Where do new species come from?
Evolution is considered a long and slow process, especially when it comes to the formation of new species...
Since Charles Darwin and other scientists of the 19th Century drafted the first theories of evolution, biologists have studied and partly unveiled this mysterious process that has endowed Earth with the amazing diversity of creatures we witness today. Yet, we still don’t understand entirely the intricate mechanisms that drive the formation of new species, and it has only recently become clear that, given the right conditions, new species can appear quite rapidly.
For the first evolutionary biologists such as Darwin and Alfred Russell Wallace, the emergence of new species - speciation - was principally visible on a continental scale. They observed that populations separated over large distances and long periods of time had adapted differently to their environments and taken separate evolutionary paths. With the lack of tools or even knowledge of genetics, evolution was a process that could only be witnessed with the eyes. Darwin understood that the birds on several islands of the Galapagos looked similar to each other and to those of the American continent, yet showed several key differences, like the sizes of their beaks. This helped him develop the idea of species changing and adapting to new environments through natural selection. It was, however, much harder to explain how new species arose in the absence of geographical barriers and through forces different from natural selection.
The study of evolution took a gigantic step forward with the arrival of genetics and the advances that have been made since Gregor Mendel devised the first principles in the garden of the monastery where he grew his famous peas. Nowadays, thanks to the enormous progress in scientific technology, we can affordably sequence entire genomes in a matter of days, grow thousands of parallel cell-cultures in the laboratory and do even more insane things that were completely unimaginable fifteen years ago. We can now witness evolution on smaller and larger scales than Darwin and Wallace could with their naked eyes. It is now, more than ever, that we realise how ubiquitous and diverse evolution truly is.
As a graduate student at the Ludwig-Maximilians-Universität in Munich, I got the chance to study the process of speciation through a very interesting set of grasshopper species grouped under the genus name Chorthippus. These different species all look incredibly similar, making it practically impossible for biologists to distinguish them based on their appearances. To make the distinction even more challenging, the species co-exist in large areas of Europe, occurring side-by-side even in the very same meadows. Yet, taxonomists have classified them into separate species according to the distinctive mating songs of the males. More recent studies however, have questioned whether one could rightfully categorise Chorthippus grasshoppers into different species. First, researchers discovered that the species can mate and produce healthy hybrid offspring when forced to do so in the lab. If the animals were doing this in the wild, one could argue they were just a single species with many song variations. Secondly, phylogenetic research on the Chorthippus group showed that grasshoppers supposedly belonging to different species were genetically so closely related that it wasn’t possible to place them in different groups. Yet, this study was only based on a minute part of the gigantic grasshopper genome, turning it into the equivalent of looking at a single brush-stroke of an inmense, intricate and detailed painting.
With these different findings in mind, there were two possible scenarios for the evolutionary history of Chorthippus grasshoppers. In one, they interbred in the wild and couldn’t be considered as different species. The other possibility was that the different species formed so recently that they had not yet reached the stage of being physically unable to reproduce and hadn’t diverged enough genetically to detect it with small-scale genetic studies. If the latter case were true, a barrier must exist that prevents the different species from mating in nature. We knew the species could be found together in the same grass fields of Central and Western Europe, so unlike Darwin’s finches, it was not geography or ecology that separated them. The best separating candidates we had were the different mating songs of the grasshopper males: if the females had evolved to choose only mates who sing the song of their own, they might never or very rarely outbreed, leading to a segregation of different singing groups in the same physical space. It was now my - and my supervisor’s - task to find out which of these two possibilities is true.
