Blind Cavefish: Evolutionary Icon, or an Example of Preprogrammed Adaptation?
The blind cavefish, Astyanax mexicanus, is often cited as a textbook example of Darwinian evolution. The dramatic transformation from a pigmented surface fish with eyes to a nonpigmented cave-dwelling fish with no eyes is presented as strong evidence for unguided evolution. But scientists with a different perspective have started studying this fish to see if the predictions of Darwinian theory actually hold true. Let’s look at some recent research.
Standard Darwinian theory makes the following claims:
Random mutations changed the fishes’ pigmentation.
Random mutations deactivated the fishes’ eyes.
These changes happened over a long period of time.
Fish without eyes and pigment had a reproductive advantage in the cave environment.
Continuous Environmental Tracking
However, there is another model that could explain the transformations of the cavefish. This model is called continuous environmental tracking (CET) and it is design-based. The model presupposes that organisms actively track conditions within specific environments and self-adjust based on predesigned adaptation trajectories. Similar to human engineered agile systems, organisms can make internal changes within a range in response to external changes. This model posits:
Genetic changes are directed and repeatable, not random.
Adaptation may be based on epigenetic or gene expression changes.
Adaptation is rapid since it is programmed and not dependent upon the accumulation of random changes.
A sensory mechanism exists to determine when the fish should undergo these changes.
According to CET, adaptive outcomes are expected to be highly regulated, rapid, repeatable, and, in some cases, reversible. Adaptations are anticipated to encompass a spectrum ranging from physiological changes that happen to an individual within a single lifetime to generational changes, which happen more slowly across multiple generations. This model views environmental changes as triggers for organismal sensing rather than as a selective agent.
Similar Adaptations for Different Cave-Dwelling Animals
Organisms that live in caves are called troglobites (different from the more familiar troglodyte, a human who lives in a cave). They commonly exhibit similar traits to the blind cavefish, including: loss of pigmentation, reduced or absent eyes, enhanced non-visual senses, slower metabolism, specialized reproductive strategies, and extended longevity. This suggests these traits are a non-random, purposefully designed response to the cave environment.
Recent Research on Cavefish
First, cavefish populations are now known “to exhibit repeated, independent evolution for a variety of traits including eye degeneration, pigment loss, increased size and number of taste buds and mechanosensory organs, and shifts in many behavioral traits.” (McGaugh et al. 2014) Are these repeatable, parallel changes an incredible display of natural selection acting on random mutation in the same ways over and over again — i.e., “convergent evolution”? Or is something else going on? Observations of repeatable changes in traits in independent populations are more consistent with a model of preprogrammed adaptability, whereby a designer frontloads genetic variability for different environments at the population level.
One hypothesis is that there exists distributed information within the population whereby different individuals represent different optimizations for unique environments. A while back, I covered morphological changes in guppies, where more research has been done than with the cavefish. For the guppy, distributed information (i.e., variation — though not necessarily generated by random mutation) at the population level seems to be the favored current hypothesis. Guppy traits change based on baked in genetic variation, which allows changes along certain predesigned trajectories in different environments. Importantly, because the novelty generation is not “happening before our eyes,” I don’t find this evidence convincing for neo-Darwinian theory, which requires that variation arise due to random processes.
Second, developing vision turns out to be a very metabolically expensive process, accounting for as much as 15 percent of the resting metabolic rate early in development. Thus, loss of the visual system reduces the amount of energy necessary for development, which is important in the nutrient restricted cave environment. (Moran, Softley, and Warrant 2015) This means there is a purposeful reason why cavefish lose their eyes. Loss of eyesight appears to be a necessary trade-off given the extreme nutrient deprivation of the cave environment.
Third, an individual non-pigmented, eyeless cavefish was shown to nearly revert to a pigmented state after exposure to surface-like conditions (daily cycles of high-intensity, full-spectrum light for five months). (Boyle et al. 2023) This suggests color changes are probably not genetic, but more likely epigenetic and can happen to an adult fish over a period of five months.
Future Research Directions
The next step for researchers is to investigate the molecular mechanisms underlying these changes. Some of this work is already underway. Researchers used an approach called QTL mapping, where homozygous individuals for the trait of interest are crossed to produce an F1 generation. The F1 generation is then interbred or backcrossed to create the F2 or further generations. These generations have a combination of genetic material from the parents. The phenotypic information from these generations is logged and correlated with the genotypic information. This allows researchers to observe which regions of the genome segregate with the traits in question. When this was done for the guppy eye loss trait, QTL mapping implicated 2,408 genes out of a total of 23,042 genes. (McGaugh et al. 2014) Using some other techniques, the researchers lowered their gene list to 30 genes involved in these changes. But, even at 30 genes, it’s hard to imagine how random accumulation of 30 different mutations enabled this phenotypic change in multiple independent populations. Indeed, when many coordinated allele changes are observed as necessary for the development of a phenotype, this data is more consistent with movement of an organism along a predesigned trajectory of adaptation where, given the situation, the fish trades off certain things, like eyes, to function better in its new environment.
Conclusion
Recent research on the blind cavefish has shown that their transformations are reproducible, challenging the idea of random mutation accumulation. Other research has also revealed a significant functional reason for eye loss in cavefish. Additionally, studies have demonstrated that these transformations can occur much faster than previously believed. For example, a single cavefish was observed to reverse pigmentation within five months when exposed to daylight cycles. Recent QTL mapping has identified at least 30 genes involved in eye loss, which means transformations involve multiple genes, suggesting coordination.
While much work remains to be done, the current research trajectory aligns more with a design-based CET model. A deeper understanding of these processes will provide insights into whether the adaptations observed in the blind cavefish result from Darwinian evolution or preprogrammed adaptive responses. The current evidence, though, is highly suggestive.
References
Boyle, Michael J., Brian Thomas, Jeffery P. Tomkins, and Randy J. Guliuzza. 2023. “Testing the Cavefish Model: An Organism-Focused Theory of Biological Design.” Proceedings of the International Conference on Creationism 9 (1): 17.
McGaugh, Suzanne E., Joshua B. Gross, Bronwen Aken, Maryline Blin, Richard Borowsky, Domitille Chalopin, Hélène Hinaux, et al. 2014. “The Cavefish Genome Reveals Candidate Genes for Eye Loss.” Nature Communications 5 (October): 5307.
Moran, Damian, Rowan Softley, and Eric J. Warrant. 2015. “The Energetic Cost of Vision and the Evolution of Eyeless Mexican Cavefish.” Science Advances 1 (8): e1500363.
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