The Ever Evolving Theories of Darwin

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Biologists have documented a vast amount of gene-swapping among single-celled organisms--which happen to make up most of the diversity of life on Earth. There are 10,000 species of bacteria in a spoonful of dirt, twice as many species as all the mammals in the world. In the genome of a typical microbe, most of the genes hopped from one species to another at some point in the history of life. In some ways, the history of life is indeed like a tree, sprouting new branches. But in some ways, it's also like a tapestry, emerging from a loom, its genetic threads woven together in new combinations.

In the mid-1900s, biologists succeeded in merging the newest biological developments at the time into a new vision of evolution known as the Modern Synthesis. Today a number of biologists argue that it's time for a new understanding of evolution, one that Pigliucci has called the Extended Evolutionary Synthesis. For now, they are fiercely debating every aspect of that synthesis--how important gene-swapping is to the course of evolution, for instance, and how gene networks get rewired to produce new traits.

Some researchers argue that many patterns of nature--such as the large number of species in the tropics--cannot be reduced to the effect of natural selection on individuals. They may be following rules of their own. "Which of these ideas is going to actually survive and prove fruitful is anybody's guess," says Pigliucci. "I don't see things coalescing for at least a decade or more."

Darwin predicted this. "We can dimly foresee that there will be a considerable revolution in natural history," he wrote at the end of On the Origin of Species. He saw his work not as the end of biology but as a beginning.

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DARWIN TODAY Species share a common ancestry, like branches on a tree Genetic studies confirm that different species have evolved from common ancestors. But DNA has also jumped from one species to another--turning parts of the tree of life into a web Humans evolved from apes in Africa Evidence from DNA indicates that chimpanzees and bonobos are the closest living relatives to humans. Fossils document the course of human evolution in Africa from apelike ancestors over the past 7 million years Natural selection is a powerful force driving evolution Natural selection's fingerprints can be detected in the human genome. But many mutations have spread thanks to pure chance (a process known as genetic drift) Complex traits like eyes can evolve through a series of intermediate steps Fossils have documented some of those steps in structures such as limbs and ears. Studies on DNA have shown how genes for building old organs have been "borrowed" to help build new ones

Zimmer is the author of the forthcoming book The Tangled Bank: An Introduction to Evolution