"So nat'ralists observe, a flea
Hath smaller fleas that on him prey,
And these have smaller fleas that bite 'em,
And so proceed ad infinitum."
Has λ phage evolved a new IC system?
In the last post in this series, we discussed the results of an experiment done by the Lenski group on λ phage evolution, and some of its implications for the argument from irreducible complexity (IC). We now turn our attention to the question of whether the new function observed to arise in this experiment constitutes a new IC system. Recall that in Darwin’s Black Box, Behe (a) introduces IC systems as something beyond the abilities of evolution to produce gradually, and (b) defines them as follows:
What type of biological system could not be formed by “numerous, successive, slight modifications”? Well, for starters, a system that is irreducibly complex. By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. (pg. 39)
For λ phage, the original system of protein J binding to LamB on the outer membrane of E. coli is one interaction out of several that are essential for its replication. These interactions are a single system that requires well-matched parts (for precise protein binding) that contribute to the basic function (replication of the phage). If either LamB or protein J are disrupted, the system ceases to function (the phage loses the ability to replicate). By Behe’s definition, this is an IC system.
The new system is similarly IC: protein J now binds to OmpF on the outer membrane, and this interaction requires well-matched, interacting parts. The other steps in the phage replication cycle are the same, and use the interactions present in the previous IC system. As before, removal of either protein J or its binding partner (now OmpF) causes the system to cease functioning (since at this point in the experiment LamB is already missing). The new system is also IC according to Behe’s criteria.
But here’s the rub: in this experiment, the researchers observed the new IC system form step by step through “numerous, successive, slight modifications” to the previous IC system. At no point did those modifications remove the function of the original system, but in fact improved it (the modified protein J binds to its original partner, LamB, even more effectively than before).
Has λ phage jumped the Edge?
Beyond its troubling ability to generate new IC complexes by modifying old ones, λ phage evolution, as we have seen, is similarly troubling for the probability argument Behe lays out in his book Edge of Evolution. The key section (that we have discussed previously) is as follows:
Although two or three missing steps doesn’t sound like much, that’s one or two more Darwinian jumps than were required to get chloroquine resistance in malaria. In chapter 3 I dubbed that level a “CCC”, a “chloroquine complexity cluster,” and showed that its odds were 1 in 1020 births… Now suppose that, in order to acquire some new, useful property, not just one but two new protein-binding sites had to develop. A CCC requires, on average, 1020, a hundred billion billion, organisms – more than the number of mammals that has ever existed on earth. So if other things were equal, the likelihood of getting two new binding sites would be what we called in Chapter 3 a “double CCC”- the square of a CCC, or one in ten to the fortieth power. Since that’s more cells than likely have ever existed on earth, such an event would not be expected to have happened by Darwinian processes in the history of the world.
Admittedly, statistics are all about averages, so some freak event like this might happen – it’s not ruled out by force of logic. But it is not biologically reasonable to expect it, or less likely events that occurred in the common descent of life on earth. In short, complexes of just three or more different proteins are beyond the edge of evolution… And the great majority of proteins in the cell work in complexes of six or more. Far beyond that edge. (pp. 134-135)
The key point being that mutations, in Behe’s model, are simultaneous mutations, not sequential ones.
Drawing the argument together from both Darwin’s Black Box and Edge of Evolution, we can summarize Behe’s argument with three statements:
- Evolution of protein-protein binding sites cannot proceed by multiple, simultaneous mutations because the probability of such events is too rare to be expected to happen during the history of life on earth.
- Evolution must proceed only by multiple, simultaneous mutations to create new protein-protein binding sites. Gradual modifications to produce new IC systems are not possible.
- Thus when we observe complexes with more than three protein-protein binding partners, we can infer that they were not produced by evolution.
