Small island spider sheds unnecessary DNA to survive

On the volcanic slopes of Gran Canaria, a small orange spider has done something scientists didn’t expect. Over millions of years, Dysdera tilosensis has cut its genome nearly in half.

Most animals isolated on islands expand their DNA, but this one went the opposite way. It simplified. The discovery has forced researchers to rethink what isolation really does to evolution.

Spider downsizes its DNA

The findings, published in Molecular Biology and Evolution, reveal that Dysdera tilosensis holds a genome of just 1.7 billion base pairs.

Its mainland cousin, Dysdera catalonica, carries around 3.3 billion. That’s almost double. Yet the smaller genome shows more genetic variation – a twist that puzzled the research team.

“The genome downsizing of the spider D. tilosensis, associated with the colonization process of the Canary Islands, is one of the first documented cases of drastic genome downsizing using high-quality reference genomes,” said Professor Julio Rozas, the study’s co-author.

His team’s work shows that a reduced genome doesn’t necessarily mean a simpler life form.

Evolution on island time

The Canary Islands act as a living experiment for evolution. Separated by ocean, their habitats push species to change fast. In this isolated setting, nearly 50 unique Dysdera spiders have evolved.

Scientists from the University of Barcelona, the University of La Laguna, the Spanish National Research Council, and the University of Neuchâtel joined forces to trace what made Dysdera tilosensis different.

The analysis revealed that the island spider lost massive chunks of repetitive DNA, while the mainland species kept most of it.

The deleted parts mainly included transposable elements – bits of DNA that can copy and move around the genome without serving any clear purpose. In Dysdera tilosensis, evolution appears to have trimmed the unnecessary code.

Spider DNA cleaned naturally

Losing half a genome sounds risky, yet Dysdera tilosensis managed it cleanly. Large, stable populations helped natural selection act efficiently. Over time, selection removed pieces of DNA that offered no benefit.

The study supports what’s known as the Mutational Hazard Hypothesis, which argues that when populations stay large and stable, natural selection weeds out redundant genetic material.

The deletions mostly hit the autosomes, leaving the X chromosome largely unchanged. That detail gives scientists a clue: the process wasn’t random. It followed patterns shaped by recombination and selection pressure.

“Interestingly, despite having a smaller genome, the species from the Canary Islands shows greater genetic diversity,” said Rozas. It’s a strange but elegant outcome – less DNA, yet more variety.

DNA shaped by peace

At first, the team wondered whether the smaller genome made the spider more efficient. Maybe it helped with metabolism or reproduction. But that idea didn’t hold up. Both spiders live in similar habitats, eat similar prey, and look alike.

“Differences in genome size cannot easily be attributed to ecological or behavioral factors,” said Professor Sara Guirao, co-author of the study. “The common ancestor had a large genome. This indicates that the drastic genome reduction occurred during or after the arrival on the islands.”

That means the change wasn’t an adaptation to the environment. It was more of a long-term cleanup.

“Populations in the Canary Islands remained numerous and stable for a long time. This would have made it possible to maintain a strong selective pressure and eliminate unnecessary DNA,” said doctoral student Vadim Pisarenco. Stability, not struggle, gave the spider’s genome room to slim down.

Erasing redundancy, not function

The research shows that most deletions came from intergenic regions – the stretches between genes that don’t code for proteins. Those sections often fill up with repeated DNA sequences.

When unequal recombination occurs, entire fragments can vanish. Birds with small, efficient genomes show the same pattern. Over thousands of generations, Dysdera tilosensis likely experienced that kind of steady trimming.

It’s evolution by deletion. No catastrophe, no crisis – just continuous editing that left the genome compact but functional.

“This study supports the idea that, rather than direct adaptation, genome size in these species depends primarily on a balance between the accumulation and removal of this repetitive DNA,” the authors conclude.

Spider DNA shows simplicity

The case of Dysdera tilosensis flips a familiar story. Evolution doesn’t always add complexity. Sometimes it cuts back.

The island spider didn’t grow new features – it refined old ones. That kind of change doesn’t grab headlines like dramatic mutations, but it’s just as powerful.

In the end, the spider teaches a quiet lesson: survival doesn’t always mean adding more. It can also mean knowing what to lose. In that sense, Dysdera tilosensis didn’t just adapt to its island – it perfected the art of genetic editing.

The study is published in the journal Molecular Biology and Evolution. 

Image Credit: University of Barcelona

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