Tag Archives: speciation


Two raven lineages that tied the knot yield evidence of ‘speciation reversal’

After some two million years of walking their own path, two species of common raven are tying the knot — this process of “speciation reversal” shows just how indifferent evolution is to taxonomical constraints.


Image via Pixabay.

Speciation is a pretty handy tool in evolution’s hand; in short, it allows one species to branch out into two or more other species. A new study now looks at the other side of this process — called speciation reversal. Drawing on almost 20 years of research, the study shows how two previously distinct lineages of common raven can merge back together, and what role this process plays in the grand evolutionary scheme of things.

One big happy species

“The bottom line is that [speciation reversal] is a natural evolutionary process, and it’s probably happened in hundreds, or almost certainly thousands, of lineages all over the planet,” said co-author Kevin Omland, professor of biological sciences at University of Maryland, Baltimore County (UMBC).

“One of our biggest goals is to just have people aware of this process.”

The study’s roots stretch back to almost 20 years ago, when Omland started studying the common raven (Corvus corax). He soon began to suspect that species was, in fact, two separate species. He set out to identify the two, thinking perhaps it was a question of a “new world” and an “old world” raven — but the reality was more complicated. By the turn of the millennium, he reported that there is one common raven lineage concentrated in the southwestern United States, which he dubbed “California,” and another, found everywhere else (including the U.S., Norway, and Russia), which he christened “Holarctic”.

His latest paper peers more closely into the evolutionary history of the two groups. A genetic analysis of 400 birds, spanning the geographical range of the two lineages, he identified suggests that the California and Holarctic groups split some one to two million years ago. However, it also suggested that the two have been merging and hybridizing together for some tens of thousands of years now. The pure California type no longer exists and the population is now made up of pure Holartics and a group of hybrids from the two original lineages.

“The extensive genetic data reveals one of the best supported examples of speciation reversal of deeply diverged lineages to date,” said Arild Johnsen, a professor of zoology and evolutionary biology at University of Oslo and another co-author. “The biggest thing is the degree to which we’ve caught them in the act.”

The analysis also revealed that mitochondrial DNA differences between the Holartics and hybrids amount to about 4%, which the team said was twice as much as would normally be seen for birds to be considered separate species. Still, despite being genetically distinct, the two groups look the same, sound, and behave the same.

The paper further notes that a third group of ravens — which branched off from the California lineage and are known as Chihuahuan ravens — have remained separate from the Holarctics and their hybrids. The Chihuahuans refuse to interbreed with the two groups despite the fact that their geographical ranges overlap over large swaths of land. Omland is unsure what causes this.

“The Chihuahuan raven doesn’t want to play,” said Omland. “It stays by itself and doesn’t interbreed with the others.”

Right now, the team is trying to determine what made the two lineages merge, including whether humans inadvertently played a part. They’re also looking at genetic data from ravens that lived in the early 1900s to determine if the hybridization process picked up speed since then.

The paper “Genomic evidence of speciation reversal in ravens” has been published in the journal Nature Communications.


Sleeping around makes it hard to speciate by mixing genes, paper shows

Promiscuity may work against speciation by homogenizing a population’s gene pool, a paper from the University of Bath’s Milner Centre for Evolution reports.

Wild Rabbits.

I’ll let you make the joke for this one.
Image credits Holger Langmaier.

In order for one group to speciate (split off into new species), there has to be a certain level of genetic isolation going on. Darwin said that over time, as environmental factors collide with the genetic make-up of each population, natural selection would favor the best-adapted individuals, who would pass on their genes more readily — thus favoring speciation. This is really good if you want to become adapted to the particular conditions in your plot of land.

But efficient Darwinism isn’t the only hand shaping genomes around the world. Other models, such as Fisher’s runaway selection models, take into account sexual selection. In short — males like females, females like what they like, so strong horns make way for flashier horns to give males a better shot at getting some action. This way, arbitrary traits which can actually make animals less adapted (think of the peacock’s ridiculously impractical drab) become valuable assets as they improve access to mates.


Image credits Christy Mesker.

Theories like Fisher’s brought into discussion the fact that sexual preferences, especially on the part of the females who tend to be pickier in regards to mates, can become a huge driver of speciation. Preferences can vary quite wildly from place to place and as genomes get tuned more towards delivering the goods at the expense of efficiency, overall genetic diversity rises setting the stage for speciation — or at least that’s how the theory goes.

