Rosa Rubicondior: Refuting Creationism - Evolutionary Divergence and Hybridization in Mediterranean Shearwaters

The Balearic shearwater (Puffinus mauretanicus, above) and the Mediterranean shearwater (Puffinus yelkouan, below) are two shearwater taxa that are genetically similar despite displaying morphological and migratory differences. The new study shows that these two taxa have undergone recurrent episodes of divergence and hybridization during the Pleistocene glacial and interglacial cycles, respectively. This serves as a key example of how hybridization can help preserve genetic diversity and evolutionary potential in threatened taxa.

Credit: Victor Paris

Hybridization between species has been crucial to the survival of Europe’s most threatened seabird - Current events - University of Barcelona

Like the story of human evolution, that of two seabirds in the shearwater family living in the Mediterranean shows a history of divergence, followed by episodes of hybridization and genetic remixing. Normally, from a conservation perspective, hybridization is considered a problem because it can erode the distinctiveness of threatened species, diluting unique traits.

However, in the case of the Mediterranean shearwater (Puffinus yelkouan) and the critically endangered Balearic shearwater (Puffinus mauretanicus), hybridization has had the opposite effect. By introducing genetic diversity into the Balearic shearwater, it may have improved the species’ chances of survival, potentially making the difference between persistence and extinction.

This evolutionary story sits uneasily with the worldview of Bible-literalist creationists. The very fact of species interbreeding undermines the notion of fixed, separately created “kinds.” It also contradicts the idea that a few thousand years ago all life was destroyed in a global flood, followed by a period of hyper-rapid diversification into the species we see today—a process which, according to creationist claims, left no trace in the fossil record. If hybridization had occurred during that supposed burst of post-Flood diversification, it would simply have blurred the lines between “kinds,” creating variation within species rather than producing the multitude of distinct species we observe today. In other words, the evidence shows that life’s history is one of gradual divergence, occasional genetic exchange, and long-term adaptation—not sudden, discrete acts of “special creation.”

A recent study led by Professors Julio Rozas, Marta Riutort, and Jacob González-Solís of the Faculty of Biology and the Biodiversity Research Institute of the University of Barcelona (IRBio), together with Joan Ferrer Obiol of the University of Milan, has revealed how this evolutionary pattern unfolded. Using genomic analysis, they showed that the two shearwater species have gone through repeated cycles of divergence and hybridization, largely driven by the glacial–interglacial rhythms of the Pleistocene. Each cycle of isolation and secondary contact left genetic signatures that are still visible today.

The researchers found a clear gradient of genetic differentiation across the Mediterranean, with hybrid populations becoming increasingly common as one moves westwards, culminating in the Balearic Islands of Ibiza and Formentera. Intriguingly, the shearwater population on Menorca is genetically closer to P. yelkouan than to P. mauretanicus, reflecting its history of admixture. Far from being detrimental, this interbreeding has enriched the Balearic shearwater’s genome, reducing inbreeding risks and helping to preserve adaptive traits that may otherwise have been lost.

Background^ Shearwaters and Their Evolution. Taxonomy and Family Tree

Shearwaters belong to the order Procellariiformes, the “tube-nosed” seabirds, which also includes petrels, prions, and albatrosses.
Within this order, they are part of the family Procellariidae, closely related to fulmars and gadfly petrels.
Their distinctive tubular nostrils on the beak help them excrete excess salt — an adaptation to life at sea.

The Genus Puffinus

The genus Puffinus contains the smaller shearwaters, as opposed to the larger Calonectris shearwaters.
The group has a widespread distribution, with species in the Atlantic, Pacific, and Mediterranean.
Genetic studies suggest complex evolutionary histories, with frequent long-distance dispersal followed by local divergence.

Mediterranean Shearwaters

Until the late 20th century, the Balearic (P. mauretanicus) and Yelkouan (P. yelkouan) shearwaters were often treated as subspecies of the “Mediterranean shearwater.”
Advances in molecular genetics and detailed behavioural studies have since confirmed that they represent distinct species, albeit with ongoing hybridization in overlapping ranges.
This taxonomic complexity reflects a broader pattern in seabirds: isolation on islands often drives divergence, but migratory behaviour and wide-ranging foraging bring species back into contact, leading to gene flow.

Evolutionary Significance

The evolutionary story of shearwaters exemplifies how isolation, migration, and hybridization interact over glacial timescales to shape species.
Far from being fixed or immutable, seabird lineages are dynamic — splitting, merging, and exchanging genes in response to changing climates and habitats.

The study’s findings, which carry important implications for conservation, are detailed in a paper published in Proceedings of the National Academy of Sciences of the USA (PNAS). A summary is also available in a news release from the University of Barcelona.

