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.Īs Wolf and I spoke, a pair of crows landed on my balcony in Berlin. (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. 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. But consensus was crumbling by the two-thousands, when Wolf confronted the species problem. 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.įor decades, Mayr’s arguments dominated evolutionary thought. 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. His imprecision, however, did not sit well with all of his successors. “We shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species,” he wrote. He encouraged biologists to treat species as “merely artificial combinations made for convenience,” which would never map perfectly onto nature. Are they separate species, subspecies, or simply locally adapted populations of a single type?ĭarwin thought that the blurriness of species boundaries was a clue that the living world was not a divine creation but actually changing over time. 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. It is difficult to find a definition of “species” that works for organisms as different as goshawks and spruce trees. “It is really laughable to see what different ideas are prominent in various naturalists’ minds.” To an extent, the same holds true today. “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. 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. By comparing the genomes of European crows, Wolf wanted to bring fresh data to one of biology’s oldest and most intractable debates. He had also collected samples from falconers whose goshawks hunt the birds. Over the years, Wolf had climbed many similar trees to gather genetic material from crow nests. Through the branches, Wolf could see a female crow sitting on her eggs. The nest was well hidden at the top of a tall spruce tree. “I have a crow nest right in front of me,” Wolf said, from his rooftop terrace. Still, a reminder of his research had followed him from the office. Germany was under lockdown, and his lab, at Ludwig Maximilian University, in Munich, had been closed for weeks. The evolutionary biologist Jochen Wolf was working from home when we first spoke, in April, 2020.
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