But this conventional wisdom might not be spot-on, according to a new paper published by researchers at the University of Bath, Cardiff University, and the Max Planck Institute for Ornithology. By tracking microsatellite data for 79 populations of 10 plover species (Charadrius) and the genetic structure of 136 shorebird species, the team found that a propensity for promiscuity makes a population’s genes less diverse. They report that polygamous bird species (those who breed with several partners each season) are less genetically diverse than monogamous (only one partner per season) species — lowering the chances of speciation.

“Our findings suggest that because of the pressure to find more than one mate, polygamous shorebirds may search large areas and therefore spread their genes as they go,” said first author Josie D’Urban Jackson, who is jointly supervised at University of Bath and Cardiff University.

“This means they effectively mix up the gene pool by diluting any genetic differences between geographically distant locations, so that populations are less likely to diversify into new species over time.”

Monogamous species only have to find one mate to partner up with each season, and generally tend to re-visit the same breeding sites throughout their life. This means they’re better adapted to local conditions and more genetically isolated as there is little to no influx of outside genes. All this makes monogamous species more likely to speciate.

Promiscuous species on the other hand travel far and wide to sow as many oats as they possibly can — but it’s not a case of first come, first served. Jackson’s supervisor at the University of Bath’s Milner Centre for Evolution, Tamás Székely, says that polygamous birds can travel hundreds of kilometers at a time to find a mate they like.

“You might think that birds choose mates arbitrarily if they are promiscuous, but most individuals prefer a certain type, just as some humans might prefer blonde or dark hair in a partner,” said Professor Tamás Székely.

This makes promiscuous populations mix their genes even over huge distances, while their monogamous counterparts can remain relatively distinct a stone’s throw away.

“For example, in Madagascar, we found that the polygamous plovers were similar across the whole island, whereas the monogamous plovers have distinct genetic composition between nearby locations — showing the same pattern that our larger scale study just confirmed.”

The paper “Polygamy slows down population divergence in shorebirds” has been published in the journal Evolution.


Seahorses have the fastest evolving genome

Seahorses have a highly specialized morphology, with several characteristics unique in the animal kingdom. Now, researchers have found another remarkable trait of seahorses – their genome evolves extremely fast.

A different species of seahorse. Image credits: Maëlick Seahorse.

The genome project, comprises six evolutionary biologists from Professor Axel Meyer’s research team from Konstanz and researchers from China and Singapore. They chose the tiger tail seahorse Hippocampus comes. The assembled genome is 502Mb or about 1/6th the size of the human genome. The reason why they chose this species is pretty straightforward – just look at it.

It doesn’t look like any other fish – or any other creature for that matter – nor does it behave like any other creature. We don’t know if its features are adaptive or not, but we do know its evolutionary history to some extent. So scientists looked at 4,122 orthologous genes (segments of DNA with shared ancestry because of a speciation event) and calculated mutation rates, finding that the rate of change is extremely high.

Figure 1 of the paper. Adaptations and evolutionary rate of H. comes.

Researchers believe that their study may partly explain why seahorses evolved the way they did.

“Our genome-wide analysis highlights several aspects that may have contributed to the highly specialized body plan and male pregnancy of seahorses,” they write in the study. “These include a higher protein and nucleotide evolutionary rate, loss of genes and expansion of gene families, with duplicated genes exhibiting new expression patterns, and loss of a selection of potential cis-regulatory elements.”

Seahorses possess one of the most highly specialized morphologies and reproductive behaviours. Their bodies include a toothless tubular mouth, a body covered with bony plates, a male brood pouch, and the absence of caudal and pelvic fins. Another surprising evolutive feature is male pregnancy. When mating, the female seahorse deposits up to 1,500 eggs in the male’s pouch. The male carries the eggs for 9 to 45 days until the seahorses emerge fully developed, but very small. After that, the male’s role is done but even so, it is a truly spectacular and almost unique trait (another type of fish does it – pipefishes).

Particularly noteworthy is the loss of the pelvic fins. Evolutionarily, you can consider them as analogous to human legs. The gene responsible for this feature (tbx4) is present in nearly all vertebrates, but is missing from the seahorse’s genome. Also, while some genes were absent, some genes were duplicated. When a gene is duplicated, it can fulfill a completely different role. In the seahorse, this is probably how a part of the newly created gene makes male pregnancy possible.

