Planet Earth

Deep Time In a Bird’s Beak

The LoomBy Carl ZimmerSep 14, 2004 8:27 AM


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Evolution works on different scales. In a single day, HIV's genetic code changes as it adapts to our ever-adapting immune system. Over the course of decades, the virus can make a successful leap from one species to another (from chimpanzees to humans, for example). Over a few thousand years, humans have adapted to agriculture--an adult tolerance to the lactose in milk, for example. Over a couple million years, the brains of our hominid ancestors have nearly doubled. Sometimes scientists distinguish between these scales by calling small-scale change microevolution and large-scale change macroevolution. Creationists have seized on these terms and used them to build one of their central canards: that they accept microevolution but can then reject macroevolution. That's a bit like accepting microeconomics--how households and firms make decisions and interact in markets--but then denying macroeconomics--how entire societies produce goods, how inflation rises and falls, and so on. Evolutionary biologists debate fiercely about how macroevolutionary change emerges from microevolution. But they continue to find abundant evidence that the two are a package deal.

I was reminded of the interwoven scales of evolution last week when, just before leaving on vacation, I read a wonderful new paper about how the beaks of baby birds develop. As I drove off sans laptop, I was sure that it would be heavily blogged and reported while I was away. But when I returned I found almost complete silence. So I thought I would do my small part to keep this research from disappearing into the data smog.

After all, these baby birds are not just any birds. They belong to a group of some 13 species collectively known as Darwin's finches. Charles Darwin first encountered the birds in 1835 when he visited the Galapagos Islands. He thought at first that they were belonged to various groups of birds, such as wrens and blackbirds. After all, their beaks were dramaticall different from one another--some blunt, some narrow, some curved. Not surprisingly, the birds use these different beaks to get different kinds of food--cracking nuts, drinking nectar, and so on. Darwin was shocked to learn later that all of the birds were finches. He struggled to understand why such an unparalleled diversity of finches existed only on a remote archipelago. That struggle helped lead him to his theory of evolution by natural selection.

As Jonathan Weiner recounts in his excellent The Beak of the Finch, later generations of biologists came back to the Galapagos to study the birds. Living in near isolation, they are a natural experiment in evolution. Today the leading experts on the finches are Peter and Rosemary Grant of Princeton University. They and their colleagues have shown that the birds originate from a few settlers who arrived on the islands two to three million years ago. These founders gave rise to different lineages, each of which adapted to the islands with a special beak shape of its own. This evolutionary change is remarkably fast compared to most other animals, and it continues today. As droughts and heavy rains hit the islands every few years, natural selection favors different beak sizes. Meanwhile, populations of the finches become separated from one another as they develop unique mating songs. Sometimes this divergence produces a new species. In other cases, closely related species may interbreed and fuse back together.

The Grants wondered what sort of mutations were fueling this extraordinary evolution of beaks on the Galapagos. They joined forces with developmental biologists at Harvard to study the genes that build the finch body within the egg--in particular, genes known as growth factors that stimulate cells to divide and differentiate. They found that a gene called bone morphogenetic protein 4 (BMP-4) played a key role. Big-beaked birds such as the ground finch made a lot of BMP-4 early on in development in the cells of their jaws. The slender-beaked cactus finch produces less BMP-4, and does so later. Each species they studied had its own unique pattern of BMP-4 activity, while the other growth factors behaved pretty much the same.

BMP-4 has a number at the end because it belongs to a family of genes. Originally, there was one BMP-like gene, and at some point it was accidentally duplicated. Those copies were duplicated again and again. The copies evolved differences in their sequences, and some eventually mutated into gibberish. It turns out that the first gene of this family evolved a long time ago. A huge range of animals have BMP-like genes, ranging from vertebrates to sea urchins to insects. The genes are so similar that you can destroy the insect version of BMP-4 in a fruit fly, replace it with a frog's BMP-4 gene, and the frog gene will cooperate perfectly well to build a fly. The simplest explanation for this similarity is that all these animals (known as bilaterians) inherited their BMP-like genes from a common ancestor some 700 million years ago. In early bilaterians, BMP-like genes probably helped lay out the front and back of a developing body. In vertebrates, it is active along the abdomen side, where the digestive system grows. Insects run their digestive system along their back, and in insect larva, that's where BMP-like genes are active.

These BMP genes belong to an entire network of body-building genes that have survived for 700 million years. Some of them switch on BMP genes, while others block their activity. And BMP genes in turn switch on and shut down other genes. This network has been borrowed many times in the course of evolution to build new structures in animal bodies. As vertebrates evolved skeletons made of bone, the BMP network took on a new role helping to build it. (BMP encourages bone to grow, and also to heal--making it the object of a lot of interest in medical circles.) But its role was not limited to ribs and vertebrae. As new sorts of vertebrates evolved, the BMP network was coopted yet again. In birds, for example, feathers grow under the guidance of the BMP network. And so to, the Grants and their colleagues have found, do bird beaks.

So here we have a network of genes that has played a major role in evolution at many scales. It emerged as part of an animal toolkit, which could be used to construct bodies as different as that of a fly and a fish. It was then borrowed and redeployed in new ways, building new structures. And because this network controls many other genes, a small tweak to it can produce some significant changes even within a single species. Alter the timing of BMP ever so slightly in a finch's developing beak, and it may be prepared to survive a drought by cracking hard seeds. Thanks to the relative ease by which beaks can evolve, these sorts of generation-to-generation changes have helped Darwin's finches explode into 13 new species over the past couple million years. Micro and macro, in other words, are bound together into one extraordinary whole.

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