by Oh dear, so many canards, so little time.
Let's deal with some of these canards in detail, shall we?
I'm having to split this post into two parts, because of the upper size limit on posts. This is Part 1.
First of all, let's nail the stupidity that is "irreducible complexity", and the farcical notion that the bacterial flagellum somehow could not have arisen via evolution.
"Irreducible complexity" as proposed by Behe et al is a canard. It has been known to be a canard for over six decades. The evolutionary biologist Hermann Joseph Müller alighted upon the idea as far back as 1918, and proposed it not as a problem for evolutionary biology, but as a natural outcome of evolutionary processes. The relevant paper is this one:
Genetic Variability, Twin Hybrids and Constant hybrids in a Case of Balanced Lethal Factors by Hermann Joseph Müller, Genetics, 3(5): 422-499 (1918)
I cite from the paper, downloadable from here as follows (starting at the bottom of page 464):
Hermann Joseph Müller wrote:Most present-day animals are the result of a long process of evolution, in which at least thousands of mutations must have taken place. Each new mutant in turn must have derived its survival value from the effect upon which it produced upon the 'reaction system' that had been brought into being by the many previously formed factors in cooperation; thus, a complicated machine was gradually built up whose effective working was dependent upon the interlocking action of very numerous different elementary parts or factors, and many of the characters and factors which, when new, were originally merely an asset finally became necessary because other necessary characters and factors had subsequently become changed so as to be dependent upon the former. It must result, in consequence, that a dropping out of, or even a slight change in any one of these parts is very likely to disturb fatally the whole machinery.
In other words, "irreducible complexity" was arrived at by Müller before Behe was born and was posited by Müller not as a problem for evolution, but as a natural outcome of evolutionary processes. The so-called "Müllerian Two Step" is summarised succinctly as follows:
[1] Add a component;
[2] Make it necessary.
This was placed upon a rigorous footing by Müller himself, along with others such as Fisher, by the 1930s, and so Behe didn't even find a gap for his purported god to fit into. Biologists have known that Behe's "irreducible complexity" nonsense has been errant nonsense for six decades.
As for the bacterial flagellum, the idea that this could not have evolved is nonsense, pure and simple.
Those nice people over at TalkRational pointed me to a very interesting blog. Namely the blog of Mark Pallen, who was co-author with Nick Matzke of at least one peer reviewed paper in Nature on the bacterial flagellum (and indeed probably wrote more - I just happen to be aware of the one I have saved to my hard drive). That paper is the following one:
From The Origin Of Species To The Origin Of Bacterial Flagella by Mark J. Pallen & Nicholas J. Matzke, Nature Reviews Microbiology, 4(10): 784-790 (October 2006).
I shall return to this paper shortly, but first, a little preamble is needed.
For those unfamiliar with the background, Nick Matzke was the author of an interesting article, namely this one, which hypothesised that the various proteins that are found in the bacterial flagellum would be found to be homologous with other proteins belonging to other metabolic systems, and that as a consequence, the bacterial flagellum would eventually be found to be the result of co-opting existing, earlier systems and re-using them for another purpose - a classic evolutionary process. Needless to say, a lot of noise was emitted by the ID brigade to the effect that Matzke's ideas were "speculation", and the rest of it, but, the point here is that Matzke made testable predictions in his article, and in doing so provided evolutionary biologists with real substance that they could pursue in the laboratory. The following quote from the abstract of Matzke's original paper is apposite:
Matkze, 2001 wrote:A new model is proposed based on two major arguments. First, analysis of dispersal at low Reynolds numbers indicates that even very crude motility can be beneficial for large bacteria. Second, homologies between flagellar and nonflagellar proteins suggest ancestral systems with functions other than motility. The model consists of six major stages: export apparatus, secretion system, adhesion system, pilus, undirected motility, and taxis-enabled motility. The selectability of each stage is documented using analogies with present-day systems. Conclusions include: (1) There is a strong possibility, previously unrecognized, of further homologies between the type III export apparatus and F1F0-ATP synthetase. (2) Much of the flagellum’s complexity evolved after crude motility was in place, via internal gene duplications and subfunctionalization. (3) Only one major system-level change of function, and four minor shifts of function, need be invoked to explain the origin of the flagellum; this involves five subsystem-level cooption events. (4) The transition between each stage is bridgeable by the evolution of a single new binding site, coupling two pre-existing subsystems, followed by coevolutionary optimization of components. Therefore, like the eye contemplated by Darwin, careful analysis shows that there are no major obstacles to gradual evolution of the flagellum.
Now, note that specific predictions were made with respect to the homologies involved, namely that homologies would be found between flagellar proteins and those of the Type 3 Secretory System, plus an enzyme called F1F0-ATP synthetase. I'll leave the latter enzyme aside for a moment, but return to it because this one turns out to play an important role. Stay tuned for the fun revelations!
Now, first of all, the paper from Nature Reviews Microbiology I cited above by Matzke & Pallen itself dispenses wholesale with the idea of the bacterial flagellum being "irreducibly complex", because, lo and behold, there are bacteria with flagella that are missing numerous components. From that paper, I copy the following details with respect to the presence or absence of specific flagellar proteins in various bacteria possessing flagella:
FlgA (P ring) - Absent from Gram-Positive bacteria
FlgBCFG (Rod) - universal
FlgD (Hook) - universal
FlgE (Hook) - universal
FlgH (L Ring) - Absent from Gram-Positive bacteria
FlgI (P Ring) - Absent from Gram-Positive bacteria
FlgJ (Rod) - FlgJ Rod N-terminal domain absent from some systems
FlgK (Hook-Filament Junction) - universal
FlgL (Hook-Filament Junction) - universal
FlgM (Cytoplasm & Exterior) - Absent from Caulobacter
FlgN (Cytoplasm) - Undetectable in some systems
FlhA (T3SS apparatus) - universal
FlhB (T3SS apparatus) - universal
FlhDC (Cytoplasm) - Absent from many systems
FlhE (Unknown) - Mutant retains full motility
FliA (Cytoplasm) - Absent from Caulobacter
FliB (Cytoplasm) - Absent from Escherichia coli
FliC (Filament) - universal
FliD (Filament) - Absent from Caulobacter
FliE (Rod/Basal Body) - universal
FliF (T3SS apparatus) - universal
FliG (Peripheral) - universal
FliH (T3SS apparatus) - Mutant retains some motility
FliI (T3SS apparatus) - universal
FliJ (Cytoplasm) - Undetectable in some systems
FliK (Hook/Basal Body) - universal
FliL (Basal body) - Mutant retains full motility
FliM (T3SS apparatus) - universal
FliN (T3SS apparatus) - universal
FliO (T3SS apparatus) - Undetectable in some systems
FliP (T3SS apparatus) - universal
FliQ (T3SS apparatus) - universal
FliR (T3SS apparatus) - universal
FliS (Cytoplasm) - Absent from Caulobacter
FliT (Cytoplasm) - Absent from many systems
FliZ (Cytoplasm) - Absent from many systems
MotA (Inner membrane) - universal
MotB (Inner membrane) - universal
So, the mere fact that there are in existence bacteria with missing proteins from the above list whose flagella still function rather makes a mockery of the "irreducible complexity" assertion to begin with. But, this is only part of the story. The same paper continues with the following:
Pallen & Matzke, 2006 wrote:Many paths to motility
Although the evolution by random mutation and natural selection of something as complex as a contemporary bacterial flagellum might, in retrospect, seem highly improbable, it is important to appreciate that probabilities should be assessed by looking forward not back2. For example, from studies on protein design it is clear that creating proteins from scratch that, like flagellin, self-assemble into filaments is not very difficult39,40. Furthermore, it is clear that there are many other filamentous surface structures in bacteria that show no apparent evolutionary relationship to bacterial flagella41,42. In other words, there are plenty of potential starting points for the evolution of a molecular propeller. Evolution of something like the flagellar filament is therefore far less surprising than it might at first seem. In fact, microorganisms have adopted other routes to motility besides the bacterial flagellum43. Most strikingly, although archaeal flagella superficially resemble bacterial flagella, in that they too are rotary structures driven by a proton gradient, they are fundamentally distinct from their bacterial counterparts in terms of protein composition and assembly.