But not everyone is convinced. Writing on his website, Larry Moran, a Professor in the Department of Biochemistry at the University of Toronto, says that the accuracy of the study can’t be assessed without going deep into the big data algorithm they used.

“They looked at a set of 4,122 orthologous genes and calculated mutation rates. The results are shown in Figure 1 in the paper. The differences in distance are quite small, ranging from 94% tp 99% of the seahorse value (0.463) in the major clade. Nevertheless, the authors claim the difference is statistically significant. They also looked specifically at neutral changes and found the same thing—faster in seahorses. The implication is that the strange morphological differences between seahorses and other species of fish can be explained by a faster mutation rate.”

“Here’s the problem. I have no idea how they came up with these numbers. I can’t possibly evaluate the quality of their data to know whether it’s believable or not. Clearly the Nature referees thought it was good enough to publish. Those referees must be experts in this kind of analysis. Can someone out there help me understand the quality of this analysis?”

This indicates an issue with many modern studies. Even experts in respective fields can’t assess the validity of certain papers, because of the reliance on complex algorithms and big data. In other words, if you want to really understand the study, you often have to go through hundreds of lines of code (which are not readily accessible), in a language you are perhaps not familiar with – that’s simply not doable. Hopefully, reviewers are doing their job.

Journal Reference: Qiang Lin et al. The seahorse genome and the evolution of its specialized morphology. Nature, 2016; 540 (7633): 395 DOI: 10.1038/nature20595

The Mexican grizzly bear became extinct in 1964. Credit: Wikipedia

Humanity is driving thousands of species extinct, but there’s a flip side — we also create new species

The Mexican grizzly bear became extinct in 1964. Credit: Wikipedia

The Mexican grizzly bear became extinct in 1964. Credit: Wikipedia

We’re all used to the depressing headlines of yet another species having gone extinct (or is just about to) due to human interference. Logging, pollution, hunting, urban expansion — these and much more take their toll on wildlife and only a select few can adapt.

Sometimes, though, humans can act as a prime driver for speciation. One example includes a new species of mosquito that dwells in the undergrounds of London and can’t breed with mosquitos that fly at the surface. It’s an entirely new species, shaped by an ecosystem that we created.

The  ‘London Underground mosquito’ is not alone, as other examples are plentiful. Writing in a new report titled ‘How humans drive speciation as well as extinction’, two researchers discuss the mechanisms that drive the formation of human-mediated speciation.

The London Underground Mosquito, found in underground systems worldwide. Presumed to have evolved from standard house mosquito. (Credit: Wikimedia Commons)

The London Underground Mosquito, found in underground systems worldwide. Presumed to have evolved from standard house mosquito. (Credit: Wikimedia Commons)

There are around five to eight million eukaryotic species living on this planet, and 1.0–2.2% of these species become extinct every decade or so. At this rate, many scientists warn, the world is headed for a sixth mass extinction — the only one that won’t be caused by nature. Humans are the obvious culprit, but the authors of the new study say humans also drive the formation of new species. They argue that there are various ways that mediate this process, like human induced physical barriers or selective pressures applied to specific members of a species.

“The prospect of ‘artificially’ gaining novel species through human activities is unlikely to elicit the feeling that it can offset losses of ‘natural’ species. Indeed, many people might find the prospect of an artificially biodiverse world just as daunting as an artificially impoverished one” study author Joseph Bull from the Center for Macroecology, Evolution and Climate at the University of Copenhagen, said in a statement. 

Of course, the most obvious way humans create new species is through domestication. Since the advent of agriculture 8,000 years ago countless species of plants, be them crops or flowers, have been selected and interbred until they look and behave nothing like the original wild species. Wild boars became pigs, wolves became dogs, and “at least six of the world’s 40 most important agricultural crops are considered entirely new” explains Joseph Bull.

[panel style=”panel-info” title=”Humans shaping extinction and speciation alike” footer=”Center for Macroecology, Evolution and Climate Dept. of Food and Resource Economics University of Copenhagen”]Since the last Ice Age, 11.500 years ago, it is estimated that 255 mammals and 523 bird species have gone extinct, often due to human activity. In the same period, humans have relocated almost 900 known species and domesticated more than 470 animal and close to 270 plant species.[/panel]

While domestication is deliberate, most new species which the researchers have identified as being speciated by human activity are the result of unnatural selection. Hunting, for instance, can lead to the adaptation of new traits in animals, which eventually leads to new species. Relocating species, either deliberately or by accident, can also lead to the hybridization of species. In the last three centuries in Europe, at least, due to the relocation of species more new plant species have appeared than those who went extinct.