Hybridization between species has been crucial to the survival of Europe’s most threatened seabird
When individuals from two different species interbreed, hybrid organisms may emerge that display characteristics from both genetic lineages. While hybridization is a common natural phenomenon, it is often seen as a challenge in biodiversity conservation. Now, a study published in the journal Proceedings of the National Academy of Sciences (PNAS) reveals that hybridization between the Mediterranean shearwater and the Balearic shearwater — a seabird at critical risk of extinction — has been decisive in maintaining genetic diversity and facilitating the survival of the most threatened seabird in Europe.
With doctoral student Guillem Izquierdo-Arànega as the main driving force, the research was led by professors Julio Rozas, Marta Riutort and Jacob González-Solís from the Faculty of Biology and the Biodiversity Research Institute of the University of Barcelona (IRBio), along with Joan Ferrer Obiol from the University of Milan (Italy). Cristian Cuevas-Caballé, recently awarded his PhD at the UB, is also a co-author of the article, along with other experts.

The new research emphasizes that hybridization should be considered in conservation and management plans, especially at a time when human pressure on biodiversity is accelerating the decline and extinction of life forms across the planet. In the specific case of shearwaters, there is also an urgent need to reduce the accidental bycatch of seabirds in fishing gear to prevent the future extinction of these iconic species. The Balearic shearwater: an extinction foretold?

The Balearic shearwater (Puffinus mauretanicus), a taxon endemic to the Balearic Islands, is critically endangered according to the International Union for Conservation of Nature (IUCN). Its breeding area is limited to the Balearic Islands, it has a low reproductive rate, and suffers high additional mortality due to accidental bycatch in longline fisheries. Several simulations suggest that it could become extinct in the coming decades if urgent action is not taken.

The new study investigates the dynamics of hybridization between the Balearic shearwater and the Mediterranean shearwater (Puffinus yelkouan) and its effect on the genomic diversity of the Balearic shearwater — a poorly studied issue that has limited conservation strategies. These two genetically close lineages have undergone recurrent episodes of divergence and hybridization during the Pleistocene glacial cycles, creating a geographical gradient of hybridization and genetic differentiation from the Eastern Mediterranean to the Pityusic Islands (Ibiza and Formentera). The study also finds that the Menorca population is genetically closer to the yelkouan lineage than to the mauretanicus.
Although hybridization was once seen as a problem in conservation management due to the potential impact of non-adaptive genetic variants, this view is increasingly being challenged. The new study offers a benchmark example of the evolutionary importance of hybridization in protecting a critically endangered flagship species.

Hybrids can introduce genetic diversity and adaptive potential, thereby benefiting small and threatened populations. Using simulations to predict the population dynamics of the Balearic shearwater, we show the value of preserving hybrid populations to avoid problems stemming from inbreeding (increased homozygosity, reduced genetic diversity, etc.). Hybridization may counteract these effects through natural genetic rescue, helping prevent a population from reaching extinction thresholds. The evolutionary history of these shearwaters also exemplifies how hybridization can be critical for maintaining genetic diversity in threatened taxa.

Guillem Izquierdo-Arànega, first author.
Departament de Genètica, Microbiologia i Estadística
Universitat de Barcelona
Barcelona, Spain.

And Joan Ferrer Obiol, co-corresponding author. Dipartimento di Scienze e Politiche Ambientali
Università degli Studi di Milano
Milano, Italy.

Avoiding bycatch in fishing equipment

Actions focused solely on preserving genetic diversity are not enough to ensure the survival of the Balearic shearwater, warn experts. Beyond conservation measures, current demographic projections suggest the species could become extinct within the next hundred years.

Longline fishing is a highly selective technique, but it occasionally catches individuals of all three endemic shearwater species in the Mediterranean, amounting to thousands of birds each year. This phenomenon has been known for decades, and there are effective measures that can completely eliminate this accidental bycatch — all that’s missing is the political will to implement them.

Professor Jacob González-Solís, co-author
Departament de Biologia Evolutiva
Ecologia i Ciències Ambientals
Universitat de Barcelona
Barcelona, Spain.

Earth BioGenome Project

In this context, it’s important to highlight that the genome of the Balearic shearwater — the first obtained under the Catalan Initiative for the Earth BioGenome Project (CBP) — was sequenced and annotated in the Rozas and Riutort laboratories. This initiative, which began within the Catalan Society of Biology, a branch of the Institute for Catalan Studies (IEC), is supported by the IEC through the Biogenome–IEC project, funded by the Catalan Government. It is co-chaired by Marta Riutort (UB-IRBio) and forms a network of experts with the mission to sequence reference genomes of eukaryotic species found in Catalan-speaking regions.
This study shows that reference genomes have become a crucial tool in biodiversity conservation efforts. Having a well-annotated genome of the Balearic shearwater was key for conducting analyses on hybridization with its sister taxon and identifying the affected genomic regions, as well as for conducting population-level genomic studies that reveal the past history of the two lineages and allow us to model their future under different protection scenarios. At the CBP, this encourages us to continue our efforts to increase the number of reference genomes and, most importantly, to promote this kind of research, which should contribute decisively to improving the state of our environment.

Marta Riutort, co-author.
Departament de Genètica, Microbiologia i Estadística
Universitat de Barcelona
Barcelona, Spain.

