How do similar animals evolve to the point of forming different species that are unable to hybridize? The question plagues researchers in evolutionary biology from Darwin.
Some imagine a rapid process, the result of modifications to a few key genes for the choice of partners or the ecology: hybridization then becomes detrimental, because the offspring, even viable, will be neither attractive nor adapted to parental environments because of their intermediate characters.
Others see it more as the effect of the gradual differentiation of genomes: when populations remain isolated for millions of years, all of their genes diverge under the effect of the mutation and gradually become incompatible, causing problems. development and fertility in hybrids.
Hybrid zones as a laboratory
Each species of frog produces a “unique” song that attracts only members of the same species. On very rare occasions, however, this filtering mechanism fails. Here, a Dendropopophus Minutus riding a Dendropophas Marmoratus © Santiago Ron / Flickr CC BY-ND 2.0
To confront these two major hypotheses, we looked at hybrid amphibian zones: the geographic regions where genetic lines of toads, frogs or even more or less divergent tree frogs meet, and mix with such affinity. The idea is to measure the degree of divergence necessary to limit hybridization, and the number of genes potentially responsible.
For this, it was necessary to collect DNA in natural populations, which required a considerable field effort during numerous trips through Europe, North Africa and the Middle East, with the complicity of close collaborators abroad for nearly 5 years. These samples were then analyzed with tools known as “genomics”, which provide access to thousands of regions of the genome.
The rewards were significant: the analyzes first led to the discovery of new species, and clarified many questions about the evolution of European herpetofauna. In particular, the application of what are called “molecular clocks” on DNA sequences has made it possible to date the moment when the species appeared, that is to say when they diverged from their cousins. In all, nearly forty natural geographic transitions were compared, representing all the known genres of the old continent.
Genetic analyzes have made it possible to clarify the diversity and evolution of many groups of amphibians. Among the alytes (small toads whose male carries the eggs until they are ready to hatch), we were able to discover a new species for France © Christophe Dufresnes (via The Conversation)
It takes time to make a new frog
The results, published in PNAS, show that it is the entire genome, which by diverging over millions of years, gradually becomes incompatible between emerging species, and not a few key genes alone. Old lines therefore have more trouble hybridizing than younger lines, because a greater number of genes have lost their compatibility.
Thus, the lines isolated during the last ice ages still manage to hybridize without concern, and traces of them can be found for hundreds of kilometers. A sign that their still very similar genomes have lost nothing in compatibility. This is particularly observed in the Iberian and French forms of pelodyte punctuated, which hybridize throughout Catalonia (see figure).
The Catalan coast is a land of encounters! But the consequences are different for the pelodytes, whose Iberian (red) and French (blue) lines mix very well, while for the alytes, they only mix over a few kilometers © Christophe Dufresnes (via The Conversation)
Several million years later, however, genetic exchanges are much more restricted. For example, midwife alytes and
almogavarre only mix for a few kilometers (see figure). Because hundreds of genes have become incompatible, and together, reduce the viability and fertility of hybrids. This is what researchers call ”
barrier genes Because they are the source of reproductive barriers.
The formation of species in amphibians. Genetic incompatibilities gradually develop as the lines diverge. Thus, the genomes initially remain compatible, then barrier genes (which cause concern in hybrids) accumulate, to the point that they eventually become incompatible, and hybridization no longer works. We can then qualify them as species in the taxonomic sense © Christophe Dufresnes (via The Conversation)
At the beginning, we are wrong
As they are initially maintained by essentially genetic mechanisms (incompatibilities with many genes) which take millions of years to set up, amphibian species therefore evolve relatively slowly. For the rest, they nevertheless remain identical in our eyes: same ecology, same morphology, same songs of mating. We are talking about “cryptic” species, identifiable only thanks to molecular tools.
But since they still look very similar, these cryptic species always seek to try to hybridize when they cross, so wrongly. It will be necessary to wait again so that sufficiently marked external differences do not evolve, this so that they avoid mating with the wrong partner.
Young species of tree frogs have remained similar despite incompatibilities between their genomes. Thus, unsuccessful hybridizations are frequent. It is later in the course of the evolution that we will see notable differences, for example at the level of coloring and mating song © Christophe Dufresnes (via The Conversation)
A new approach to classify biodiversity
By showing that species formation was a gradual process that can be quantified at the genetic level, the study paves the way for new concepts to categorize evolutionary lineages into subspecies or species.
More specifically, below two million years of divergence, frogs and toads remain compatible and therefore corresponded at best to subspecies. It is beyond five million years that we are almost certainly in the presence of real species. In between, there is a “gray area”, where the lines are nevertheless classifiable by inspecting the number of
barrier genes already accumulated.
With the discovery of hundreds of species each year around the world, this type of standardized approach should help establish taxonomic lists more objective and therefore more stable for amphibians, but also for all animal groups, which is necessary to decide on the priority species to be protected.
This analysis was written by Christophe Dufresnes, professor of zoology at the Forestry University of Nanjing (China) and Pierre-André Crochet, director of biodiversity research at the University of Montpellier.
The original article was published on the website of The Conversation.
