The Beak of the Finch: Chapter 13 Summary & Analysis

Back in Princeton, the Grants are well into their sabbatical year. Their data has revealed that since the El Niño of 1982-83, the adaptive landscape on the islands has changed dramatically. There is far less Tribulus and cactus, but more soft, small seeds. So the ground finches who feast on those seeds are doing well, while the cactus finch population is suffering. There are only about 100 left—the lowest their numbers have been since the finch watch began in the 1960s. Yet the cactus finches themselves haven’t changed or adapted—they have no other “adaptive peak” to fly to.

The medium ground finches, too, have been affected—they are flexible in what they can eat, but only to a certain point. Fortis with deeper, wider beaks are dying—but those born after the flood with narrower beaks were doing better. They have been able to adapt, survive, and nearly replace themselves. After the flood, hybrids, too have been thriving, showing that in a place that changes as rapidly as Daphne Major, it can be advantageous to be on the slope of an adaptive peak rather than firmly at the top.

When an adaptive landscape changes dramatically—when a new species wanders into very different territory, for instance—the genetic changes that will allow that species to suitably adapt over time are very large. Two evolutionists, Richard Lewontin and L.C. Birch, published a study in 1966 concerning the adaptive capabilities of fruit flies who migrated from warmer weather to cooler weather. The flies changed as they moved farther from home—and Lewontin and Birch hypothesized that because the original species once lived side by side with a similar species, the two different species passed genes back and forth. The mixture of genes, the two researchers suggested, allowed the flies to expand their landscape.

The Grants have taken this research one step further—they’ve begun to believe that while hybrid offspring might not normally be fit for a given environment, a “rare event” like a drought, plague, or flood can so transform the adaptive landscape that which traits are favored shift immensely and there is a slow fusion of populations. The pendulum swings constantly: sometimes hybrids are at a disadvantage, other times at an advantage. It’s hard for fusion to fully take place, though, before fission—environmental change—swings the other way. The Grants have witnessed these swings numerous times over the decades, and they’ve concluded that hybridization between species “provides favorable conditions for major and rapid evolution to occur.”

Hybridization among birds was once thought to be very rare—but among the 9,600-odd species of bird alive today, interbreeding is far more common than it once seemed. New hybridization most commonly takes place between different varieties of plants, and it’s also been observed often among toads, insects, and fish—generally, within populations that stay in the same place. But in a case like the Galápagos finches, it’s becoming clear that every so often, a certain lineage needs “an infusion of fresh variation”.

“Reticulate evolution” is the name for a kind of evolution that appears more like a tangled thicket and less like the distinct branches of a tree. This kind of evolution doesn’t bind lineages together forever—so it’s been historically overlooked. But the Grants are taking care to call attention to how their finches engage in this kind of evolutionary behavior. This messiness creates the “bountiful diversity” that defines life on Earth. The forms of living things are constantly in flux—in spite of the invisible borders around them.