A unique spider found only in the Canary Islands is making scientists rethink their ideas about evolution. Unlike most island species that typically expand their genomes, the spider known as Dysdera tilosensis has actually reduced its DNA content by half. This remarkable downsizing in an animal’s genome is now drawing attention.
Researchers from the University of Barcelona, along with the Spanish National Research Council (CSIC) and the University of Neuchâtel, recently published findings in Molecular Biology and Evolution. They discovered that despite its smaller genome, D. tilosensis shows high genetic diversity, which is unexpected for island species.
When comparing D. tilosensis from Gran Canaria to Dysdera catalonica, found in mainland regions like Catalonia and southern France, the differences were striking. The genome of D. catalonica consists of about 3.3 billion base pairs, while that of D. tilosensis has only 1.7 billion. This reduction is accompanied by an unusual maintenance of genetic diversity.
As Professor Julio Rozas explains, “Despite having a smaller genome, the species from the Canary Islands shows greater genetic diversity.” This runs counter to the typical belief that isolated populations face genetic bottlenecks, leading to lower diversity.
The study also highlighted differences in chromosome numbers between the two species. While D. catalonica has four autosomes and one X sex chromosome, D. tilosensis features six autosomes plus the X. This unique arrangement emphasizes the spider’s different evolutionary path.
Traditionally, researchers believed that island species would accumulate extra DNA over time due to reduced selection pressure. However, the case of D. tilosensis flips this theory. Professor Sara Guirao, a senior researcher on the project, noted that the common ancestor of these two spiders likely had around 3 billion base pairs—indicating that the downsizing happened after arriving on the islands.
This phenomenon of genome shrinkage is rare in evolutionary history, especially in animals. It suggests that D. tilosensis underwent a form of genomic streamlining, losing repetitive DNA sequences that were not beneficial for survival.
Vadim Pisarenco, a doctoral researcher, mentioned, “We observed the opposite: island species have smaller, more compact genomes with greater genetic diversity,” highlighting how this case challenges existing assumptions.
Ultimately, these findings imply that natural selection may remain robust in island habitats. Instead of sprawling their genetic material, species like D. tilosensis may adapt by efficiently trimming down unnecessary DNA. This suggests that the population might have been stable and sizable for an extended period after settling in Gran Canaria.
This unexpected twist in evolutionary biology not only opens new avenues for research but also underscores the complexities of nature. For anyone fascinated by evolution or biodiversity, this study presents intriguing insights into how life adapts in unique environments.
