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The Surprising Origins of Evolutionary Complexity

Scientists are exploring how organisms can evolve elaborate structures without Darwinian selection

One advantage of this definition is that you can measure complexity in many ways. Our skeletons have different types of bones, for example, each with a distinctive shape. Even the spine is made up of different types of parts, from the vertebrae in the neck that hold up our head to the ones that support our rib cage.

In their 2010 book Biology's First Law, McShea and Brandon outlined a way that complexity defined in this way could arise. They argued that a bunch of parts that start out more or less the same should differentiate over time. Whenever organisms reproduce, one or more of their genes may mutate. And sometimes these mutations give rise to more types of parts. Once an organism has more parts, those units have an opportunity to become different. After a gene is accidentally copied, the duplicate may pick up mutations that the original does not share. Thus, if you start with a set of identical parts, according to McShea and Brandon, they will tend to become increasingly different from one another. In other words, the organism's complexity will increase.

As complexity arises, it may help an organism survive better or have more offspring. If so, it will be favored by natural selection and spread through the population. Mammals, for example, smell by binding odor molecules to receptors on nerve endings in their nose. These receptor genes have repeatedly duplicated over millions of years. The new copies mutate, allowing mammals to smell a wider range of aromas. Animals that rely heavily on their nose, such as mice and dogs, have more than 1,000 of these receptor genes. On the other hand, complexity can be a burden. Mutations can change the shape of a neck vertebra, for instance, making it hard for the head to turn. Natural selection will keep these mutations from spreading through populations. That is, organisms born with those traits will tend to die before reproducing, thus taking the deleterious traits out of circulation when they go. In these cases, natural selection works against complexity.

Unlike standard evolutionary theory, McShea and Brandon see complexity increasing even in the absence of natural selection. This statement is, they maintain, a fundamental law of biology—perhaps its only one. They have dubbed it the zero-force evolutionary law.

The Fruit-Fly Test

Recently McShea and Leonore Fleming, a graduate student at Duke, put the zero-force evolutionary law to the test. The subjects were Drosophila flies. For more than a century scientists have reared stocks of the flies to use in experiments. In their laboratory homes, the flies have led a pampered life, provided with a constant supply of food and a steady, warm climate. Their wild relatives, meanwhile, have to contend with starvation, predators, cold and heat. Natural selection is strong among the wild flies, eliminating mutations that make flies unable to cope with their many challenges. In the sheltered environment of the labs, in contrast, natural selection is feeble.

The zero-force evolutionary law makes a clear prediction: over the past century the lab flies should have been less subject to the elimination of disadvantageous mutations and thus should have become more complex than the wild ones.

Fleming and McShea examined the scientific literature for 916 laboratory lines of flies. They made many different measures of complexity in each population. In the journal Evolution & Development, they recently reported that the lab flies were indeed more complex than wild ones. Some of the insects had irregular legs. Others acquired complicated patterns of colors on their wings. The segments of their antennae took on different shapes. Freed from natural selection, flies have reveled in complexity, just as the law predicts.

Although some biologists have endorsed the zero-force evolutionary law, Douglas Erwin, a leading paleontologist at the Smithsonian National Museum of Natural History, thinks it has some serious flaws. “One of its basic assumptions fails,” he argues. According to the law, complexity may increase in the absence of selection. But that would be true only if organisms could actually exist beyond the influence of selection. In the real world, even when they are pampered by the most doting of scientists, Erwin contends, selection still exerts a force. For an animal such as a fly to develop properly, hundreds of genes have to interact in an elaborate choreography, turning one cell into many, giving rise to different organs, and so on. Mutations may disrupt that choreography, preventing the flies from becoming viable adults.

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