Where Do Species Come From?

The evolutionary biologist Jochen Wolf was working from home when we first spoke, in April, 2020. Germany was under lockdown, and his lab, at Ludwig Maximilian University, in Munich, had been closed for weeks. Still, a reminder of his research had followed him from the office. “I have a crow nest right in front of me,” Wolf said, from his rooftop terrace. The nest was well hidden at the top of a tall spruce tree. Through the branches, Wolf could see a female crow sitting on her eggs.

Over the years, Wolf had climbed many similar trees to gather genetic material from crow nests. He had also collected samples from falconers whose goshawks hunt the birds. By comparing the genomes of European crows, Wolf wanted to bring fresh data to one of biology’s oldest and most intractable debates. Scientists have named more than a million different species, but they still argue over how any given species evolves into another and do not even agree on what, exactly, a “species” is. “I have just been comparing definitions of species,” Charles Darwin wrote to a friend, three years before he would publish “On the Origin of Species,” in 1859. “It is really laughable to see what different ideas are prominent in various naturalists’ minds.” To an extent, the same holds true today. It is difficult to find a definition of “species” that works for organisms as different as goshawks and spruce trees. Similarly, it can be hard to draw a line between organisms among whom there are only small differences, such as the goshawks in North America, Europe, and Siberia. Are they separate species, subspecies, or simply locally adapted populations of a single type?

Darwin thought that the blurriness of species boundaries was a clue that the living world was not a divine creation but actually changing over time. He encouraged biologists to treat species as “merely artificial combinations made for convenience,” which would never map perfectly onto nature. “We shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species,” he wrote. His imprecision, however, did not sit well with all of his successors. One of the most influential evolutionary biologists of the twentieth century, a German-born ornithologist named Ernst Mayr, attacked Darwin for failing “to solve the problem indicated by the title of his work.” Darwin had shown how natural selection honed a species to its niche, but he’d “never seriously attempted a rigorous analysis of the problem of the multiplication of species, of the splitting of one species into two,” Mayr wrote, in 1963. Mayr, who spent much of his career at Harvard, called speciation “the most important single event in evolution,” and proposed reproductive isolation as an “objective yardstick” for understanding it: individuals of a sexually reproducing species could procreate with one another but not with individuals of other species.

For decades, Mayr’s arguments dominated evolutionary thought. But consensus was crumbling by the two-thousands, when Wolf confronted the species problem. Wolf had learned Mayr’s “biological species concept” as a student, but he’d also discovered dozens of competing species concepts with alternative criteria, such as an animal’s form, ecology, evolutionary history, and ability to recognize potential mates. (Philosophers had joined the debate, too, with head-scratching questions about the ontological status of a species.) “The more you looked into it, the more confused you got,” Wolf said. Mayr had written that “the process of speciation could not be understood until after the nature of species and of geographic variation had been clarified.” But, in time, Wolf had come to believe the opposite: the nature of species could not be understood until the process of speciation—the ebb and flow of genetic differences between populations, and the evolution of reproductive isolation—had been clarified.

As Wolf and I spoke, a pair of crows landed on my balcony in Berlin. Since the beginning of the pandemic, I had been feeding them hard-boiled eggs and peanuts; that morning I’d set out a jar of water, thinking that they might enjoy a drink as well. The birds were a good example of the species problem. In Munich, Wolf was watching all-black carrion crows; in Berlin, I was watching hooded crows with gray breasts and bodies. During the most recent ice age, glaciers in the Alps and Scandinavia divided an ancestral population of crows into two. At that time, all the crows were probably black. The eastern population found refuge in the Balkans or the Middle East and turned gray for some reason—perhaps lighter feathers helped them stay cool—whereas the western population retreated into Spain. Twelve thousand years ago, when the glaciers melted, the two populations reconvened, in Central Europe. Carrion and hooded crows sometimes interbred and produced fertile and healthy offspring in a narrow hybrid zone running through cities such as Dresden and Vienna. But the birds retained distinct identities on both sides of this feather curtain. “It’s black and gray crows with a hybrid zone exactly through the center of Germany,” Wolf said. Mayr himself had wondered why the two groups did not recombine into a single population. Others saw them as a case that contradicted Mayr’s ideas.

Wolf believed that, using powerful new genome-sequencing technology, he could find an answer to the mystery of the crows’ species. But his first results, published in the journal Science, in 2014, suggested that any concept of species was inadequate for capturing the reality of the crows’ evolution and, by extension, the complexity of the natural world. “No matter how hard we try, there cannot be a robust, all-inclusive, objective species concept,” the Dutch geneticist Peter de Knijff wrote, in a commentary that appeared alongside Wolf’s study. Instead, an emerging field, speciation genomics, was leaving the species concept behind.

A few weeks after my phone call with Wolf, Germany eased its travel restrictions, and I took a train from Berlin to Munich to visit him. The route traversed the crow hybrid zone, but I did not see any birds until I disembarked on the other side. A pair of carrion crows rested on a rooftop antenna across the street from my hotel room. After months of watching hooded crows from my apartment in Berlin, I thought the carrion crows seemed as spectacular as hyacinth macaws.

“I think it’s an age thing,” Wolf said, when I told him how much I had come to enjoy my observations of the crows. Tall and fit, with shoulder-length hair and a strong jaw, Wolf had started studying local birds in his thirties. Born outside of Munich, in 1976, he spent his boyhood in the Bavarian forests that skirt the Alps, and he was drawn to biology as a way to keep on playing in the woods. Later, he studied the behaviors of wolves in Poland, black bears in Maine, and sea lions in the Galápagos Islands. In the Galápagos, it was natural to grow interested in Darwin and evolution. “You start asking, Why does this pup thrive and get fat and healthy and the other one doesn’t?” Wolf said. As a Ph.D. student, in the early two-thousands, he wanted to find out whether sea lions associated mainly with their family members or also “made friends” with nonrelatives. Wolf snuck up on beached sea lions and wrestled them to the ground while a colleague nicked a sample from their flippers; he would later determine the sea lions’ blood relations, using a method patented by a German geneticist named Diethard Tautz. (The method was similar to the paternity tests used by American talk-show hosts in popular “You Are Not the Father!” segments.)

Wolf travelled to Cologne to analyze the tissue samples in Tautz’s lab. Beginning in the nineteen-eighties, Tautz had spent his career sequencing DNA, focussing on only a few hundred base pairs at a time. He was looking to see whether DNA might solve the species puzzle. Decades earlier, Mayr had argued that reproductive isolation can only develop in geographic isolation, after an impassable physical barrier, such as a mountain range or a river, divides a population in two; without migration the two populations would evolve into different species that could remain separate even when the barrier dried up or crumbled. This model, which Mayr called allopatric, or other-place, speciation, became the textbook standard of speciation, even though plenty of organisms appeared to have evolved without a geographic barrier. Some African lakes, for example, contain hundreds of species of colorful fish called cichlids; it was hard to imagine each species evolving in isolation, but Mayr and other mid-century leaders of evolutionary biology were dismissive of alternative ideas. (“These species have come into contact only after they had evolved,” Mayr wrote, of the fish.) For Tautz, the question was not whether allopatric speciation was valid—everyone agreed it was—but whether it was the only way species could diversify. “The allopatric paradigm was based on a few facts, a lot of faith, and on paradigmatic despots ruling the field,” he wrote.

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