More on the White Space in Evolutionary Thinking: A Critic Responds to How to Build a Worm
Getting to adulthood isn't so simple. An egg does not morph into a chick in a single leap, or a chick into a hen. Rather, the fertilized egg must undergo a series of cell divisions accompanied by complex molecular interactions that bit by bit restrict what each cell lineage can do, and these lineages work together to construct the final integrated functional form. For a hen's egg the result is a hen; in the case of the butterfly egg it's a butterfly. And for C. elegans it's a microscopic worm. The new video from Discovery Institute, How to Build a Worm, featuring philosopher of biology Paul Nelson, illustrates this restriction of lineages beautifully.
However, biologist and blogger PZ Myers took exception to this video, claiming that it demonstrates an insufficient understanding of developmental biology. His objection appears to have been its emphasis on teleology -- that the worm's development is directed toward the final adult form -- and the conclusion that only intelligent agents can produce such a goal-directed process.
Curiously, Myers's response avoids engaging Paul Nelson on teleology except to flatly deny it. Myers says that with regard to development, many of the molecular details for C. elegans have been worked out and they are entirely mechanistic and natural, not the work of an intelligent designer. He gives a few examples of how cell fates get specified, and he acknowledges the complexity of the mechanisms involved. He agrees that the more we learn, the more complex things appear. Nonetheless, he says, certain principles can be deduced from what we have learned from other organisms besides C. elegans. Myers summarizes, saying very succinctly: "Development is both hierarchical and incremental."
Myers does not indicate how such developmental pathways evolved, however. What he does say is that the same molecules involved in cellular interactions in C. elegans are found in the vast array of animal forms we see now. He does not say but implies that those molecules are used in analogous ways in different species, and are therefore proof of descent with modification, otherwise known as evolution.
He acknowledges that the unit of selection (the stage of an organism's life cycle that natural selection selects) is the individual capable of reproduction -- in other words, the adult. And I infer from this that he believes there must have been a step-wise selectable pathway from a single cell to multicellular adult, each step of which was both viable and capable of reproduction.
Yes, the details of development that have been worked out for C. elegans are indeed solely natural, though I might dispute the word mechanistic. (See here for more on that.) Yes, the more we know, the more complicated things get. Yes, development is both hierarchical and incremental. (It's worth noting that both of these adjectives describe a pattern that could be due to the nature of the developmental process, but there's reason to think that a designer would use these same principles to organize things.)
As for his point about shared molecules across animal phyla, the fact that there are important developmental regulators that are present throughout animal phyla should not come as a surprise. There are two possible explanations. One is implied by Myers, that these shared molecules are evidence of common descent. Perhaps. But another explanation is possible. Life is a very complex integrated system with hierarchical layers of regulation and gene expression, similar to the programs and sub-programs of computer software but much more sophisticated. When human programmers work with complex systems they simplify matters by creating abstractions -- interfaces that specify all the detailed instructions below them -- allowing the architect to work across platforms and manipulate programs with similar functions in multiple environments. By analogy, it may be that the important developmental regulators present in various animal phyla are like the abstractions created by human designers, put in place to allow cross-platform manipulation.
But the main problem with Myers's argument is that there are enormous gaps between his explanation of development and how it might have evolved. You can imagine a simple evolutionary pathway, but when you get down to the nitty-gritty it's far from simple. I discussed the hypothetical evolutionary sequence in a recent article here, "The White Space in Evolutionary Thinking."
The genus Volvox is a favorite with evolutionary biologists as an example of the evolution of multicellularity. Volvox species are a group of organisms that share the same basic life history. Each organism is comprised of photosyntheitic cells stuck together in a gel-like matrix. In some Volvox species, all the cells help the organism to swim using flagella and to photosynthesize. Then at a cue they stop swimming and the cells divide to produce more Volvox. Other species of Volvox have a different method of reproduction, though. Most of their cells are dedicated to swimming and photosynthesis, but a subset is dedicated to reproduction. The vast majority of cells provide nutrients for the few that reproduce the next generation. It's a simple division of labor controlled by a handful of genes. There are speculative theories as to how such a simple bifurcating path of development may have arisen.
What is absent from the theories are all the things necessary to get to this stage of evolution, things present in eukaryotes but not in bacteria: the nucleus, with its controlled flow of information in and out, a different mechanism of duplicating and segregating DNA to daughter nuclei, cell adhesion molecules that allow the cells to stick together and form junctions between them, the matrix in which the cells are embedded, and the cytoskeleton that holds things in place and permits cell division to occur. Evolving each of these things would involve the evolution of multiple interacting proteins, either by cooption or by de novo creation.
Beyond Volvox, at each new evolutionary stage new things would need to be made -- new cell types, new body plans, new life histories, and new modes of reproduction, all produced by the creation of new proteins or the cooption of old ones. However, our own research has shown that unless a protein can be coopted to a new function within three or four mutations, it isn't going to happen. And that number of mutations is highly unlikely to be able to create new functions for old proteins. It's even harder to get an entirely new protein from random sequence -- it's next to impossible.
Furthermore, there is the problem of selection for new functions or new forms -- new organs or new body plans. Getting a new form or function typically involves multiple genes and proteins, even when stripped down to a bare minimum of steps. The transition to a new form or function won't occur unless each step on the pathway provides some benefit to the organism, which is rarely the case. And you can't coopt something to a new function unless it already exists and serves some purpose. That's the kicker -- where did all these cells and proteins come from in the first place?
Learning about the development of organisms is a wonderful thing. In the last few decades, biologists have made enormous strides in discovering how a wide variety of organisms develop from egg to adult. They are finding themes that occur repeatedly. But to say that organisms' similarities demonstrate that they evolved step by step from simpler forms is to miss exactly what it would take, and how utterly improbable such evolution would be.