Intermediate forms
What about intermediate forms between bacterial flagella and other biological entities? Darwin encountered a similar argument about gaps in the fossil record, and in response he pointed out how improbable fossilization was, so that little of any extinct biosphere could ever be expected to appear in the fossil record14. Although fossils are of no use in reconstructing flagellar evolution, similar arguments might be made at the molecular level. Despite a decade of bacterial genome sequencing, we have scarcely begun to sample the molecular diversity of the biosphere. Yet even with the scant coverage of genome sequence data to date, several curiosities have already been revealed. For example, there is growing evidence that flagellin and the flagellar filament are homologous to the NF T3SS protein EspA and the EspA filament, respectively35,44–48. The EspA filament therefore provides a model for how the ancestral flagellar filament might have functioned for purposes other than locomotion (adhesion or targeted protein secretion). Furthermore, the EspA protein from E. coli initially seemed to be one of a kind. However, thanks to genome sequencing, related proteins have been identified in several bacteria occupying diverse niches, including: S. typhimurium, Edwardsiella ictaluri, Shewanella baltica, Chromobacterium violaceum, Yersinia frederiksenii, Yersinia bercovieri and Sodalis glossinidius. In addition, proteins that resemble flagellar components but that are encoded in the genomes of bacteria that do not engage in flagellar motility have also been identified. The first example of these potential ‘missing links’ came from the chlamydias49. More recently, flagellar-related genes have been detected in the genome of the soil bacterium Myxococcus xanthus, which uses gliding rather than flagellar motility35. It seems likely that other examples of potential evolutionary intermediaries will be found as we sequence an increasing proportion of the biosphere.
The paper continues with:
Pallen & Matzke, 2006 wrote:Towards a plausible evolutionary model
From the above discussions of sequence homologies and modularity, it is clear that designing an evolutionary model to account for the origin of the ancestral flagellum requires no great conceptual leap. Instead, one can envisage the ur-flagellum arising from mergers between several modular subsystems: a secretion system built from proteins accreted around an ancient ATPase, a filament built from variants of two initial proteins, a motor built from an ion channel and a chemotaxis apparatus built from pre-existing regulatory domains (FIG. 1). As we have seen, each of these function in a modular fashion and share ancestry with simpler systems — thereby answering the question ‘what use is half a flagellum?’ Furthermore, it is not hard to envisage how an ancestral crude and inefficient flagellum, if it conferred any motility at all, could function as the starting material for natural selection to fashion today’s slicker flagellar apparatus.
However, one could still question how, from such bricolage, natural selection could lock on to an evolutionary trajectory leading to an organelle of motility in the first place, when none of the components alone confer the organism with a selective advantage relevant to motility. The key missing concept here is that of exaptation, in which the function currently performed by a biological system is different from the function performed while the adaptation evolved under earlier pressures of natural selection50. For example, a bird’s feathers might have originally arisen in the context of selection for, say, heat control, and only later have been used to assist with flight51,52. Under this argument, a number of slight but decisive functional shifts occurred in the evolution of the flagellum, the most recent of which was probably a shift from an organelle of adhesion or targeted secretion, such as the EspA filament, to a curved structure capable of generating a propulsive force.
Now, as a slight tangential diversion, which along the way provides yet more evidence for evolutionary hypotheses, one avenue of attack being considered with respect to the development of the bacterial flagellum is the reconstruction of earlier, more ancient versions of the proteins responsible for the construction of this structure. Precedents already exist with respect to the reconstruction of ancient genes, and the following four papers are examples thereof:
Crystal Structure Of An Ancient Protein: Evolution By Conformational Epistasis by Eric A. Ortlund, Jamie T. Bridgham, Matthew R. Redinbo and Joseph W. Thornton, Science, 317: 1544-1548 (14 September 2007)
Resurrecting Ancient Genes: Experimental Analysis Of Extinct Molecules by Joseph W. Thornton, Nature Reviews: Genetics, 5: 366-375 (5 May 2004)
Resurrection Of DNA Function In Vivo From An Extinct Genome by Andrew J. Pask, Richard R. Behringer and Marilyn B. Renfree, PLoS One, 3(5): e2240 (online version, May 2008)
The Past As The Key To The Present: Resurrection Of Ancient Proteins From Eosinophils by Steven A. Benner, Proc. Natl. Acad. Sci. USA., 99(8): 4760-4761 (16 April 2002)
From the paper by Pask et al above, we have:
Pask et al, 2008 wrote:
There is a burgeoning repository of information available from ancient DNA that can be used to understand how genomes have evolved and to determine the genetic features that defined a particular species. To assess the functional consequences of changes to a genome, a variety of methods are needed to examine extinct DNA function. We isolated a transcriptional enhancer element from the genome of an extinct marsupial, the Tasmanian tiger (Thylacinus cynocephalus or thylacine), obtained from 100 year-old ethanol-fixed tissues from museum collections. We then examined the function of the enhancer in vivo. Using a transgenic approach, it was possible to resurrect DNA function in transgenic mice. The results demonstrate that the thylacine Col2A1 enhancer directed chondrocyte-specific expression in this extinct mammalian species in the same way as its orthologue does in mice. While other studies have examined extinct coding DNA function in vitro, this is the first example of the restoration of extinct non-coding DNA and examination of its function in vivo. Our method using transgenesis can be used to explore the function of regulatory and protein-coding sequences obtained from any extinct species in an in vivo model system, providing important insights into gene evolution and diversity.
So scientists are already resurrecting ancient proteins and testing their functionality in model organisms. Indeed, one of the results in the scientific literature comes courtesy of this paper:
Resurrecting The Ancestral Steroid Receptor: Ancient Origin Of Oestrogen Signalling by J.W. Thornton, E. Need and D. Crews, Science, 301: 1714-1717 (2003)
in which the scientists determined that the modern receptors for steroid hormones in modern organisms are traceable to an ancestral receptor dating back 600 million years, and reconstructed the ancestral steroid receptor in the laboratory to determine that it worked.
