An organism can exist without external selection—without the environment determining who wins and loses in the evolutionary race—but it will still be subject to internal selection, which takes place within organisms. In their new study, McShea and Fleming do not provide evidence for the zero-force evolutionary law, according to Erwin, “because they only consider adult variants.” The researchers did not look at the mutants that died from developmental disorders before reaching maturity, despite being cared for by scientists.
Another objection Erwin and other critics have raised is that McShea and Brandon's version of complexity does not jibe with how most people define the term. After all, an eye does not just have many different parts. Those parts also carry out a task together, and each one has a particular job to do. But McShea and Brandon argue that the kind of complexity that they are examining could lead to complexity of other sorts. “The kind of complexity that we're seeing in this Drosophila population is the foundation for really interesting stuff that selection could get hold of” to build complex structures that function to aid survival, McShea says.
As a paleobiologist, McShea is accustomed to thinking about the kind of complexity he can see in fossils—bones fitting together into a skeleton, for example. But in recent years a number of molecular biologists have independently begun to think much as he does about how complexity emerges.
In the 1990s a group of Canadian biologists started to ponder the fact that mutations often have no effect on an organism at all. These mutations are, in the jargon of evolutionary biology, neutral. The scientists, including Michael Gray of Dalhousie University in Halifax, proposed that the mutations could give rise to complex structures without going through a series of intermediates that are each selected for their help in adapting an organism to its environment. They dubbed this process “constructive neutral evolution.”
Gray has been encouraged by some recent studies that provide compelling evidence for constructive neutral evolution. One of the leaders in this research is Joe Thornton of the University of Oregon. He and his colleagues have found what appears to be an example in the cells of fungi. In fungi, such as a portobello mushroom, cells have to move atoms from one place to another to stay alive. One of the ways they do so is with molecular pumps called vacuolar ATPase complexes. A spinning ring of proteins shuttles atoms from one side of a membrane in the fungus to another. This ring is clearly a complex structure. It contains six protein molecules. Four of the molecules consist of the protein known as Vma3. The fifth is Vma11 and the sixth Vma16. All three types of protein are essential for the ring to spin.
To find out how this complex structure evolved, Thornton and his colleagues compared the proteins with related versions in other organisms, such as animals. (Fungi and animals share a common ancestor that lived around a billion years ago.)
In animals, the vacuolar ATPase complexes also have spinning rings made of six proteins. But those rings are different in one crucial way: instead of having three types of proteins in their rings, they have only two. Each animal ring is made up of five copies of Vma3 and one of Vma16. They have no Vma11. By McShea and Brandon's definition of complexity, fungi are more complex than animals—at least when it comes to their vacuolar ATPase complexes.
The scientists looked closely at the genes encoding the ring proteins. Vma11, the ring protein unique to fungi, turns out to be a close relative of the Vma3 in both animals and fungi. The genes for Vma3 and Vma11 must therefore share a common ancestry. Thornton and his colleagues concluded that early in the evolution of fungi, an ancestral gene for ring proteins was accidentally duplicated. Those two copies then evolved into Vma3 and Vma11.