February 2009

Where's the evolution?

On his trip to the Galapagos, Darwin collected nine finch species (scientists now recognize 14 or 15) — but he only accurately identified six of them as finches. Some of the birds were so different from standard finches that Darwin assumed them to be wrens or members of the oriole family. He only discovered his error when he returned to England and showed his specimens to a leading ornithologist, who informed Darwin that all the birds actually all belonged to the same close-knit finch group. Eventually — after his ideas on evolution had begun to take shape — Darwin recognized that the finches, with their distinct sizes, beak shapes, and plumage colors, might perfectly exemplify his ideas about adaptation via natural selection and common ancestry. He didn’t know much about the evolution of these birds, but surmised that they had all evolved from a common ancestor that had somehow reached the Galapagos and diversified into a variety of specialized forms. In 1845, Darwin summed up his views: “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.”

Darwin’s hypotheses about the birds’ evolution were based on fairly casual observations (his mislabeling of the finch specimens and localities has caused endless headaches for historians!) and powerful theory. Subsequent research has backed him up on all counts and has illuminated new details. What have we learned about the evolution of the Galapagos finches since Darwin? Lots! Here are just a few of the twists and turns in the finches’ evolutionary story that recent research has brought to light:

Who are their ancestors? DNA collected from the Galapagos finches tells us that they are most closely related to seed-eating tanager finches that live in the Caribbean, Central America, and South America. Based on the amount that the finches’ genetic sequences have diversified, scientists reason that they must have arrived on the islands two to three million years ago and begun speciating soon after that. We still aren’t sure (and may never know) why the original Galapagos finches ventured across the 900 kilometers of ocean that separate the islands from the mainland, but we do know that a fair number of birds (probably 30 or more) made the journey. Biologists have studied the genes that encode parts of the finches’ immune system (the major histocompatibility complex) and have found them to be quite diverse. Since populations that evolved from a very small number of founders tend to be more uniform in this respect, we can infer that the finches must have had a more-than-minimal starting population.

How are they related? The genetic sequences of the Galapagos finches and the Cocos finch suggest that they form a monophyletic clade — that is, that they represent all the living descendents of a single common ancestor. Their genetic sequences also clarify the relationships between species, as shown in the phylogeny here. Finches with similar feeding approaches tend to be closely related to one another. For example, the two species of finches that specialize on cactus fruit are close evolutionary cousins. From geologic and other lines of evidence, we know that the finches speciated during a turbulent time. Over the past 2 million years, volcanic activity has added 14 new islands to the Galapagos archipelago, sea level has changed (altering the islands’ sizes and the distance between them), and the climate of the islands has shifted. The finches’ ancestor probably arrived on a lush, tropical paradise — which became much cooler and dryer over the subsequent millennia. The changing ecology, geology, and geography of the Galapagos provided many opportunities for isolation and specialization of finch populations.

How do they evolve? Rapidly! For more than 30 years, Biologists Peter and Rosemary Grant, along with their colleagues and students, have been carefully documenting the ups and downs of the finches on the Galapagos island of Daphne Major. Even in this relatively short amount of time, they have directly observed the evolution of body size, beak size, and beak shape in response to changing ecological conditions, such as droughts and El Nino events (which both change the proportions of different sorts of seeds available for food) and the invasion of another finch species that competes with locals for food.

What is the genetic basis of this evolutionary change? Darwin was impressed by the morphological diversity of the finches’ beaks, which range from narrow probes to powerful crushing machines. These different beak shapes are encoded in the finches’ genes. With the latest genetic and developmental techniques, biologists have now identified two of the genes involved in reshaping beaks over evolutionary time. When the Bmp4 gene is turned on early and at high levels in the beak of a developing bird, the beak becomes deeper and broader. When the CaM gene is turned on at high levels in the beak of a developing bird, the beak becomes elongated. These genes are fairly similar to one another in different finch species — but they are turned on and off at different times. For example, particularly high levels of Bmp4 during development seem to result in the robust beak of Geospiza magnirostris. The evidence suggests that, over evolutionary time, changes in the patterns of expression of these and other genes (e.g., when and where they are turned on and off) evolved, producing the diverse set of finch beak shapes we see today.

In the past 150 years, scientists have learned a remarkable amount about the evolution of the Galapagos finches. Unaware of genes, DNA, isotopic dating, and host of other implements in the toolkits of modern evolutionary biologists, Darwin couldn’t have imagined the advances to come in our understanding of evolution — that we might, for example, reconstruct evolutionary relationships among a group of organisms simply by sampling their blood and sequencing their genes. We’ve certainly come a long way! Nevertheless, science is an ongoing process, and even as we reflect on Darwin’s bicentennial, biologists continue to look ahead. Scientists studying the Galapagos finches are currently trying to learn more about the changing ecology of the Galapagos, how and why finch populations stopped breeding with one another, the genetic changes that accompanied these splits, and much more. We imagine that Darwin couldn’t have asked for a better 200th birthday present than to see his life’s work so powerfully extended.