Why Evolution Goes Wild on Islands: The Science of Adaptive Radiation
Story by Irby Lovette; Design by Jillian Ditner
December 20, 2018
Normally, bird identification begins by mentally assessing similarities to other familiar birds: Is it a finch, tanager, wren, or sparrow? Experience allows for an educated guess and turning right to the relevant section of a field guide where the unknown bird’s likely family is illustrated.
But there are places in the world where even very closely related birds look incredibly different from one another. For example, on the Galápagos Islands one bird species might look like a warbler, another like a grosbeak, and a third like no other bird on the planet. Yet despite their different shapes, sizes, and colors, these three birds are all in the same family—close descendants of the same avian ancestor.
The process that creates these many different forms stemming from the same original bird is called “adaptive radiation,” and it has confounded past generations of seasoned naturalists and expert ornithologists. Yet however perplexing, a deeper look at adaptive radiations reveals that these birds are wondrous examples of the power of evolution to create new forms. Among the most spectacular extremes of bird evolution, these explosions of avian diversity are worth understanding, celebrating, and conserving.
Evolution goes into overdrive to fill unoccupied niches
Scientists apply the term “adaptive radiation” to groups in which one ancestor species has rapidly evolved into many descendant species, each with its own specialized way of life.
Here, the word “adaptive” invokes the process of natural selection that has fine-tuned each species to use its environment in a different way. One bird species might evolve a long, curved beak that is particularly well suited for sipping nectar from within deep flowers, while another species might evolve a massively robust beak that allows it to crack open the hardest of seeds. Each of these adaptations allows its respective species to use its environment in a different way.
The second word—“radiation”—refers to the particularly rapid diversification of many species that evolve from a single common ancestor. One hallmark of adaptive radiation is the unusually fast pace of species forming and diverging from one another.
Some of the clearest examples of these evolutionary explosions have occurred on remote island archipelagos that offer freedom from competition and access to a variety of habitats.
Because land birds rarely make it to isolated islands, the few pioneering species that first arrive and colonize find a wide-open ecosystem with many vacancies for exploiting food resources. The absence of competitors makes it possible for the colonizing species to be fruitful and multiply, evolving and spiraling off into many descendant species that each starts to specialize in different food types.
Darwin’s Finches: A classic example of adaptive radiation
Adaptive radiation confused Charles Darwin. When first observing the finches of the Galápagos Islands, Darwin classified some species as wrens or warblers, others as grosbeaks, some as finches, others as blackbirds. It was only after expert ornithologists back in England examined his specimens that Darwin realized all of these birds were closely related, rather than members of different avian families. Today we refer to this entire group as “Darwin’s finches,” even though Darwin’s first impressions of them were mistaken.
Genetic evidence has since revealed another surprise: Darwin’s famous “finches” are actually tanagers! Their ancestor was a small tanager that colonized the archipelago about 2 million years ago. Nobody knows exactly how that first colonist arrived; perhaps a small flock was blown far out to sea in a storm. That single ancestor has since radiated into 17 or so descendant species with a great variety of bill shapes, ranging from short, pointed bills in the insect-eating warbler-finches to the massive, seed-cracking bill of the Large Ground-Finch. Some of these finch species no longer interbreed, yet others still hybridize frequently.
Malagasy Vangas: Evolution creates a woodpecker from a warbler
The vangas of Madagascar represent an adaptive radiation of notable antiquity. The ancestor of all vangas colonized the island of Madagascar about 20 million years ago and was most likely a bird that gleaned insects off vegetation like a warbler. From that single species evolved 21 descendant vanga species, representing a great variety of feeding strategies—an aerial, flycatcher-like bird that snaps up insects out of mid-air; a bird that probes into bark like a woodpecker; and many others that forage in different ways.
Many vanga species are so different from one another that for centuries they were classified into different bird families: vanga species vary greatly in coloration, size, feeding behavior, and bill shape. Thanks to genetic studies, ornithologists have discovered that these vangas are all part of a single, grand adaptive radiation.
Hawaiian Honeycreepers: Adaptive radiation goes to extremes
Many ornithologists tout the Hawaiian honeycreepers as the most spectacular avian example of adaptive radiation. From a single ancestor, this group evolved into more than 50 honeycreeper species spanning an incredible variety of bill shapes and feeding behaviors. This adaptive radiation was fostered by the absence of competing species: the Hawaiian archipelago is so remote that very few other landbirds ever found their way there, leaving many habitats and food types open for the honeycreepers.
The ancestor of the honeycreepers was a rosefinch-like bird, most likely from Asia, that first colonized the archipelago about 6 to 7 million years ago. Over the following millennia, these finchlike colonists diversified into an incredible variety of forms, with honeycreeper species that mirror nearly all the bill shapes found in passerine songbirds around the world—nectar sippers, seed eaters, tree bark foragers, and more—plus several bill shapes not found in any other birds anywhere else on the planet.
Selection at work in your backyard
Although the most famous adaptive radiations of birds have occurred on islands, the same processes of evolution are happening among birds everywhere. If you have a bird feeder nearby, take a close look at the beaks of your avian visitors.
The birds with the largest and heaviest bills—like cardinals or grosbeaks— are particularly well adapted for opening large and hard seeds. For example, a cardinal can easily crack a tough sunflower seed. At the other extreme, the diminutive siskins and redpolls use their tiny, sharp-pointed bills to eat small seeds, like the black thistle or nyjer seeds that require a special type of bird feeder with small openings. In the middle of the range are many seedeating birds such as finches, juncos, sparrows, and goldfinches with medium-sized bills adapted for cracking and eating medium-sized seeds.
None of these species originally evolved their particular bill size in response to the cultivated seeds we now provide in bird feeders, but their behavior at feeders is a good indication of how they have evolved to specialize in different foods. Cardinals are great at cracking large seeds, but their large bills make them far less efficient than siskins at eating the smallest seeds. In turn, siskins can easily and efficiently manipulate tiny seeds, but they would starve before being able to crack open a tough sunflower seed. Given the popularity of bird feeding, it seems entirely possible that these birds are experiencing new kinds of evolutionary selection from these abundant food sources.
Irby Lovette is the Fuller Professor of Ornithology at Cornell University and lead editor of the Cornell Lab of Ornithology Handbook of Bird Biology. Jillian Ditner is Living Bird graphics artist.