An Evolutionary Enigma: How a Canary Island Spider Halved Its Genome and Rewrote the Rules of Evolution
Over the course of a few million years, the spider Dysdera tilosensis, a species found exclusively in the Canary Islands, has achieved something that defies long-standing assumptions in evolutionary biology. This small arachnid, isolated on an oceanic island, has cut its genome size nearly in half — a phenomenon rarely observed in nature. What makes this discovery even more extraordinary is that despite this drastic reduction, the spider’s genome retains higher genetic diversity than that of its close mainland relatives.
Published in Molecular Biology and Evolution, the study represents the first documented case of such a dramatic genome reduction in an animal species during island colonization. Led by Professors Julio Rozas and Sara Guirao from the University of Barcelona’s Faculty of Biology and the Biodiversity Research Institute (IRBio), with first author Vadim Pisarenco, the research sheds new light on one of evolution’s most enduring mysteries: why genome size changes so drastically across species — and sometimes, in such unexpected directions.
Challenging the Traditional Narrative of Island Evolution
For decades, scientists have believed that island colonization often leads to an expansion of genomes. This idea stems from the “island rule,” which suggests that isolation and reduced selective pressures on islands allow for the accumulation of repetitive DNA and non-coding sequences, leading to genome enlargement. The new findings, however, overturn this long-held view.
The Dysdera tilosensis case shows the opposite pattern: an island species with a significantly smaller and more efficient genome. This not only challenges traditional evolutionary theory but also opens up a profound new discussion on how genomes evolve under isolation and selective pressure.
According to Professor Julio Rozas, “The species D. catalonica has a genome of 3.3 billion base pairs (3.3 Gb), nearly double that of D. tilosensis (1.7 Gb). Despite its smaller genome, the island species exhibits greater genetic diversity.” Such findings upend the notion that small populations or restricted habitats necessarily lead to reduced genetic variation.
The Dysdera Spiders: Evolution in Isolation
The genus Dysdera is a remarkable evolutionary lineage, particularly in the Canary Islands. This archipelago, often dubbed a “natural laboratory” for evolution, has fostered the diversification of nearly 50 endemic Dysdera species — representing about 14% of the genus’s total known diversity worldwide. These spiders have radiated into various ecological niches over a few million years, adapting to the islands’ unique environmental pressures.
Researchers compared Dysdera tilosensis, native to the island of Gran Canaria, with Dysdera catalonica, a related species found in parts of Catalonia and southern France. Using advanced sequencing technologies, they discovered that while the two species share many genetic similarities, their genome sizes diverge sharply. The mainland species possesses a much larger genome with abundant repetitive elements, whereas the island species’ genome is far more compact and streamlined.
Unraveling the Genomic Puzzle
Genome downsizing is a rare evolutionary event, especially among animals. The sequencing revealed that D. catalonica has a haploid chromosome number of four autosomes plus one X chromosome, whereas D. tilosensis has six autosomes and one X chromosome — a difference suggesting complex chromosomal rearrangements during evolution.
“The genome downsizing of D. tilosensis, associated with its colonization of the Canary Islands, is one of the first well-documented cases of drastic genome reduction using high-quality reference genomes,” explained Professor Rozas. “This phenomenon is now described in detail for closely related animal species for the first time.”
Such a case is exceptional because most evolutionary studies, particularly in plants, have documented the opposite pattern — genome expansion through polyploidy (whole-genome duplication). Plants frequently exhibit increased genome size via duplication events, while animals rarely undergo such transformations. Therefore, the reduction in genome size of D. tilosensis offers an unprecedented glimpse into an alternative evolutionary pathway.
What Causes a Genome to Shrink?
Identifying the forces behind genome reduction is not straightforward. In species with similar lifestyles, diets, and habitats — such as D. tilosensis and D. catalonica — ecological and behavioral factors cannot fully explain the difference. Phylogenetic analyses suggest that their common ancestor possessed a large genome of approximately 3 billion base pairs. Thus, the genome reduction in D. tilosensis likely occurred after its ancestors colonized the Canary Islands.
Professor Sara Guirao notes that this finding is paradoxical for two main reasons. First, genome size reductions of this magnitude are rare, particularly over such short evolutionary timescales. Second, the island species’ compact genome contradicts the expectation that reduced selective pressure on isolated islands leads to genome expansion and the accumulation of repetitive DNA.
Instead, the researchers found the opposite: D. tilosensis maintained strong selective pressures that favored the elimination of unnecessary DNA. This implies that populations on the islands remained stable and relatively large for long periods, enabling natural selection to efficiently remove superfluous or redundant genetic material.
A Non-Adaptive Mechanism with Profound Implications
The team proposes that genome downsizing in D. tilosensis may not result from direct adaptation to the island environment but from non-adaptive genomic mechanisms. These include the balance between the accumulation of repetitive elements, such as transposable DNA sequences, and their subsequent removal by selective forces.
Doctoral researcher Vadim Pisarenco explains: “In the study, we observed that the island species not only had smaller, more compact genomes but also exhibited greater genetic diversity. This pattern suggests that non-adaptive processes, combined with sustained selective pressure, could drive the elimination of redundant DNA.”
Such findings challenge the simplistic view that genome size is solely a function of organismal complexity or environmental demands. Instead, genome size may depend more fundamentally on the dynamic balance between the insertion and deletion of repetitive DNA — a tug-of-war that shapes genomic architecture over evolutionary time.
Shedding Light on the Evolutionary Mystery of Genome Size
One of the enduring mysteries in biology is why some species accumulate vast amounts of repetitive DNA while others maintain lean, efficient genomes. Genome size can vary dramatically even among closely related species with comparable morphology and physiology. For example, some amphibians and plants possess genomes dozens or even hundreds of times larger than the human genome, while certain insects and birds have much smaller ones.
The Dysdera study underscores that genome size evolution is not a linear or uniform process. Instead, it reflects the complex interplay of genetic drift, mutation, selection, and environmental stability. In the case of D. tilosensis, strong selection pressures on a stable island population appear to have favored the streamlining of its genetic material — making its genome both compact and efficient.
A New Chapter in Evolutionary Genomics
The discovery of genome reduction in Dysdera tilosensis not only refines our understanding of how genomes evolve in isolated ecosystems but also challenges fundamental assumptions about the direction of evolutionary change. The idea that island species necessarily accumulate “genetic baggage” is now open to reevaluation.
This research may inspire broader inquiries into whether other island-dwelling animals have undergone similar genomic streamlining — and whether such reductions confer hidden advantages, such as increased genetic efficiency or enhanced adaptability.
As Professor Rozas concludes, “Rather than direct adaptation, genome size in these species depends primarily on the equilibrium between the accumulation and removal of repetitive DNA.” This insight brings evolutionary biology one step closer to understanding the enigmatic relationship between genome structure and environmental change — and reveals that, sometimes, evolution’s most powerful innovations emerge not from addition, but from subtraction.
Story Source: University of Barcelona.
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