So, given that precedents already exist for the successful reconstruction of ancient proteins and the genes coding for them, this avenue of attack is likely to prove highly instructive with respect to the bacterial flagellum. Indeed, Pallen & Matzke make this very observation in their paper:
Pallen & Matzke, 2006 wrote:But obviously, one cannot model millions of years of evolution in a few weeks or months. So how might such studies be conducted? One option might be to look back in time. It is feasible to use phylogenetic analyses to reconstruct plausible ancestral sequences of modern-day proteins, and then synthesize and investigate these ancestral proteins. Proof of principle for this approach has already been demonstrated on several NF proteins69–75. Similar studies could recreate plausible ancestors for various flagellar components (for example, the common ancestor of flagellins and HAP3 proteins). These proteins could then be reproduced in the laboratory in order to examine their properties (for example, how well they self-assemble into filaments and what those filaments look like). An alternative, more radical, option would be to model flagellar evolution prospectively, for example, by creating random or minimally constrained libraries and then iteratively selecting proteins that assemble into ever more sophisticated artificial analogues of the flagellar filament. Another experimental option might be to investigate the environmental conditions that favour or disfavour bacterial motility. The fundamental physics involved (diffusion due to Brownian motion) is mathematically tractable, and has already been used to predict, for example, that powered motility is useless in very small bacteria76,77.
However, let us move on to the more recent developments.
Now, back in 2006, Pallen & Matzke listed some known homologies, and once again, I reproduce their results from the table in the paper:
FlgA (P ring) - CpaB
FlgBCFG (Rod) - FlgBCEFGK
FlgD (Hook) - none specified
FlgE (Hook) - FlgBCEFGK
FlgH (L Ring) - none yet known
FlgI (P Ring) - none yet known
FlgJ (Rod) - none yet known
FlgK (Hook-Filament Junction) - FlgBCEFGK
FlgL (Hook-Filament Junction) - FliC
FlgM (Cytoplasm & Exterior) - none yet known
FlgN (Cytoplasm) - none yet known
FlhA (T3SS apparatus) - LcrD/YscV
FlhB (T3SS apparatus) - YscU
FlhDC (Cytoplasm) - Other activators
FlhE (Unknown) - none specified
FliA (Cytoplasm) - RpoD, RpoH, RpoS
FliB (Cytoplasm) - none specified
FliC (Filament) - FlgL, EspA
FliD (Filament) - none yet known
FliE (Rod/Basal Body) - none yet known
FliF (T3SS apparatus) - YscJ
FliG (Peripheral) - MgtE
FliH (T3SS apparatus) - YscL, AtpFH
FliI (T3SS apparatus) - YscN, AtpD, Rho
FliJ (Cytoplasm) - YscO
FliK (Hook/Basal Body) - YscI
FliL (Basal body) - none yet known
FliM (T3SS apparatus) - FliN, YscQ
FliN (T3SS apparatus) - FliM, YscQ
FliO (T3SS apparatus) - none
FliP (T3SS apparatus) - YscR
FliQ (T3SS apparatus) - YscS
FliR (T3SS apparatus) - YscT
FliS (Cytoplasm) - none yet known
FliT (Cytoplasm) - none yet known
FliZ (Cytoplasm) - none yet known
MotA (Inner membrane) - ExbB, TolQ
MotB (Inner membrane) - ExbD, TolR, OmpA
Now, as Pallen states in his blog entry as linked above, out of this list of proteins, only two were listed as being both essential to all bacterial flagella AND possessing no known homologues in 2006. Those proteins were FliE and FlgD. From the 2006 update of Matzke's original 2003 paper, we read:
Matzke, 2003 wrote:Many of the homologous and/or inessential proteins found in Table 1 of Pallen and Matzke 2006 were cited in the 2003 paper, but the 2006 table is an authoritative update and supercedes what is said here. The important overall point, as discussed in my blog post, is that of the 42 proteins in Table 1 of Pallen and Matzke, only two proteins, FliE and FlgD, are both essential and have no identified homologous proteins. This is substantially more impressive than the situation in 2003, and means that the evidence for the evolutionary origin of the flagellum by standard gene duplication and cooption processes is even stronger than in 2003. Important specific updates include: a homolog of FlgA has been confirmed (along the lines that I suggested in 2003); FliG has no homolog in NF-T3SS or the Exb/Tol systems, rather it may be homologous to the magnesium transporter MgtE; and the flagellar filament protein FliC (and its sister FlgL) is probably homologous to EspA and other pilus proteins found in NF-T3SS. I still suspect that all of the axial proteins (including FliE and FlgD) are homologous to each other and therefore to pilus proteins in NF-T3SS, but only the confirmed homologies are reported in Pallen and Matzke 2006.
At least, this was the situation back in 2006. However, science moves on!
First, take a look at this site, which is the site devoted to ATP synthase. Now, one of the homologies that Matzke originally hypothesised was that at least one of the flagellar proteins would prove to be homologous to proteins in the ATP synthase group, in particular the awkwardly named F1F0-ATP synthetase. Now it turns out that ATP synthases are themselves complex entities, and indeed F1-ATPase rotates on an axis as it performs its synthesis! However, as this paper:
Axle-Less F1-ATPase Rotates In The Correct Direction by Shou Furuike, Mohammad Delawar Hossain, Yasushi Maki, Kengo Adachi, Toshiharu Suzuki, Ayako Kohori, Hiroyasu Itoh, Masasuke Yoshida and Kazuhiko Kinosita, Jr., Science, 319: 955-958 (No. 5865, 15 February 2008)
reveals very succinctly, dismantling this structure so that it no longer has an axle to rotate about does not stop it from functioning! Here's the abstract:
Furuike et al, 2008 wrote:F1–adenosine triphosphatase (ATPase) is an ATP-driven rotary molecular motor in which the central γ subunit rotates inside a cylinder made of three α and three β subunits alternately arranged. The rotor shaft, an antiparallel α-helical coiled coil of the amino and carboxyl termini of the γ subunit, deeply penetrates the central cavity of the stator cylinder. We truncated the shaft step by step until the remaining rotor head would be outside the cavity and simply sat on the concave entrance of the stator orifice. All truncation mutants rotated in the correct direction, implying torque generation, although the average rotary speeds were low and short mutants exhibited moments of irregular motion. Neither a fixed pivot nor a rigid axle was needed for rotation of F1-ATPase.
Another blow to "irreducible complexity" (Hermann Müller would doubtless have smiled wryly over this!), but this isn't all. Returning to Pallen's blog, we find this:
Pallen, 2008 blog post wrote:Since the early 1990s, it has been known, from sequence comparisons, that the flagellar ATPase (FliI) is homologous to the alpha and beta subunits of the F-type ATPase, a transmembrane protein complex (see figure) found in bacteria, mitochondria and chloroplasts (see http://www.atpsynthase.info).
