Jim Shapiro has been outlining his views on evolution over at the Huffington Post, including a posting entitled, What Is the Key to a Realistic Theory of Evolution?
Not surprisingly, Jerry Coyne does not like it and weighs in with a posting entitled, A colleague wrongfully disses modern evolutionary theory.
Let me focus on a key point of their disagreement.
Shapiro takes issue with the conventional notion that all of evolution has occurred gradually. He notes:
Darwin put it this way in Chapter 6: “If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find no such case.”
And then cites examples of such cases:
Many genome changes at key stages of evolution have been neither small nor gradual.
Perhaps the most important evolutionary step of all took place at least one billion years ago, when two or more cells fused to produce the first “eukaryotic” cell having a defined nucleus. This nucleated cell was apparently the progenitor of all “higher” forms of life, including plants and animals. Such cell mergers are known as “symbiogenesis,” long championed as an evolutionary force by the recently deceased biologist, Lynn Margulis .
Coyne responds as follows:
We all know, thanks to Lynn Margulis, that the evolution of the eukaryotic cell involved two rapid evolutionary events: the acquisition of mitochondria via the ingestion of one bacterium by another cell, and, in plants, the origin of chloroplasts via a similar route. Centrioles (a group of microtubules involved in cell division) may have originated via symbiosis. And some species, like lichens, are actually a mixture of two distinct species—in the case of lichens, an alga (or cyanobacterium) and a fungus. That fusion probably happened quickly as well.
Margulis theorized that symbiosis was not only important in evolution, but ubiquitous, involved in nearly all cases of speciation and macroevolution. She was wrong. We know now that the rapid origin of new taxa by symbiosis, while critical for some evolutionary transformations, is rare. It can hardly be used to discount the notion that “Darwinian” evolution is usually gradual.
At this point, it becomes clear Coyne cannot follow the argument. Shapiro was not talking about the origin of mitochondria and chloroplasts. He was talking about the origin of Eukarya, where bacteria and archaea fused to generate the eukaryotic cell plan.
What’s more, Shapiro is not arguing that Margulis was right about symbiosis being behind all speciation. He is arguing that Darwin was wrong in insisting that all biological features were formed by numerous, successive, slight modifications. Coyne’s confusion leads him to think that by noting Margulis was wrong, Darwin was right.
We have an example where the most radical evolutionary transition of all time did not occur through numerous, successive, slight modifications – the eukaryotic cell. And even if Coyne is correct in noting that birds, mammals, whales, etc. arose entirely through numerous, successive, slight modifications, they could only do this because they were built from eukaryotic cells (there are no prokaryotic bird-like or mammal-like organisms). This means that Coyne’s fossil evidence, as evidence of gradualism, is of secondary concern. What is of primary concern is the origin of the cell type that made that evolution possible.
So when we turn to the origin of eukarya, we can ask if there is any evidence that such a transition would (or could) occur without symbiosis? Answer – none. Random mutations and natural selection alone would not have spawned the eukaryotic cell. We are on safe ground in insisting that symbiosis was a necessary step in evolving eukaryotes.
Then we need ask if this necessary step was a fluke, or whether it was something that was bound to happen. The “fluke” hypothesis makes no predictions and thus can always be maintained as someone’s metaphysical preference. The “bound to happen” hypothesis predicts that bacteria and archaea were endowed with preadaptations such that the emergence of euakarya was poised to happen. And there is a growing list of evidence that preadaptation was important in the appearance of the eukaryotic cell plan and its organelles.