Barbara J. King is emerita professor of anthropology at the College of William and Mary. Her latest book is “Personalities on the Plate: The Lives & Minds of Animals We Eat.”
Evolutionary biologist Jonathan B. Losos has spent a lot of time on tropical islands chasing small lizards called brown anoles, lassoing them with waxed dental floss and measuring their legs. As it turns out, anole species that use broader surfaces to get around have evolved longer legs compared with their narrow-substrate counterparts. In “Improbable Destinies: Fate, Chance, and the Future of Evolution,” Losos amusingly recounts the times during his research when he was outwitted by these “pea-brained” anoles or coped with surprises such as roaches lurking in his cereal box.
But Losos spins a serious science story through the lens of lizard legs, with remarkable skill. He reaches beyond the behavior of the plants and animals filling his pages to tackle a question central to today’s practice of evolutionary biology: “whether natural selection inevitably produces the same evolutionary outcomes or whether the particular events a lineage experiences — the contingencies of history — affect the end result.”
The revered figure of Harvard evolutionary biologist and science popularizer Stephen Jay Gould looms behind this question. Gould, who died in 2002, took a hard-line stance for contingency and against evolutionary predictability. Replay the tape of life, he famously wrote, and nothing would unfold in the same way it first had: We humans wouldn’t even be here. Too many tiny variables would cascade through the millennia to allow the same evolutionary pathway to occur twice.
At first, by offering abundant instances of convergent evolution under natural conditions, Losos veers toward an anti-Gouldian perspective. The caffeine molecule in coffee, tea and cacao, three plants not closely related, evolved independently. Eyes of humans and octopuses are “nearly identical” although their shared ancestry is a half-billion years back. In the Greater Antilles, anole lizards on four different islands — again, not close kin — carve up the habitat in the same ways, as do mouse-eared bats across different regions in many parts of the world.
Soon, Losos beautifully complicates this story. Species often fail to converge even when faced with similar selection pressures. Cape buffaloes’ horns, impalas’ long legs, porcupines’ spines and the projectile venom of the spitting cobra are all adaptations to sharing habitat with predators such as lions. An animal’s existing structure and physiology constrain what evolutionary pathways may be taken. As Losos reminds us, natural selection, lacking foresight, “won’t favor a detrimental feature just because it is an early step on a path leading to an ultimately superior condition.”
Dueling lists, offered either in support of or against evolutionary predictability, yield no definitive answer. How about experiments that test the question of evolutionary determinism directly? Here, “Improbable Destinies” really soars, achieving at times the pace and wow factor of thrill fiction.
From Darwin’s time until about 50 years ago, it had been thought that evolution occurred at such a slow pace that it would be impossible to field-test hypotheses relating to it. Losos takes some poetic license in suggesting that we now know evolution may operate “at light speed,” but from a perspective of eons of geological time, he’s not far wrong. Working in Trinidad in the 1970s, zoologist John Endler moved guppies from streams dense with pike cichlids, “a streamlined torpedo with teeth that makes its living eating guppies,” to other streams containing killifish, which take a far more lackluster approach to consuming guppies. In a mere two years, the guppies evolved from drab-colored with small spots to colorful with larger spots in response to the great lessening of predation pressure.
Losos mentions in passing “several potential harms” that come with relocating animals like this. Indeed, throughout the book scientists are shown relentlessly interfering with animals’ lives, as when the mortality of light- and dark-colored deer mice in Nebraska zooms up because they were moved around for an adaptation experiment. Animals routinely face all sorts of death pressures in the wild, of course; whole lizard populations were washed away by hurricanes during Losos’s research. When scientists intervene with apparent aplomb, however, ethical concerns deserve a fuller hearing.
Whatever their pros and cons, from these experiments on numerous species, the take-home message became clear: Evolution can be rapid and, to some notable degree, predictable. So was Gould wrong? If we could rewind the tape of life, would evolution unfold again in much the same way? Settling for no easy answers, Losos adds layer upon layer of nuance as he shows just how challenging it is to answer this question conclusively. A laboratory experiment with E. coli run by Rich Lenski, now nearing its third continuous decade and involving more than 64,000 generations, encapsulates why. At the start, all the E. coli were genetically identical, meaning that when they were made to encounter new experimental conditions, mutation was the sole available pathway for evolutionary change. After 14 years, the data indicated that “faced with the same selective environment, populations independently evolved in the same way.” Then came the stunner: One population evolved an adaptation, the ability to feed on citrate as well as glucose in the presence of oxygen, that had apparently never occurred before. “So much for predictability and parallel evolution!” Losos exclaims.
It may take many generations for evolutionary unpredictabilities to show up, then. Both random and highly (though not completely) determined paths occur, and it’s scientists’ job to figure out the relative contributions of each, in each individual case.
Why does any of this matter outside of the science classroom? Knowledge of evolutionary pathways may help us fight the potential catastrophe of increased antibiotic resistance or design custom treatments for cystic fibrosis patients who suffer from bacterial rampages in their lungs. Maybe it can even help us think through animals’ and plants’ responses to global warming patterns or the potential forms extraterrestrial life might take. Yet science need not be applied to our own well-being to matter. With an ideal combination of clarity and comedy, scholarly caution and infectious enthusiasm, Losos shows us how evolutionary biology opens up for each of us the glorious workings of our world, with surprises around every corner.
By Jonathan B. Losos
Riverhead. 368 pp. $28