Life is complex. This is true not just in a rhetorical sense but also biologically in that living things are complicated arrangements of multicellular life. So how did this complexity arise? The most common explanation given is that it is a natural outgrowth of natural selection and that once a self-replicating molecule formed, the selection pressures would be such that given sufficient time multicellular life would be an almost inevitable consequence, though the exact form it would take would depend on many contingencies.
But via Machines Like Us, I came across this article by Carl Zimmer about a new study that suggests that complexity can arise as ‘neutral evolution’ even in the absence of having the kind of selection advantage required by natural selection. In other words, complexity can just happen and does not have to be pushed.
One question to be addressed is that of what exactly constitutes complexity and how it can be measured. The authors of the study suggest that it should not just be the number of cells that make up a organism but also the number of types of cells.
Our bodies are made of 10 trillion cells. If they were all of one type, we would be featureless heaps of protoplasm. Instead we have muscle cells, red blood cells, skin cells, and so on. Even a single organ can have many different cell types. The retina, for example, has about 60 different kinds of neurons, each with a distinct task. By this measure, we can say that we humans are, indeed, more complex than an animal such as a sponge, which has perhaps only six cell types.
The authors of the study have postulated a new law of biology.
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.
They base their conclusions on 916 biological lines of laboratory fruit flies and find that by their measure laboratory fruit flies are more complex than wild ones, even though laboratory flies are subjected to less evolutionary pressures than those in the wild.
Other scientists question whether this conclusion is really warranted by their data and point out that fruit flies encounter selection pressures even in the laboratory.
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.
There is a lot more interesting stuff about the nature of complexity in Zimmer’s article.