In 2003, Nick Matzke (then at the NCSE and so a couple of years later science adviser to the plaintiffs in the Dover trial) wrote an essay summarising plausible evolutionary scenarios for the origin of the bacterial flagellum. He noted a couple of previous suggestions that the proto-flagellum might have originated from the F-type ATPase. Crucially, he predicted that additional homologies would be found between components of the F-type ATPase and the flagellar protein export apparatus, for example between the b subunit of the ATPase and FliH and between the delta subunit and FliJ.
In 2006, I confirmed one of Nick's hunches through homology searches, showing that part of FliH was homologous to the b subunit. However, things turned out slightly different from Nick's predictions in that FliH is actual of a fusion of domains homologous to the b subunit and the delta subunit.
Last year Namba's group published the structure of FliI and confirmed the striking homology with the F-type ATPase enzymatic subunits. At that stage in the game, it had become clear that the ATPase was a universal component not just of flagellar export systems but also of non-flagellar type III secretion systems. Also, if it was also clear that if one knocked out the gene for FliI, one abolished flagellar biosynthesis. Thus, just about everyone in the field accepted that FliI was an essential component of the flagellar apparatus and that it energised secretion of proteins through the protein export system. In other words, if there were anything to the idea, put forward by Behe and others in the ID movement, that the flagellum showed "irreducible complexity", even experts might have accepted that FliI was one of the "irreducible" components!!
BUT earlier this year, Minamino and Namba (and independently a team headed by Kelly Hughes in the US) overturned all our assumptions by showing that it was perfectly possible to make flagella without FliI--what you needed to do was knock out FliH at the same time. Somehow or other FliH, which usually interacts with FliI, gums up the export apparatus in the absence of FliI. So, bang goes another pillar of support for the ID argument! In fact, it appears that flagellar protein export is powered not primarily by the ATPase but by the proton-motive force.
So, the FliI protein appeared on the face of it to be essential, because knocking out the gene for FliI synthesis destroyed flagellar biosynthesis. But, and here's the part that really throws the spanner into "irreducible complexity" as espoused by Behe, if you knock out the gene coding for FliI, but in addition knock out the gene for FliH, flagellar biosynthesis returns! This rather buggers up "irreducible complexity" in a spectacular manner.
Yet even this is not the whole story. Believe it or not, there is more! Returning to Pallen's blog, we read:
Pallen, 2008 blog post wrote:Namba and colleagues have now solved the structure of FliJ, another protein that interacts with FliI and FliH. And what they found was clear evidence of homology with yet another protein from the F-type ATPase--the gamma subunit!
So, now we have deep and broad homologies between the flagellum and the F-type ATPase, just as Nick predicted. This provides another nail in the coffin of the idea that flagellum was intelligently designed. If the flagellum were the product of intelligent design, particularly by an omniscient deity, the designer could have custom-built it from scratch, so it need not resemble anything else in nature. By contrast, the processes of evolution tends to cobble together and tweak already existing components (something Francois Jacob called bricolage)--and slowly but steadily it is become clear that the flagellum has been built this way.
Incidentally, the paper covering the homology between FliI and the alpha and beta subunits of the F-type ATPase is this paper:
Salmonella typhimurium Mutants Defective In Flagellar Filament Regrowth And Sequence Similarity Of FliI to F0F1, Vacuolar, And Archaebacterial ATPase Subunits by Alfried P. Vogler, Michio Homma, Vera M. Irikura and Robert M. McNab, Journal of Bacteriology, 173(11): 3564-3572 (June 1991) [Full paper downloadable from here]
so this homology had actually been known even before Behe made his assertions about "irreducible complexity", something he would have known if he had bothered to perform a basic literature search. After all, he has institutional access, whereas I don't currently, yet I was able to find this paper once pointed in the right direction. This paper also covers the knocking out of the gene for FliI and the effect on flagellar biosynthesis.
More pertinently, the following paper:
Evolutionary Links Between FliH/YscL-Like Proteins From Bacterial Type III Secretion Systems And Second-Stalk Components Of The F0F1 And Vacuolar ATPases by Mark J. Pallen, Christopher M. Bailey and Scott A. Beatson, Protein Science, 15: 935-941 (2006) [Full paper downloadable from here]
is the one containing the confirmation by Pallen of one of Matzke's predictions as cited above. Another homology was confirmed courtesy of this paper:
Structural Similarity Between The Flagellar Type III ATPase FliI And F1-ATPase Subunits by Katsumi Imada, Tohru Minamino, Aiko Tahara and Keiichi Namba, Proceedings of the National Academy of Sciences of the USA, 104(2): 485-490 [Full paper downloadable from here]
This paper:
Distinct Roles Of The FliI ATPase And Proton Motive Force In Bacterial Flagellar Protein Export by Tohru Minamino and Keiichi Namba, Nature, 451: 485-489 (24th January 2008) [Full paper downloadable from here]
is the paper that covers the knocking out of FliH and FliI resulting in restoration of flagellar biosynthesis. The experimental work documented in that paper verifies the Müllerian Two Step empirically.
So, now the only two proteins remaining to find homologies for are FliE and FlgD, and you can bet that this is being worked upon as I type this.
So, another massive nail in the coffin for ID is hammered home. I'll raise a glass of claret to that.
So, that's that nonsense out of the way.
Next, the so-called "Cambrian explosion". Oh dear, the tiresome canards that are erected with respect to this by creationists are legion. First of all, scientists have unearthed Precambrian fossils in considerable numbers. There are numerous species of Precambrian organisms known to science. So the idea that the Cambrian pseudo-explosion was something special is not supported by the hard evidence from observational reality. Indeed, I've presented in the past the scientific paper describing Bangiomorpha pubescens, a sexually reproducing multicellular eukaryote organism that was discovered in a stratum dated to 1.2 billion years before present, almost 700 million years before the Cambrian pseudo-explosion. Let's take a look at this paper in more detail shall we?
Bangiomorpha pubescens n. Gen., n. sp., Implications For The Evolution Of Sex, Multicellularity, And The Mesoproterozoic/Neoproterozoic Radiation Of Eukaryotes by Nicholas J. Butterfield, Paleobiology, 26(3): 386-404 (7th February 2000)
Butterfield, 2000 wrote:Abstract.—Multicellular filaments from the ca. 1200-Ma Hunting Formation (Somerset Island, arctic Canada) are identified as bangiacean red algae on the basis of diagnostic cell-division patterns. As the oldest taxonomically resolved eukaryote on record Bangiomorpha pubescens n. gen. n. sp. provides a key datum point for constraining protistan phylogeny. Combined with an increasingly resolved record of other Proterozoic eukaryotes, these fossils mark the onset of a major protistan radiation near the Mesoproterozoic/Neoproterozoic boundary.
Differential spore/gamete formation shows Bangiomorpha pubescens to have been sexually reproducing, the oldest reported occurrence in the fossil record. Sex was critical for the subsequent success of eukaryotes, not so much for the advantages of genetic recombination, but because it allowed for complex multicellularity. The selective advantages of complex multicellularity are considered sufficient for it to have arisen immediately following the appearance of sexual reproduction. As such, the most reliable proxy for the first appearance of sex will be the first stratigraphic occurrence of complex multicellularity.
