Rosa Rubicondior: Refuting Creationism - Evolution By LOSS of Genetic Information

Dysdera catalonica

Dysdera tilosensis

Fotografies: Marc Domènech and Pedro Oromí

Deciphering the mechanisms of genome size evolution - Current events - University of Barcelona

For years, creationists have confidently assured anyone who’ll listen that evolution can’t possibly work, because losing genetic material is always disastrous — rather like claiming a book can’t be edited without collapsing into meaningless gibberish. Yet nature has an unhelpful habit of ignoring such pronouncements and getting on with things regardless. And now, a tiny spider living quietly in the Canary Islands has delivered another inconvenient data point: it’s been shedding DNA at a remarkable rate, and doing perfectly well in the process.

Researchers led by Julio Rozas and Sara Guirao, from the Faculty of Biology and the Biodiversity Research Institute (IRBio) at the University of Barcelona, have shown that a spider endemic to the Canary Islands has lost almost half its genome in only a few million years.

The spider, Dysdera tilosensis, is a close relative of the mainland species D. catalonica and the familiar British woodlouse-hunter, D. crocata, yet is morphologically almost identical to both.

The findings have been published in the journal Molecular Biology & Evolution.

This discovery runs counter to a general pattern in evolutionary biology, in which adaptation to oceanic island environments often involves increases in genome size. Rather than undermining evolution, this unexpected result enriches the scientific debate over how and why genome size changes during evolution.

It also raises awkward questions for creationist dogma. Why would an intelligent designer equip spiders with almost twice as much genetic material as they actually need? And how would one distinguish such closely related species or show a transition from one to the other in the fossil record, if genome size — the key difference — leaves no trace in fossils?

Background^ The Dysdera Spiders.

Dysdera crocata

Joseph Berger, Bugwood.org CC BY

The genus Dysdera comprises more than 280 known species of ground-dwelling spiders, found primarily across Europe, North Africa, and nearby islands, including the Canary archipelago. They are commonly known as woodlouse-hunters, reflecting their specialised diet: many species feed predominantly on woodlice, using powerful jaws to pierce their tough exoskeletons.

Dysdera spiders are easily recognised by their reddish bodies, elongated chelicerae, and pale abdomens. They are typically nocturnal, hiding under stones, logs, and leaf litter during the day. Most species do not build webs to catch prey; instead, they actively hunt, relying on stealth and strength rather than silk.

Despite striking variation in chromosome number and genome size across the genus, Dysdera species are often remarkably similar in external appearance. This makes them a useful group for studying evolutionary processes, including ecological specialisation, island colonisation, and genome evolution. In many cases, species have evolved in isolation on islands, producing clusters of closely related yet ecologically distinct forms — a pattern seen in the Canary Islands, Madeira, and the Mediterranean region.

The research is described in a news release from the Universitat de Barcelona.

Deciphering the mechanisms of genome size evolution
In a few million years, the spider Dysdera tilosensis — a species endemic to the Canary Islands — has reduced the size of its genome by half during the process of colonization and adaptation to its natural habitat. In addition to being smaller, this genome is more compact and contains more genetic diversity than that of other similar continental spiders. The discovery, published in the journal Molecular Biology and Evolution, reveals for the first time how an animal species managed to reduce its genome by almost half during oceanic island colonization.
This study contradicts the more traditional evolutionary view — on island-colonizing species, whose genomes are larger and often have more repetitive elements — and expands the scientific debate on a major puzzle in evolutionary biology: how and why does genome size change during the evolution of living beings?

The study is led by Julio Rozas and Sara Guirao, experts from the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the University of Barcelona. The paper, whose first author is Vadim Pisarenco (UB-IRBio), also involves teams from the University of La Laguna, the Spanish National Research Council (CSIC) and the University of Neuchâtel (Switzerland).

This research offers a surprising perspective to explain a phenomenon that has puzzled scientists for decades: the size of the genome — the total number of DNA base pairs encoding an organism’s genetic information — varies enormously between species, even those with similar biological complexity.

A smaller genome in an island species: an evolutionary paradox?

Spiders of the genus Dysdera have diversified spectacularly in the habitats of the Canary Islands. This archipelago is considered a veritable natural laboratory in which to analyse how species and their genomes evolve in a context of geographical isolation. Nearly 50 endemic species — 14% of all the species of this genus described in the world — have emerged since the islands were formed a few million years ago.

Applying advanced DNA sequencing technologies, the team has analysed the genomes of two evolutionarily close spider species: Dysdera catalonica, a continental species, present in the northern half of Catalonia and southern France, and D. tilosensis, which is endemic to the island of Gran Canaria.

The species D. catalonica has a genome of 3.3 billion base pairs (3.3 Gb, the letters of DNA), which is almost double that of the species D. tilosensis (1.7 Gb). Interestingly, despite having a smaller genome, the species from the Canary Islands shows greater genetic diversity.

Professor Julio Rozas, co-corresponding author
Departament de Genètica, Microbiologia i Estadística
Universitat de Barcelona
Catalunya, Spain.

Genomic sequencing also reveals that D. catalonica has a haploid chromosome number of four autosomes and one X sex chromosome, while D. tilosensis has six autosomes plus the X chromosome.
The genome downsizing of the spider D. tilosensis, associated with the colonization process of the Canary Island, is one of the first documented cases of drastic genome downsizing using high-quality reference genomes. This phenomenon is now being described for the first time in detail for phylogenetically closely related animal species.

Professor Julio Rozas.

How can genome reduction be explained?