The difficulty with this argument is that Behe has not established the second point, but merely asserted it: that evolution must proceed only by multiple, simultaneous mutations to create new protein-protein binding sites. Indeed, Darwin’s original prediction was that “new” systems would not really be “new” at all, but rather gradually modified from previously existing systems through accumulation of small changes. If protein-protein complexes (including IC ones) can be built up over time by gradual addition of new components, then Behe cannot claim statement 3, even if statement 1 remains uncontroversial.
The evidence we have seen from λ phage is doubly problematic for the argument from IC: not only has a new IC system been formed, but its stepwise formation, one amino acid substitution at a time, was repeatedly documented in detail.
λ phage, that tiniest of fleas, has inflicted quite a bite on the argument from IC.
What about other lines of evidence?
As compelling as the evidence from this detailed experiment is, it is not the only evidence that points to the conclusion that new protein-protein interactions, functions and systems can arise gradually. Other lines of evidence we have examined include:
The similarities between whole genomes that speak to gradual change at the nucleotide and amino acid levels over large evolutionary times: the sequence differences between the human and chimpanzee genomes, for example, are tiny. Yet despite these minute differences at the genomic level, we (obviously) exhibit many traits that chimpanzees do not. The obvious conclusion is that the accumulation of small genetic changes to complex systems can have dramatic effects on function.
The gradual emergence of anaerobic citrate metabolism in an experimental population of E. coli bacteria: As we saw when we examined this system, a new, advantageous trait arose piecemeal, eventually cobbling together a new function from mutations that occurred tens of thousands of generations apart in some cases. This evidence further undermines the contention that evolution requires simultaneous mutations.
The recently duplicated p24-2 gene in Drosophila: Whatever p24-2 and Éclair are doing, exactly, there is good evidence to suggest they are not merely duplicates of each other, but have distinct functions. It’s highly likely that p24-2 has developed new protein-protein interactions of some kind (unless we wish to posit that p24-2 does its job without interacting with any other proteins at all, which is highly unlikely). We do not yet know if these genes are involved in IC systems, though the fact that they are required for survival suggests that they have important roles. Interestingly, Behe’s reply to my argument (via comment #70322) in part discusses how existing IC systems might add components:
Now what we have here is an already-intact transport system. At best, the amino acid changes in p24-2 would be analogous to re-arranging the magnets at the end of the oval-shaped object, so that it could transport a different object than before. But that is just taking advantage of an already-existing IC system; it is not a new one.
I would argue that observing new components adding themselves to already complex systems, and then becoming essential, is just the type of gradualism that evolutionary theory predicts. Behe continues:
A final point is that there are five specific amino acid differences between Eclair and p24-2. Professor Venema seems not to comprehend that they may not have arisen by random mutation. Once one gets beyond one or two random mutations, one can’t assume that multiple further mutations arose by chance. In other words, as far as anyone knows, those five point mutations may have required guidance or design in their appearance.
But as we have seen, the (repeated) accumulation of five or more amino acid substitutions in protein J of λ phage was no problem at all, despite the astronomical odds of all the needed mutations happening at once (note too that Behe’s argument depends on simultaneous mutations, as we have seen). There is no need to assume that the differences between p24-2 and Éclair arose by chance, but rather cite the evidence that multiple amino acid changes have been observed to accumulate over time in other systems. While we cannot recreate the last 3 million years of Drosophila evolution, it seems strained to argue that this time span is insufficient to generate the same amount of change that we observe in the laboratory in a matter of weeks in other, more tractable systems.
In the next installment of this series, we’ll examine some recent ID arguments from biochemistry that purport to have found a limit to evolutionary mechanisms.
For further reading:
Behe, M.J. Darwin’s Black Box: the Biochemical Challenge to Evolution. Free Press, New York, 1996.
Behe, M.J. The Edge of Evolution: the Search for the Limits of Darwinism. Free Press, New York, 2007.
Meyer, J.R., Dobias, D.T., Weitz, J.S., Barrick, J.E., Quick, R.T., and Lenski, R.E. (2012). Repeatability and contingency in the evolution of a key innovation in phage lambda. Science 335; 428-432. http://www.sciencemag.org/content/335/6067/428.abstract