The Gene: An Intimate History - Siddhartha Mukherjee Page 0,50

drives evolution.

Dobzhansky’s final flourish was to solve the “mystery of mysteries” that had preoccupied Darwin: the origin of species. The Galápagos-in-a-carton experiment had demonstrated how a population of interbreeding organisms—flies, say—evolves over time. But if wild populations with variations in genotype keep interbreeding, Dobzhansky knew, a new species would never be formed: a species, after all, is fundamentally defined by its inability to interbreed with another.

For a new species to arise, then, some factor must arise that makes interbreeding impossible. Dobzhansky wondered if the missing factor was geographic isolation. Imagine a population of organisms with gene variants that are capable of interbreeding. The population is suddenly split into two by some sort of geographical rift. A flock of birds from one island is storm-blown to a distant island and cannot fly back to its island of origin. The two populations now evolve independently, à la Darwin—until particular gene variants are selected in the two sites that become biologically incompatible. Even if the new birds can return to their original island—on ships, say—they cannot breed with their long-lost cousins of cousins: the offspring produced by the two birds possess genetic incompatibilities—garbled messages—that do not allow them to survive or be fertile. Geographic isolation leads to genetic isolation, and to eventual reproductive isolation.

This mechanism of speciation was not just conjecture; Dobzhansky could demonstrate it experimentally. He mixed two flies from distant parts of the world into the same cage. The flies mated, gave rise to progeny—but the larvae grew into infertile adults. Using linkage analysis, geneticists could even trace an actual configuration of genes that evolved to make the progeny infertile. This was the missing link in Darwin’s logic: reproductive incompatibility, ultimately derived from genetic incompatibility, drove the origin of novel species.

By the late 1930s, Dobzhansky began to realize that his understanding of genes, variation, and natural selection had ramifications far beyond biology. The bloody revolution of 1917 that had swept through Russia attempted to erase all individual distinctions to prioritize a collective good. In contrast, a monstrous form of racism that was rising in Europe exaggerated and demonized individual distinctions. In both cases, Dobzhansky noted, the fundamental questions at stake were biological. What defines an individual? How does variation contribute to individuality? What is “good” for a species?

In the 1940s, Dobzhansky would attack these questions directly: he would eventually become one of the most strident scientific critics of Nazi eugenics, Soviet collectivization, and European racism. But his studies on wild populations, variation, and natural selection had already provided crucial insights to these questions.

First, it was evident that genetic variation was the norm, not the exception, in nature. American and European eugenicists insisted on artificial selection to promote human “good”—but in nature there was no single “good.” Different populations had widely divergent genotypes, and these diverse genetic types coexisted and even overlapped in the wild. Nature was not as hungry to homogenize genetic variation as human eugenicists had presumed. Indeed, Dobzhansky recognized that natural variation was a vital reservoir for an organism—an asset that far outweighed its liabilities. Without this variation—without deep genetic diversity—an organism might ultimately lose its capacity to evolve.

Second, a mutation is just a variation by another name. In wild fly populations, Dobzhansky noted, no genotype was inherently superior: whether the ABC or CBA strain survived depended on the environment, and on gene-environment interactions. One man’s “mutant” was another man’s “genetic variant.” A winter’s night might choose one fly. A summer’s day might choose quite another. Neither variant was morally or biologically superior; each was just more or less adapted to a particular environment.

And finally, the relationship between an organism’s physical or mental attributes and heredity was much more complex than anticipated. Eugenicists such as Galton had hoped to select complex phenotypes—intelligence, height, beauty, and moral rectitude—as a biological shortcut to enrich genes for intelligence, height, beauty, and morality. But a phenotype was not determined by one gene in a one-to-one manner. Selecting phenotypes was going to be a flawed mechanism to guarantee genetic selection. If genes, environments, triggers, and chance were responsible for the ultimate characteristics of an organism, then eugenicists would be inherently thwarted in their capacity to enrich intelligence or beauty across generations without deconvoluting the relative effects of each of these contributions.

Each of Dobzhansky’s insights was a powerful plea against the misuse of genetics and human eugenics. Genes, phenotypes, selection, and evolution were bound together by cords of relatively basic laws—but it was easy to imagine that these laws could be misunderstood and distorted. “Seek