High in the hills of Kazakhstan, where the ancestors of malus domestica trees first began experimenting with the shape and color of their fruit, you can find an astounding variety of examples of what the apple could have been, from large purplish softballs to knobby green clusters. But through its countless journeys over the Silk Road many thousands of years ago, carried by generations of wayfarers, the apple adapted to meet the needs of its traveling companions, evolving to become a portable, durable conduit for sweetness.
Despite the wide variations of taste you find in different cultures, a predilection for sweetness seems to be universal. But sweetness is a quality rarely found in nature. Most apples that grow in the wild taste bitter; only a few trees produce a fruit that is sweet. But we humans have learned over the years to cultivate sweetness in the apple, mostly by grafting the trees that produced the tastiest fruit. The apple, for its part, protected its seeds by making them distasteful, even slightly poisonous, so that we wouldn't eat or digest them and they would survive to produce more trees.
At its core, each apple contains seven or eight seeds, each of which contains the genetic ingredients for a tree radically different from its parents and its siblings. More than any other trait, it is the apple's genetic variability that accounts for its ability to make itself at home in places as different from one another as New England and New Zealand, Kazakhstan and California. Wherever the apple tree goes, its offspring propose so many different variations on what it means to be an apple—at least seven per apple, several thousand per tree—that a couple of these novelties are almost bound to have whatever qualities it takes to prosper in the tree's adopted home.
When it traveled to the New World, the apple reinvented itself once again, mirroring its immigrant companions. Bitter apples, planted from seed instead of grafted, were used by homesteaders across the young country to produce hard cider—until a national campaign against alcohol threatened Americans' relationship with the apple. But with the help of early public relations pioneers crafting slogans such as "an apple a day keeps the doctor away," the plant quickly reinvented itself as a healthy foodstuff.
The quest to sweeten the typically bitter fruit ignited a grower's frenzy in the 19th century, bringing fame to hundreds of quintessentially American personalities such as the Red Delicious, the Baldwin and the Jonathan. Each iteration of the apple reflected the young country's understanding of itself as a diverse nation of transplants, a new breed of individuals digging new roots into their adoptive soil.
The apple's astounding global success has, ironically, created new problems for the plant. As we go about selecting the tastiest apple and sending it around the world, we are also shrinking the species' genetic diversity by grafting the same plant over and over, restricting its natural ability to keep adapting its defenses against the pests that prey upon it. Most of the apples grown today have been grafted from the same five or six parents. That has allowed the apple's natural insect and viral enemies to gain on it, requiring farmers to apply ever-greater amounts of pesticides to keep the predators at bay.