Bangiomorpha pubescens is the first occurrence of complex multicellularity in the fossil record. A differentiated basal holdfast structure allowed for positive substrate attachment and thus the selective advantages of vertical orientation; i.e., an early example of ecological tiering. More generally, eukaryotic multicellularity is the innovation that established organismal morphology as a significant factor in the evolutionary process. As complex eukaryotes modified, and created entirely novel, environments, their inherent capacity for reciprocal morphological adaptation, gave rise to the “biological environment” of directional evolution and “progress.” The evolution of sex, as a proximal cause of complex multicellularity, may thus account for the Mesoproterozoic/Neoproterozoic radiation of eukaryotes.
Moving on from Bangiomorpha pubescens to other organisms, Ediacaran fossils are now well known to palaeontologists. Appropriate papers on the subject include:
Anatomical Information Content In The Ediacaran Fossils And Their Possible Zoological Affinities by Jerzy Dzik, Integrative and Comparative Biology, 43(1): 114-126 (2003) [full paper downloadable from here]
Dzik, 2003 wrote:SYNOPSIS. Various modes of preservation of Ediacaran fossils in different sediments, quartz sand at Zimnie Gory in northern Russia and lime mud at Khorbusuonka in northern Yakutia, show that the sediment was liquid long after formation of the imprints and that its mineralogy did not matter. A laminated 2 mm thick microbial mat is preserved intact at Zimnie Gory. It stabilized the sediment surface allowing formation of imprints on it. The soft body impressions on the under surface of the sand bed and within it developed owing to formation of a less than 1 mm thin ‘‘death mask’’ by precipitation of iron sulfide in the sediment. Fossils of the same species or even parts of the same organism may be preserved differently. Internal organs either collapsed, their cavities being filled with sediment from above, or resisted compression more effectively than the rest of the body. This allows restoration of the original internal anatomy of Ediacaran organisms. At Zimnie Gory numerous series of imprints of Yorgia on the clay bottom surface with the collapsed body at their end represent death tracks. The environment of formation of the Ediacaran fossils was thus inhospitable to most organisms. Those adapted to it, namely the radially organized frondose Petalonamae (of possible ctenophoran affinities), anchored in the mat with their basal bulbs. They evolved towards sessile life possibly in symbiosis with photo- or chemoautotrophic microorganisms. Vagile Ediacaran organisms belong mostly to the Dipleurozoa (somewhat resembling chordates and nemerteans), characterized by a segmented dorsal hydraulic skeleton, intestine with metameric caeca, and serial gonads. Only a fraction of the actual Precambrian faunal diversity is represented in the Ediacaran biota.
Among the organisms covered in the above paper include Kimberella, Dicksonia costata, Yorgia waggoneri, Ernietta, Rangea, Pteridinium, Marywadea, Spriggina, Praecambridium, Vendia, Archaeapsis, Chondroplon and Andiva. Also mentioned is Yunnanozoon, an early Cambrian organism that is considered to have descended from one of the Ediacaran dipleurozoans. This list is incomplete (but I've provided a link to the full paper so you can download it and find the other organisms listed). Note that Yunnanozoon above is a Chordate - in other words, an organism belonging to the same taxonomic Phylum as you and I. At the time that paper was written, Yunnanozoon was considered to be the oldest known Chordate, but wait a moment, I have a nice surprise in store for you ...
There is also this paper:
The Ediacaran Emergence Of Bilaterans: Congruence Between The Genetic And The Geological Fossil Records by Kevin J. Peterson, James A. Cotton, James G. Gehling and Davide Pisani, Philosophical Transactions of the Royal Society Part B, 363: 1435-1443 (11th January 2008) [Full paper downloadable from here]
Peterson et al, 2008 wrote:Unravelling the timing of the metazoan radiation is crucial for elucidating the macroevolutionary processes associated with the Cambrian explosion. Because estimates of metazoan divergence times derived from molecular clocks range from quite shallow (Ediacaran) to very deep (Mesoproterozoic), it has been difficult to ascertain whether there is concordance or quite dramatic discordance between the genetic and geological fossil records. Here, we show using a range of molecular clock methods that the major pulse of metazoan divergence times was during the Ediacaran, which is consistent with a synoptic reading of the Ediacaran macrobiota. These estimates are robust to changes in priors, and are returned with or without the inclusion of a palaeontologically derived maximal calibration point. Therefore, the two historical records of life both suggest that although the cradle of Metazoa lies in the Cryogenian, and despite the explosion of ecology that occurs in the Cambrian, it is the emergence of bilaterian taxa in the Ediacaran that sets the tempo and mode of macroevolution for the remainder of geological time.
In other words, the "Cambrian explosion" is now regarded as a taphonomic event rather than an evolutionary one - that is, the abundance of Cambrian fossils is the result both of the presence of conditions of preservation in aquatic sediments that were not present in earlier eras, and the appearance of organisms with readily fossilisable hard tissues that were not present in earlier eras. Plus, the duration of the appearance of new animal groups within the Cambrian is regarded as a time period whose minimum duration was 5 million years, and which more recent estimates place at 18-23 milllion years, which means that these organisms hardly appeared overnight. Of course, the existence of organisms 500 million years ago does nothing to support YEC blind assertions that the entire universe is only 6,000 years old, but creationists treat science as if it were a branch of apologetics, and frequently do so in a wholly duplicitous fashion, so it shouldn't be surprising that they try to use the "Cambrian explosion" as somehow "supporting" their reality-denial masturbation fantasy of a doctrine.
And now, to the little surprise ...
The Museum of South Australia has 18 specimens of a fossil that is regarded as being the earliest known Chordate (namely, a member of the same taxonomic Phylum as you and I) dating from the Ediacaran (over 600 million years old). Read all about it here and here.