Then, of course, we have the most disruptive human-driven speciation mechanism: the creation of entirely novel ecosystems. Humans have built cities, farms and underground railways like the one the new mosquito species calls home. Finally, the researchers note that technology might drive more speciation than ever before. Examples include genetically modified organisms, which although are not new species may have the capacity to generate self-sustaining populations or hybridize with wild species. Technology may soon also allow re-creation of extinct species (de-extinction), despite deep practical and moral arguments against doing so.

“In this context, ‘number of species’ becomes a deeply unsatisfactory measure of conservation trends, because it does not reflect many important aspects of biodiversity. Achieving a neutral net outcome for species numbers cannot be considered acceptable if weighing wild fauna against relatively homogenous domesticated species. However, considering speciation alongside extinction may well prove important in developing a better understanding of our impact upon global biodiversity. We call for a discussion about what we, as a society, actually want to conserve about nature”, says Associate Professor Martine Maron from the University of Queensland.

The takeaway is that any species gone extinct at the hand of man is a tragedy. Millions of years of heritage lost forever (despite pipe dreams of cloning, de-extinction and what not). That being said, adding new species in the ecosystem, intentionally or unintentionally, does nothing to make things better. But one can only wonder, what would have biodiversity looked like today if humans lived more sustainably? We might never learn.

America’s invasive species – 450 million years ago

Land clearing and human habitation put significant pressure on local species – combine this with globalization and a general recklessness of the population, and you get a big, negative impact (both environmental and economic) from invasive plants.

invasive-species-laurentia-450-million-years invasive-species-laurentia-450-million-years

But invasive plants aren’t something new – they’ve been around for hundreds of millions of years. Scientists have now analyzed 450-million-year-old fossils of marine creatures that once dwelled in Laurentia, the continent North America once was part of. You may recall the article we posted yesterday about the 350 million year old scorpion, which included a discussion about Laurentia and Gondwana – the only two existing continents on the face of the Earth in that period.

Back then, the forerunners of the Appalachian Mountains may have opened the gates for invasive species to storm Laurentia (or Laurasia). The Taconic mountains, as they were called, left a depression behind the mountain range, flooding the area with cool, nutrient-rich water. In order to better understand how these tectonic processes affected existing life, paleontologists investigated the remains of brachiopods – clam like creatures which dominated the waters during that time.


“Our data show a very clear shift in evolutionary processes that coincides with a shift in Earth systems dynamics,” researcher Alycia Stigall, a paleontologist at Ohio University, explained.” In particular, these results shed light on the Earth system controls on how new species form, or speciation.”

As geological changes slowly took their toll in Laurentia, the fossils highlight two different patterns of evolution: vicariance and dispersal. Vicariance occurs following large-scale geophysical events such as the uplift of a mountain chain, or the separation of continents; through it, new species appear, each better suited for their new habitats.

Dispersal on the other hand involves directly invading habitats for which you are suited. Although initially biodiversity increases, in the long run, this is a very negative process, following which only a few aggressive plants dominate, thus greatly reducing biodiversity.

These findings could provide valuable insight into what drives dispersal today – as a great number of plants are threatened by invasive species.

“Only one out of 10 invaders truly become invasive species,” Stigall said in a statement. “Understanding the process can help determine where to put conservation resources.”

It’s also valuable data in the attempt to understand the emergence of new species:

“Scientists, both biologists and paleontologists, have spent a lot of time and effort studying extinction — the process by which the Earth loses species,” Stigall said. “We understand many of those controls very well — (meteor) impact, volcanism, ocean acidification, habitat destruction. It is relatively easy to envision ways to reduce a population size to zero and thereby cause a species to go extinct. “Understanding speciation is much more complex,” Stigall continued. “Species form by breakdown of gene flow between populations. This is much harder to study on short timescales and the process is explicitly tied to a geographic place and ancestors, which requires understanding both geography and evolutionary history.”

Journal Reference: PLoS ONE. David F. Wright, Alycia L. Stigall: Geologic Drivers of Late Ordovician Faunal Change in Laurentia: Investigating Links between Tectonics, Speciation, and Biotic Invasions.