In such evolutionarily similar species, which share similar habitats and diet:
… differences in genome size cannot easily be attributed to ecological or behavioural factors. Phylogenetic analysis combined with flow cytometry measurements, reveals that the common ancestor had a large genome (about 3 Gb). This indicates that the drastic genome reduction occurred during or after the arrival on the islands.

Professor Sara Guirao, co-corresponding author.
Departament de Genètica, Microbiologia i Estadística
Universitat de Barcelona
Catalunya, Spain.

This result is clearly paradoxical for two reasons. On the one hand, although less frequent in animals, the most common pattern is the increase in genome size via whole-genome duplications,
… especially in plants, where the appearance of polyploid species (with multiple chromosome endowments) is common. In contrast, such sharp reductions in genome size over a relatively short period of time are much rarer.

Professor Sara Guirao.

Secondly, the findings contradict theories that argue that, on islands, the founder effect — the process of colonization by a small number of individuals — leads to a reduction in selective pressure and, as a result, genomes should be larger and richer in repetitive elements.
In the study, we observed the opposite: island species have smaller, more compact genomes with greater genetic diversity. [This pattern suggests the presence of non-adaptive mechanisms,] whereby populations in the Canary Islands would have remained relatively numerous and stable for a long time. This would have made it possible to maintain a strong selective pressure and, as a consequence, eliminate unnecessary DNA.

Vadim A. Pisarenco, first author
Departament de Genètica, Microbiologia i Estadística
Universitat de Barcelona
Catalunya, Spain.

Deciphering one of the great enigmas in evolutionary biology

It is still a mystery why, in similar species, some genomes accumulate numerous repetitive DNA sequences while others are more compact. The study could provide a fresh perspective on solving this open question in evolutionary biology.

According to some hypotheses, these changes in the genome are of direct adaptive value. Other explanations propose non-adaptive mechanisms, in which genome size is the result of a balance between the accumulation of repetitive elements (such as transposons) and their removal by purifying selection.

“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 researchers conclude.

Publication:
Vadim A Pisarenco, Adrià Boada-Figueras, Marta Olivé-Muñiz, Paula Escuer, Nuria Macías-Hernández, Miquel A Arnedo, Pablo Librado, Alejandro Sánchez-Gracia, Sara Guirao-Rico, Julio Rozas,
How Did Evolution Halve Genome Size During an Oceanic Island Colonization?
Molecular Biology and Evolution 42(9) 2025, msaf206, https://doi.org/10.1093/molbev/msaf206

Abstract
Red devil spiders of the genus Dysdera colonized the Canary Islands and underwent an extraordinary diversification. Notably, their genomes are nearly half the size of their mainland counterparts (∼1.7 vs. ∼3.3 Gb). This offers a unique model to solve long-standing debates regarding the roles of adaptive and nonadaptive forces on shaping genome size evolution. To address these, we conducted comprehensive genomic analyses based on three high-quality chromosome-level assemblies, including two newly generated ones. We find that insular species experienced a reduction in genome size, affecting all genomic elements, including intronic and intergenic regions, with transposable element (TE) loss accounting for most of this contraction. Additionally, autosomes experienced a disproportionate reduction compared to the X chromosome. Paradoxically, island species exhibit higher levels of nucleotide diversity and recombination, lower TE activity in recent times, and evidence of intensified natural selection, collectively pointing to larger long-term effective population sizes in species from the Canary Islands. Overall, our findings align with the nonadaptive mutational hazard hypothesis, supporting purifying selection against slightly deleterious DNA and TE insertions as the primary mechanism driving genome size reduction.

Vadim A Pisarenco, Adrià Boada-Figueras, Marta Olivé-Muñiz, Paula Escuer, Nuria Macías-Hernández, Miquel A Arnedo, Pablo Librado, Alejandro Sánchez-Gracia, Sara Guirao-Rico, Julio Rozas,
How Did Evolution Halve Genome Size During an Oceanic Island Colonization?
Molecular Biology and Evolution 42(9) 2025, msaf206, https://doi.org/10.1093/molbev/msaf206

Copyright: © 2025 The authors/ Society for Molecular Biology and Evolution.
Published by Oxford University Press. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

For creationists, this result is doubly awkward. Not only does it show that large-scale loss of genetic material can be entirely compatible with survival and adaptation, but it also illustrates that evolution is not a one-way march toward ever-greater complexity. Nature streamlines as readily as it elaborates, and the direction of change depends on environment, opportunity, and selective pressure — not on metaphysical assumptions about what organisms “ought” to do. If genomic pruning produces a spider perfectly capable of thriving in its niche, reality wins over doctrine.

And there’s a deeper issue still. If two species can be almost indistinguishable in form yet differ by nearly half their genome, then the simple “design” narrative collapses into hand-waving. Why create redundant DNA in the first place? Why remove it later? And how, precisely, does one detect such changes in the fossil record — the only evidence available for most of life’s history? Evolutionary theory anticipates unexpected patterns and welcomes revision when new evidence arrives. Creationism, by contrast, must simply ignore this spider, as it must every other data point that refuses to fit a prefabricated answer. Nature, inconveniently, keeps providing them.

If proponents of “intelligent design” wish to salvage their argument here, the challenge is straightforward: explain why an all-knowing designer would equip a spider lineage with nearly twice as much genetic material as required, only to remove it later — silently, without trace in the fossil record, and without any apparent functional disadvantage along the way.

A coherent explanation would need to address the timing, mechanism, purpose, and detectability of such a “design” strategy, and be testable against real genetic data. Until then, this spider stands as yet another reminder that nature, like science, follows evidence, not ideology.

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