Evolution Doesn’t Just Create New Species – It Can Also Reverse It

When Charles Darwin pondered how a single species could, over time, become two distinct ones, he realized that natural selection could act differently on separate populations, gradually driving them apart. This insight was so powerful that it gave rise to an entirely new scientific field: evolutionary biology.

To this day, the emergence of new species remains a central concept in evolutionary biology, and understanding the mechanisms behind it is one of the field’s key research questions. At the same time, hybridisation between populations that have evolved in isolation can influence the pace of speciation—either speeding it up, slowing it down, or even reversing it altogether.

A recent study by Ingo Müller and colleagues delved into the evolutionary history of Melidectes, a bird genus found in New Guinea that includes six species living in various mountainous regions at different elevations. Most of these species occupy separate areas, but two of them meet and hybridise in the central mountain range.

Despite this overlap, scientists had assumed that differences in plumage and adaptations to elevation would help maintain clear species boundaries. However, Müller and his team used genomic and genetic analyses to show that these visible differences do not, in fact, prevent gene flow between the two species. On the contrary, the researchers discovered that their genes are mixing extensively across most of their shared range.

This makes the bird community formed by these two species quite unusual. It follows a pattern where the most genetically distinct and least interbreeding populations are found at the geographical edges of their ranges. Remarkably, this is the case even though their maternally inherited mitochondrial genomes still sort neatly into two deeply divergent lineages, consistent with their traditional species identities.

These findings led the researchers to conclude that the two incipient species have recently begun to merge—possibly due to increased contact brought on by climate warming—resulting in nearly complete fusion into a single lineage. In other words, a speciation process that was well underway has now stalled, and the two almost-species appear to be on the path to becoming one unified species again.

Previous thoughts on the same topic:
The Dire Wolf and the Rights of Extinct Human Species
Shower Taking and Interfering Elephants
The fundamentals of conservation biology wavered

The Historical Merging of Human Groups

Understanding human evolution has progressed step by step during my lifetime. One of the most sensational discoveries was the evidence of interbreeding between Neanderthals, who lived in Europe before modern humans, and our ancestors.

Significant new information has once again been obtained on this matter. It was based on an excellent idea: previously, the focus had been on determining how much Neanderthal DNA is present in our genes, but now researchers decided to investigate how much so-called modern human DNA is present in Neanderthal genes.

This was possible using two criteria. The first was based on the fact that Neanderthals were already known to be much fewer in number than modern humans, so their genomes likely contained less variation. Thus, the more variable regions of the Neanderthal genome would be possible gene regions originating from modern humans.

When such gene regions were then found, the corresponding segments of heritage could be searched for in modern Africans, who have never interbred with Neanderthals. If they had similar genome segments to the now-found more variable regions, they must have developed in modern humans and not in Neanderthals. And indeed, this confirmed that as much as about ten percent of Neanderthal genes ultimately came from modern humans.

Based on this new information, it can also be concluded that Neanderthals may not have disappeared from the world after all but simply merged into the much more numerous populations of modern humans. And they left their genetic mark on them— a set of genes that developed in Neanderthals and have since been passed down to me and to all of you, dear readers, unless you are entirely of African descent.

Previous thoughts on the same topic:
The amazing adventures of the German cockroach
Violent demographic changes in Denmark
American black population more vulnerable to the Omicron variant of SARS-CoV-2