Oh, and as for the idea that the Cambrian pseudo-explosion was an "instantaneous" event, this too is nonsense. Scientists have stated that this event took place over a period of eighteen to twenty three million years. Which means that organisms were continually evolving during that period. Relevant scientific papers include these:
Anatomical Information Content In The Ediacaran Fossils And Their Possible Zoological Affinities by Jerzy Dzik, Integrative and Comparative Biology, 43(1): 114-126 (2003)
Can fast early rates reconcile molecular dates with the Cambrian explosion? by L.D. Bromham & M.D. Hendry, Proc. R. Soc. Lond. B 267: 1041-1047 (2000)
Estimating Metazoan Divergence Times With A Molecular Clock by Kevin J. Peterson, Jessica B. Lyons, Kristin S. Nowak, Carter M. Takacs, Matthew J. Wargo & Mark A. McPeek, Proceedings of the National Academy of Science of America, April 2004, 101, 17, 6536-6541
Evolution of Amino Acid Frequencies in Proteins Over Deep Time: Inferred Order of Introduction of Amino Acids into the Genetic Code by Dawn J. Brooks, Jacques R. Fresco, Arthur M. Lesk, and Mona Singh, Molecular Biology and Evolution 19: 1645-1655 (2002)
Fossils, Molecules And Embryos: New Perspectives On The Cambrian Explosion by J.W. Valentine, D. Jablonski & D.H. Erwin, Development, February 1998, 126(5): 851-859
Inferring The Historical Patterns Of Biological Evolution by Mark Pagel, Nature, 401: 877-884 (28 October 1999)
Interpreting the Earliest Metazoan Fossils: What Can We Learn? by Ben Waggoner, Amer. Zool., 38: 975-982
Molecular Phylogeny of Arthropods and the Significance of the Cambrian "Explosion" for Molecular Systematics by Jerome C. Regier & Jeffrey W. Schultz, AMER. ZOOL., 38: 918-928 (1998)
Precambrian Sponges with Cellular Structures by Chia-Wen Li, Jun-Yan Chen and Tzu-En Hua, SCIENCE 279(6) February 1998
Quality of the fossil record thorugh time by M.J. Benton, M.A. Wills and R. Hitchin, Nature, 403: 534-537 (3 Feb 2000)
Sr and C isotopes in Lower Cambrian carbonates from the Siberian craton: A paleoenvironmental record during the ‘Cambrian explosion’ by L.A. Derry, M.D. Brasier, R. M. Corfield, A. Yu. Rozanov & A. Yu. Zhuralev, Earth and Planetary Science Letters 128: 671-681 (1994)
Taxonomic Congruence Versus Total Evidence, and Amniote Phylogeny Inferred from Fossils, Molecules and Morphology by Douglas J. Eernisse and Arnold G. Kluge, Molecular Biology & Evolution, 10(6): 1170-1195 (1993)
Testing the Cambrian explosion hypothesis by using a molecular dating technique by Lindell Bromham, Andrew Rambaut, Richard Fortey, Alan Cooper and David Penny, Proceedings of the National Academy of Science of America, October 1998, 95: 12386-12389
The Cambrian "Explosion": Slow Fuse Or Megatonnage? by Simon Conway Morris, Proceedings of the National Academy of Science of America, April 2000, 97(9): 4426-4429
The Ediacaran Biotas in Space and Time by Ben Waggoner, Integrative & Comparative Biology, 43: 104-113 (2003)
The Ediacaran Emergence Of Bilaterans: Congruence Between The Genetic And The Geological Fossil Records by Kevin J. Peterson, James A. Cotton, James G. Gehling and Davide Pisani, Philosophical Transactions of the Royal Society Part B, 363: 1435-1443 (11th January 2008)
The Timing Of Eukaryotic Evolution: Does A Relaxed Molecular Clock Reconcile Proteins And fossils? by Emmanuel J.P. Douzery, Elizabeth A. Snell, Eric Bapteste, Frédéric Delsuc & Hervé Philiipe, Proceedings of the National Academy of Science of America, October 2004, 101, 43, 15386-15391
As for the assertion erected in this thread earlier that speciation doesn't occur, this is manifest nonsense. Once again, here is a list of relevant scientific papers documenting speciation events observed both in nature AND in the laboratory, along with theoretical papers covering speciation models:
A Model For Divergent Allopatric Speciation Of Polyploid Pteridophytes Resulting From Silencing Of Duplicate-Gene Expression by Charles R.E. Werth and Michael D. Windham, American Naturalist, 137(4): 515-526 (April 1991) - DEVELOPMENT OF A MODEL TO MATCH OBSERVED SPECIATION IN NATURE
A Molecular Reexamination Of Diploid Hybrid Speciation Of Solanum raphanifolium by David M. Spooner, Kenneth. J. Sytsma and James F. Smith, Evolution, 45(3): 757-764 - DOCUMENTATION OF AN OBSERVED SPECIATION EVENT
Chromosome Evolution, Phylogeny, And Speciation Of Rock Wallabies, by G. B. Sharman, R. L. Close and G. M. Maynes, Australian Journal of Zoology, 37(2-4): 351-363 (1991) - DOCUMENTATION OF OBSERVED SPECIATION IN NATURE
Evidence For Rapid Speciation Following A Founder Event In The Laboratory by James R. Weinberg Victoria R. Starczak and Danielle Jörg, Evolution 46: 1214-1220 (15th January 1992) - EXPERIMENTAL GENERATION OF A SPECIATION EVENT IN THE LABORATORY
Evolutionary Theory And Process Of Active Speciation And Adaptive Radiation In Subterranean Mole Rats, Spalax ehrenbergi Superspecies, In Israel by E. Nevo, Evolutionary Biology, 25: 1-125 - DOCUMENTATION OF OBSERVED SPECIATION IN NATURE
Experimentally Created Incipient Species Of Drosophila by Theodosius Dobzhansky & Olga Pavlovsky, Nature 230: 289 - 292 (2nd April 1971) - EXPERIMENTAL GENERATION OF A SPECIATION EVENT IN THE LABORATORY
Founder-Flush Speciation On Drosophila pseudoobscura: A Large Scale Experiment by Agustí Galiana, Andrés Moya and Francisco J. Alaya, Evolution 47: 432-444 (1993) EXPERIMENTAL GENERATION OF A SPECIATION EVENT IN THE LABORATORY
Pollen-Mediated Introgression And Hybrid Speciation In Louisiana Irises by Michael L. Arnold, Cindy M. Buckner and Jonathan J. Robinson, Proceedings of the National Academy of Sciences of the USA, 88(4): 1398-1402 (February 1991) - OBSERVATION OF A SPECIATION EVENT IN NATURE
Speciation By Hybridisation In Heliconius Butterflies by Jesús Mavárez, Camilo A. Salazar, Eldredge Bermingham, Christian Salcedo, Chris D. Jiggins and Mauricio Linares, Nature, 441: 868-871 (15th June 2006) - DETERMINATION OF A SPECIATION EVENT IN NATURE, FOLLOWED BY LABOARTORY REPRODUCTION OF THAT SPECIATION EVENT, AND CONFIRMATION THAT THE LABORATORY INDIVIDUALS ARE INTERFERTILE WITH THE WILD TYPE INDIVIDUALS
Speciation By Hybridization In Phasmids And Other Insects By Luciano Bullini and Guiseppe Nascetti, Canadian Journal of Zoology 68(8): 1747-1760 (1990) - OBSERVATION OF A SPECIATION EVENT IN NATURE
The Gibbons Speciation Mechanism by S. Ramadevon and M. A. B. Deaken, Journal of Theoretical Biology, 145(4): 447-456 (1991) - DEVELOPMENT OF A MODEL ACCOUNTING FOR OBSERVED INSTANCES OF SPECIATION
Indeed, I presented the Mavárez et al paper in another thread, but I'll cover that paper in detail again, since it's particularly apposite here, not least because the authors not only determined that a speciation event had taken place in the laboratory, but REPRODUCED THAT SPECIATION EVENT IN THE LABORATORY. The relevant paper, once again, is:
Speciation By Hybridisation In Heliconius Butterflies by Jesús Mavárez, Camilo A. Salazar, Eldredge Bermingham, Christian Salcedo, Chris D. Jiggins and Mauricio Linares, Nature, 441: 868-871 (15th June 2006) [Full paper downloadable from here]
Mavárez et al, 2006 wrote:Speciation is generally regarded to result from the splitting of a single lineage. An alternative is hybrid speciation, considered to be extremely rare, in which two distinct lineages contribute genes to a daughter species. Here we show that a hybrid trait in an animal species can directly cause reproductive isolation. The butterfly species Heliconius heurippa is known to have an intermediate morphology and a hybrid genome1, and we have recreated its intermediate wing colour and pattern through laboratory crosses between H. melpomene, H. cydno and their F1 hybrids. We then used mate preference experiments to show that the phenotype of H. heurippa reproductively isolates it from both parental species. There is strong assortative mating between all three species, and in H. heurippa the wing pattern and colour elements derived from H. melpomene and H. cydno are both critical for mate recognition by males.
