different islands. Each island environment acts as a natural barrier that promotes divergence.

Evolution is therefore an intricate process influenced by many interacting forces. No single theoretical framework fully captures every evolutionary pattern. Each model becomes most useful when applied to the contexts where its assumptions match biological reality.

Additional Evolutionary Terminology

Before concluding, several additional evolutionary concepts are worth mentioning.

Anagenesis refers to evolutionary change within a single lineage that does not produce branching. The entire population gradually transforms over time, making it difficult to determine where one species ends and another begins.

Cladogenesis represents the opposite pattern. Here, an ancestral species splits into two or more distinct lineages. On a phylogenetic tree, cladogenesis corresponds to branching points.

Species complex describes a group of closely related species that resemble one another so strongly that distinguishing them taxonomically becomes difficult. Killer whales are often considered an example; what is currently recognized as a single species may actually represent several closely related species. Recent studies have already identified distinct lineages in the northeastern Pacific Ocean, and further research may reveal additional species in other regions.

Ring species occur when a species spreads around a geographical barrier in a ring-like distribution. Neighboring populations interbreed successfully, but populations at opposite ends of the ring may become reproductively isolated. This situation blurs the boundary between species and illustrates how gradual evolutionary divergence can lead to speciation. A well-known example involves the herring gull complex around the Arctic. Several subspecies form a continuous chain of interbreeding populations, yet the terminal populations in Europe can no longer hybridize.

Ring species provide a particularly compelling illustration of evolutionary processes because they simultaneously display ancestral forms, intermediate populations, and newly differentiated groups within a single geographical system.

Distribution map of seven subspecies of the herring gull (Larus argentatus), showing that although the northern European and British subspecies are adjacent, there is no red arrow indicating hybridization between them.（圖片來源：Frédéric MICHEL，採用 CC BY 3.0 授權）

Author: Bai Leng

Reference:

1. Brian Charlesworth et al. (1982). A NEO-DARWINIAN COMMENTARY ON MACROEVOLUTION. Evolution

2. Voje, K.L. (2016). Tempo does not correlate with mode in the fossil record. Evolution

3. Stroud, J.T., Moore, M.P., Langerhans, R.B., Losos, J.B. (2023). Fluctuating selection maintains distinct species phenotypes in an ecological community in the wild. Proc Natl Acad Sci U S A

4. 鄒徵廷. (2024). 有的生物幾億年沒變樣，是演化真的停滯了嗎？. 返朴

5. Niles, E., Gould, S.J. (1972). "Punctuated equilibria: An alternative to phyletic gradualism." Models in Paleobiology

6. Evolution and Natural Selection. (2010). University of Michiga.