Publication:
G. Izquierdo-Arànega, C. Cuevas-Caballé, F. Giannelli, J.R. Paris, K. Bourgeois, E. Trucchi, J. González-Solís, M. Riutort, J.F. Obiol, & J. Rozas
Pervasive and recurrent hybridization prevents inbreeding in Europe’s most threatened seabird
Proc. Natl. Acad. Sci. U.S.A. 122(34) e2427223122, https://doi.org/10.1073/pnas.2427223122 (2025).

Significance
Interspecific hybridization is common across the tree of life, yet it is often viewed as a negative evolutionary force in many conservation contexts. Our study on the Critically Endangered Balearic shearwater uncovers a critical and positive role of interspecific hybridization in small population dynamics. We demonstrate that climate-driven episodes of pervasive hybridization have allowed the preservation of genetic diversity and the introgression of adaptive loci. Using forward simulations, we show the importance of preserving hybrid populations to avoid inbreeding in the near future. Overall, our research highlights the urgency of reevaluating hybridization dynamics in conservation actions to ensure species’ survival.

Abstract
Hybridization is a double-edged sword: While it can erode distinct evolutionary lineages, it can also introduce genetic diversity and adaptive potential into dwindling populations. In the Critically Endangered Balearic shearwater (Puffinus mauretanicus), this dilemma is exacerbated by a limited understanding of the extent and consequences of hybridization with the Yelkouan shearwater (Puffinus yelkouan). This knowledge gap has limited the scope of science-based conservation strategies to avoid the Balearic shearwater’s imminent extinction. Here, we investigate shearwater hybridization dynamics and their effect on genome-wide diversity in the Balearic shearwater. Divergence dating, demographic modeling, and admixture analyses suggest that these two poorly differentiated shearwater lineages have experienced recurrent episodes of divergence and widespread hybridization during glacial cycles. Selection scans reveal a 500 kb region hosting an adaptive haplotype that potentially underpins interspecific differences in migratory behavior and which has been repeatedly introgressed between the two taxa. Moreover, we show that interspecific gene flow has prevented increases in homozygosity and genetic load, and through forward simulations, we illustrate how it can buffer the negative effects of future population bottlenecks in the Balearic shearwater. Our findings illustrate how introgression can be crucial for maintaining genetic diversity in threatened taxa and highlight the need for considering the protection of hybridization in conservation plans.

G. Izquierdo-Arànega, C. Cuevas-Caballé, F. Giannelli, J.R. Paris, K. Bourgeois, E. Trucchi, J. González-Solís, M. Riutort, J.F. Obiol, & J. Rozas
Pervasive and recurrent hybridization prevents inbreeding in Europe’s most threatened seabird
Proc. Natl. Acad. Sci. U.S.A. 122(34) e2427223122, https://doi.org/10.1073/pnas.2427223122 (2025).

© 2025 National Academy of Sciences of the USA.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.

The picture that emerges from the study of these Mediterranean shearwaters is one of complexity and continuity: species diverging, reconnecting, and exchanging genes over tens of thousands of years, with the signatures of those events still traceable in their genomes today. This is exactly what evolutionary biology predicts in a world shaped by shifting climates, geographic barriers, and the ebb and flow of glacial cycles. Hybridization is not an anomaly but a well-documented process that can act as a source of genetic innovation, sometimes tipping the balance between extinction and survival.

For Bible-literalist creationists, however, such evidence poses a serious problem. Their model requires that all species alive today arose from a handful of “kinds” after a global flood just a few thousand years ago, through a period of hyper-accelerated evolution for which there is no trace in the fossil record. But if that were true, hybridization would only have blurred distinctions between those emerging species, reducing diversity rather than increasing it. The patterns we see in shearwaters—gradual divergence punctuated by gene flow over immense timescales—are wholly incompatible with such a scenario.

In contrast, these findings fit seamlessly into the evolutionary framework. They show how species boundaries can be porous, how genetic exchange can provide adaptive advantages, and how the history of life is written not in sudden, one-off acts of creation, but in the ongoing interplay of variation, selection, and time. Far from undermining evolutionary theory, shearwaters offer another elegant demonstration of its explanatory power.

Where creationism sees rigid ‘kinds,’ evolution reveals a living tapestry—woven from divergence, hybridization, and adaptation across deep time.

Advertisement

What Makes You So Special? From The Big Bang To You

How did you come to be here, now? This books takes you from the Big Bang to the evolution of modern humans and the history of human cultures, showing that science is an adventure of discovery and a source of limitless wonder, giving us richer and more rewarding appreciation of the phenomenal privilege of merely being alive and able to begin to understand it all.

Ten Reasons To Lose Faith: And Why You Are Better Off Without It

This book explains why faith is a fallacy and serves no useful purpose other than providing an excuse for pretending to know things that are unknown. It also explains how losing faith liberates former sufferers from fear, delusion and the control of others, freeing them to see the world in a different light, to recognise the injustices that religions cause and to accept people for who they are, not which group they happened to be born in. A society based on atheist, Humanist principles would be a less divided, more inclusive, more peaceful society and one more appreciative of the one opportunity that life gives us to enjoy and wonder at the world we live in.

Amazon