The authors continue with:
Mavárez et al, 2006 wrote:Homoploid hybrid speciation—hybridization without change in chromosome number—is considered very rare2–4. This has been explained by the theoretical prediction that reproductive isolation between hybrids and their parents is difficult to achieve3,5,6. However, if a hybrid phenotype directly causes reproductive isolation from parental taxa, this difficulty can be overcome. Such a role for a hybrid phenotype has been convincingly demonstrated only in Helianthus sunflowers7. In animals, the evidence for homoploid hybrid speciation is less convincing. Putative hybrid species are known with mixed genomes8–11, but in these examples shared genetic variation could also be a result of introgression subsequent to a bifurcating speciation event.
Heliconius cydno and H. melpomene are two closely related species that overlap extensively in lower Mesoamerica and the Andes12. Speciation in these butterflies has not involved any change in chromosome number13 but is instead associated with shifts in colour patterns that generate both assortative mating and postzygotic isolation due to predator-mediated selection14–17. Heliconius cydno is black with white and yellow marks, whereas H. melpomene is black with red, yellow and orange marks. Both species exhibit strong positive assortative mating based on their wing colour patterns and also differ in habitat use18 and host plant preference19, but interspecific hybrids do occur at low frequency in the wild15. Heliconius heurippa has an intermediate wing pattern, which has led to the suggestion that this is a hybrid species1,20. Its hindwing is indistinguishable from that of sympatric H. m. melpomene, whereas the yellow band on its forewing is similar to that of parapatric H. cydno cordula. Ecologically, H. heurippa is most similar to H. cydno, which it replaces geographically in the eastern Andes of Colombia. Here we first establish that H. heurippa is currently genetically isolated from its putative parents and provide evidence that its genome is of hybrid origin. A Bayesian assignment analysis using 12 microsatellite loci scored in populations from Panama, Colombia and Venezuela divides H. cydno (n = 175), H. melpomene (n = 167) and H. heurippa (n = 46) individuals into three distinct clusters (Fig. 1). Hence, H. heurippa is genetically more differentiated than any geographic race sampled of either species. Moreover, analyses of polymorphism at two nuclear genes (Invected and Distal-less) show no allele sharing between H. cydno and H. melpomene, whereas the H. heurippa genome appears as an admixture, sharing allelic variation with both putative parental species (Supplementary Fig. 2, and C.S., C.D.J. and M.L., unpublished observations).
So, the authors begin by noting that the wing pattern of Heliconius heurippa is intermediate between that of local races of Heliconius melpomene and Heliconius cydno, and ask the question whether or not this is because Heliconius heurippa is a hybrid between individuals from those two races of Heliconius melpomene and Heliconius cydno. Suspicions that this might be the case were reinforced, when a genetic analysis demonstrated that certain genes present in Heliconius heurippa were admixtures of those found in Heliconius melpomene and Heliconius cydno, whilst the genes in question show NO such admixture in the other two species.
Moving on ...
Mavárez et al, 2006 wrote:To test the hypothesis of a hybrid origin for the H. heurippa colour pattern, we performed inter-specific crosses between H. cydno cordula and H. m. melpomene to reconstruct the steps of introgressive hybridization that could have given rise to H. heurippa. The colour pattern differences between H. m. melpomene and H. cydno cordula are determined largely by three co-dominant loci controlling the red and yellow bands on the forewing and the brown pincer-shaped mark on the ventral hindwing (see Fig. 2a)21,22. Most H. cydno × H. melpomene F1 hybrids seem intermediate to both parents (Fig. 2a), with both a yellow (cydno) and a red (melpomene) band in the median section of the forewing, whereas the ventral side of the hindwing shows a reduced brown mark intermediate between the parental species.
So, the authors produced some experimental crosses, and noticed that those experimental crosses produced individuals possessing wing pattern intermediate between those of the parents. However, they didn't just produce single-generation crosses, instead, they tested the effects that would arise from multiple crossings across several generations, and the results were extremely illuminating to put it mildly! But I'm jumping the gun here a little ... let's see what the authors have to reveal to us, shall we?
Mavárez et al, 2006 wrote:Female F1 hybrids resulting from crosses between H. melpomene and H. cydno are sterile in accordance with Haldane’s rule1,23, and thus only male F1 hybrids backcrossed to either H. cydno cordula or H. m. melpomene females resulted in offspring. Backcrosses to H. melpomene produced offspring very similar to pure H. m. melpomene, and further backcross generations never produced individuals with forewing phenotypes similar to H. heurippa (Fig. 2a). However, after only two generations a phenotype virtually identical to H. heurippa (Supplementary Fig. 3) was produced by backcrossing an F1 male to an H. cydno cordula female and then mating selected offspring of this cross (Fig. 2b). In offspring of crosses between these H. heurippa-like individuals the pattern breeds true, showing that they are homozygous for the red forewing band (BB) and the absence of brown hindwing marks (brbr) characteristic of H. melpomene, and similarly homozygous for the yellow forewing band (NNNN) derived from H. cydno. The pattern of these H. heurippa-like individuals also breeds true when crossed to wild H. heurippa (Fig. 2b), implying that pattern genes segregating in our crosses are homologous with those in wild H. heurippa.
Oh, now look at that for a spectacular set of results!
First of all, the authors crossed Heliconius melpomene with Heliconius cydno to produce F1 hybrids, then back-crossed the fertile males with females of each species. Back-crossing with Heliconius melpomene resulted in melpomene wing patterns reappearing, but back-crossing the F1 hybrids with Heliconius cydno to produce the F2 generation, then mating selected offspring of the F2 generation, produced individuals that were virtually identical to Heliconius heurippa!
But it gets even better. When the laboratory produced Heliconius heurippa analogues were mated to wild type Heliconius heurippa, they produced fertile offspring and the wing patterns bred true!.
These crossing experiments, as a consequence, constitute compellingly strong evidence that Heliconius heurippa resulted from a similar process occurring among hybrid butterflies in the wild. Not only did the authors reproduce the likely crossing sequence that produced Heliconius heurippa in the wild, thus providing a repeatable test of the relevant speciation mechanism, but the laboratory crosses were interfertile with the wild type Heliconius heurippa, further strengthening the hypothesis advanced by the authors.