Although the research on this specific question was going to be entirely computer-based, I got introduced to living, wild Chorthippus grasshoppers when my supervisor invited me on a field trip to the Pyrenees for a side-project of his, shortly before starting my thesis. The plan was to drive all the way from Munich to the French-Spanish border (and back) to catch two types of grasshoppers which coexist exclusively there for breeding experiments in the lab. After two full days of driving through the Swiss lowlands, the French pre-Alps and Côte d’Azur, we finally arrived in a small Spanish mountain town where our collaborators from Madrid were waiting with a beer in the excruciating summer heat. On that same day, I got acquainted with the tiny grasshoppers and the surprisingly difficult task of telling them apart. The senior scientists in our team tried to teach me how to detect the one species we were looking for and how to tell adults and nymphs, males and females apart. But, in all honesty, until the last day I was still finding myself running over the grass following what I thought was the desired species, hunting the insect down only to discard it with a disappointed face when I realised it was in fact a very similar-looking Chorthippus cousin. Yet, no matter how inexperienced I was, and how difficult the grasshopper hunt could be, it was a real pleasure to work surrounded by such fantastic landscapes out in the open, fresh mountain air. After a day of catching insects, we would come back down the mountain, take care of our living catch and enjoy the cooler hours of the evening with the team. A couple of days later, my supervisor and I left the Pyrenees with more than 250 vividly jumping and singing grasshoppers in cooled plastic bottles filled with grass on the back seat.
After that small fieldwork adventure, it was time to get to the real business. I had become acquainted with my study species, but had now a question to answer which could not be resolved by mere fieldwork. The answer on whether the grasshoppers bred out or not hid in their DNA. However, doing genetic research nowadays requires much more preparatory work on the computer than one would imagine. Although the lab-work of extracting and sequencing DNA from animal tissue had already been done for us by different labs, we now had to sort, clean and standardise the genetic information that had been loaded onto the Internet by several different scientists around the world. Since I was learning while working, it took me around two months to have the primary DNA sequences ready for the first analyses. While it was getting increasingly dark and cold in Munich, I started to master the skills of computer coding and the first results started to roll in. My work was a long way removed from what is generally considered a biologist’s job: no damp jungles or vast savannas for me to explore. Instead, millions and millions of letters of genetic code on post-apocalyptic black screens with green digital text reminiscent of the Matrix. Yet, I was deeply enjoying the computer work: even if I wasn’t venturing deep into the natural world, I was still on a treasure hunt for answers on evolutionary questions. And, eventually, when Munich was at its coldest and not a single grasshopper could be alive in the surrounding fields of the university buildings, it seemed as though my supervisor and I had figured out whether a millennia-long love story had ever existed between the Chorthippus grasshoppers, or not.
Fantastically enough, even though they live centimetres away from each other and could perfectly mate, our set of grasshopper species seemed not to do so. Their genetic codes told us that the different species were cleanly separated, with no signs of interbreeding at all. Stranger still, we discovered that these grasshopper species were extremely "young", arising most probably within the last 12 000 years ago, during or after the last Ice Age. At this time in history, human beings were already developing agriculture and building the first settlements. On an evolutionary timescale, these species had formed in seconds. As for why the grasshoppers developed different songs and evolved into different species, the answer remains unclear. One possibility could be that they were physically segregated in the past, for example in separate refugia during the Ice Age, and were reunited once the ice pulled back. But it is equally possible that the species only started to diversify afterwards, during their expansion towards the newly-freed territories of Europe. What is fascinating about these tiny grasshopper species is that, despite their young age and great similarity, they still maintain a very clear barrier – in this case generated by the search for a mate - and not directly by the survival in a new environment.
Our grasshoppers showed us, once again, that new species can arise and be maintained in many different ways other than complete geographical isolation and natural selection. This kind of evidence for evolution was hard to perceive for Darwin and Wallace, yet, with the help of modern tools and computers, we can now understand the diversity of evolutionary paths that nature embarks upon everywhere and all the time around us. We are still far from having grasped the entire book of evolution, but it is by unravelling the origin of the tiniest - and seemingly most insignificant - creatures around us that we learn more about this astonishing process. Next time you hear a grasshopper sing in your yard, think about the blind but marvelously intricate tinkering and engineering behind it, which has composed that melody precisely so that it can be distinguished from all other sounds and songs around it. Nature is full of secrets and surprises, waiting for us to discover, and in every creature, may it be a plant, animal, or something else, we can find traces of this endlessly fascinating process.