Moving on ...
Mavárez et al, 2006 wrote:Furthermore, in a wild population of sympatric H. m. melpomene and H. cydno cordula in San Cristóbal, Venezuela, we observed natural hybrids at an unusually high frequency (8%), including some individuals very similar to our laboratory-produced H. heurippa-like butterflies (Fig. 2b). Microsatellite data show that these individuals have genotypes indistinguishable from that of H. cydno and must therefore be at least fifth-generation backcrosses (Supplementary Fig. 4). This shows that multiple generations of backcrossing can occur in the wild and that female hybrid sterility is not a complete barrier to introgressive hybridization. The fact that the H. heurippa pattern can be generated by laboratory crosses between H. melpomene and H. cydno, and is also observed in wild hybrids between the two species, establishes a probable natural route for the hybrid origin of H. heurippa.
Well, at this point, one is tempted to say, QED. The authors could hardly have asked for better, could they? Not only did their laboratory crosses reproduce virtually identical Heliconius heurippa analogues, that were furthermore interfertile with wild Heliconius heurippa, but they observed hybrids in the wild that included individuals matching both the wild type Heliconius heurippa and the authors' laboratory analogues!
Not satisfied with this, however, the authors then turned their attention to the next part of the speciation process, and performed some experiments to determine if an isolating mechanism was in place, which would reinforce speciation. Let's take a look at those experiments, shall we?
Mavárez et al, 2006 wrote:The next step in species formation is reproductive isolation. We therefore tested the degree to which H. heurippa is isolated from H. melpomene and H. cydno by assortative mating. No-choice mating experiments showed a reduced probability of mating in all interspecific comparisons, with H. heurippa females particularly unlikely to mate with either H. cydno or H. melpomene (Table 1). When a male of each species was presented with a single female, H. heurippa males were tenfold more likely to court their own females than the other species (Supplementary Fig. 5). In mating experiments with choice, there was similarly strong assortative mating, although occasional matings between H. cydno and H. heurippa were observed (Table 2). Isolation due to assortative mating, on average more than 90% between H. heurippa and H. melpomene and more than 75% between H. heurippa and H. cydno, is therefore considerably greater than that caused by hybrid sterility (about 25% isolation between H. heurippa and H. melpomene, and zero between H. heurippa and H. cydno)1 or predator selection against hybrids (about 50%)24. Therefore, strong assortative mating, in combination with geographic isolation from H. cydno and postzygotic isolation from H. melpomene has contributed to the speciation of H. heurippa.
So, the females of the new species, Heliconius heurippa, exhibited strong preference for other male Heliconius heurippa, with probabilities of out-crossing being 0.073 with Heliconius melpomene males and 0.022 with Heliconius cydno males. Male Heliconius heurippa again exhibited strong preference for female Heliconius heurippa, with probabilities of outcrossing being 0.1 with Heliconius melponeme and 0.44 with Heliconius cydno females. The table in the paper also demonstrates that the parent species also show strong assortative mating, though exhibit enough tendency to hybridise with each other to produce the offspring needed to generate Heliconius heurippa in the first place (hybridisation rate approximately 8%).
However, apart from mating experiments, the authors conducted some other experiments too. Let's take a look at these shall we?
Mavárez et al, 2006 wrote:We next investigated the role of colour pattern in mate choice. Experiments with dissected wings showed that both elements of the forewing colour pattern of H. heurippa were necessary for the stimulation of courtship (Fig. 3). H. heurippa males were less than half as likely to approach and court the H. m. melpomene or the H. cydno cordula pattern than their own (Fig. 3).When either the red or yellow bands were experimentally removed from the H. heurippa pattern, this led to a similar reduction in its attractiveness, demonstrating that both hybrid elements are necessary for mate recognition by male H. heurippa (Fig. 3).
So in this experiment, the authors demonstrated that visual cues are important to Heliconius heurippa, and that experimental manipulation of the wing pattern to mask certain features reduces their attractiveness as visual stimuli to mating.
Mavárez et al, 2006 wrote:Similar results were obtained when these experiments were replicated with printed-paper models (Fig. 3), showing that the colour pattern itself was the cue rather than pheromones associated with the dissected wings. Additional experiments showed that males of both H. m. melpomene and H. cydno cordula showed a greatly reduced probability of approaching and courting the H. heurippa pattern than their own (Supplementary Figs 6 and 7). Given the incomplete postzygotic reproductive isolation between all three species1, this pattern-based assortative mating must have a continuing role in generating reproductive isolation between H. heurippa and its relatives.
Nice. The above experiments established that visual stimuli reproduce the same pattern of assortative mating behaviour even in the absence of pheromones, demonstrating that visual cues are the primary means of stimulating courtship behaviour in these butterflies, and that those visual cues exert strong effects upon mate preference, leading to the assortative mating patterns seen above.
Mavárez et al, 2006 wrote:Novel patterns in Heliconius probably become established through a combination of genetic drift and subsequent fixation of the novel pattern driven by frequency-dependent selection25. Such an event could have established the hybrid H. heurippa pattern as a geographic isolate of H. cydno. Subsequently, the pattern was sufficiently distinct from both H. melpomene and H. cydno that mate-finding behaviour also diverged in parapatry, generating assortative mating between all three species (Supplementary Fig. 8). This two-stage process indicates a possible route by which the theoretical difficulty of a rapid establishment of reproductive isolation between the hybrid and the parental taxa could have been overcome5,6. Furthermore, because we are proposing divergence in mate behaviour in a geographically isolated population, reinforcement or some other form of sympatric divergence is not required for speciation to occur.
Our study provides the first experimental demonstration of a hybrid trait generating reproductive isolation between animal species, and the first example of a hybrid trait causing pre-mating isolation through assortative mating. None of the theoretical treatments of homoploid hybrid speciation have considered the effects of
assortative mating5,6. If variation for mate preference were incorporated, the theoretical conditions favouring hybrid speciation might not be as stringent as has been supposed. Finally, two other species, H. pachinus20 and H. timareta26, have also been proposed as having H. cydno/H. melpomene hybrid patterns, indicating that this process might have occurred more than once. However, whether these cases represent a particularity of Heliconius or a common natural process that has been undetected in other animal groups studied less intensively remains a matter of further study. Suggestively, other proposed cases of homoploid hybrid speciation in animals occur in well-studied groups such as African cichlids8–10 and Rhagoletis flies11.
So, the authors were able to reproduce a wild speciation event in the laboratory, produce laboratory analogues of the new species that were interfertile with wild type members of that species, and demonstrate the existence of assortative mating preferences producing a reproductive isolation barrier between the new species and the parents once the new species existed. Furthermore, this mechanism of speciation has been erected as a probable model in other well-studied groups of organisms, including those particular favourites of mine among the vertebrates, African Cichlid fishes.
I'd say that's Game Over.
. Buy or don't buy-it's up to the purchaser.
