I think you need to figure this out. If front-loading makes the same predictions as evolution then parsimony will be on the side of evolution.
Why would we need front-loading in order for there to be evolutionary bias? It would seem to me that evolutionary bias would occur naturally.
What does your design theory predict about using glutamate signaling period, regardless of the receptor?
Isnât glutamate toxic?
I think thereâs a difference between âtheyâre dubious, and you know itâ, and âthereâs agreement on their validity, but disagreement over their characterizationâ. But I wonât belabor the point.
Am I willing to accept big gaps in the fossil record of some of the smallest organisms known to science, in some of the oldest fossil-bearing rocks known to science? Yes I am.
Iâm trying to figure it out, by asking you. If not even supporters of conventional evolutionary biology can figure out what it predicts, Iâm not sure I would invoke parsimony.
We seem to be having a communicational barrier. You ask me to predict individual mutations, as if front-loading required individual mutations to be predictable. I explain that front-loading doesnât require predictable mutations, but rests on a recognition of evolutionary bias. At no point do I claim that we need front-loading in order for there to be evolutionary bias.
Good, because you have none. What was dubious was always their identification as eukaryotes. That was always the issue, and youâre just shedding squid ink.
Probably not a good idea. And once again we get squid ink to obscure the point.
Thatâs not what âfront-loadingâ generally means. As usual, your hypotheses are very unclear. One might suspect itâs on purpose.
I have figured out what it predicts. It predicts that specific mutations will always happen in response to specific stimuli. Since that is not what we see, then front-loading is falsified.
That is what it requires. If front-loading doesnât consistenly produce specific mutations then it canât front load anything.
Indeed, the plant and animal glutamate-gated ion channels/receptors are similar enough at the amino acid sequence level to be considered homologs. Front-loading (as well as ateleological mechanisms) would indicate that ancestral glutamate-gated ion channels have been co-opted into many, very different, physiological mechanisms. (Also, plants do not have nervous systems. Please try to understand when authors use terminologies that both catch a readerâs eye and instigate discussion.)
As with almost every front-landing scenario, in this case there really is not any practical distinction from ateleological mechanisms when it comes to outcomes.
It appears that what @Krauze means by âfront-loadingâ is what other people mean by âpre-adaptationâ or âexaptationâ.
Hmm⌠I thought all current eukaryotes seem to contain mitochondrial-related genes.
Apparently the endosymbiosis happened some time after the seeding of stem-eukaryotes onto the planet.
Thatâs your subjective opinion. And as you say elsewhere: âThatâs not how science works. Conclusions are not based on subjective opinions.â
How does the ateleological perspective predict this? Would the perspective have been in trouble if different glutamate-gated ion channels had evolved de novo for different functions?
I do understand that. I could also have written âThe common ancestor of plants and animals is not believed to have employed nervous system-like signalling âŚâ, which would have been a wordier way of making the exact same point.
âExaptionâ is just a label attached to a phenomenon, itâs not by itself an explanation for that phenomenon. The ateleoligic explanation is that a structure that was selected to function in one organismal context just happened to also work well in a different organismal context.
And that explanation may also be correct in many cases, especially those with a low degree of specificity - like the evolution of the mammalian middle ear from reptilian jaw bones, where ears can be constructed from different bones.
But when it comes to cases with a high degree of specificity - like multicellularity evolving mulitple times independently, each time using the same ancient molecular machinery, which is non-essential in unicellular organisms - that arouses my suspicion of design.
Then how can you front load evolution if you canât control which mutations happen?
So far you havenât supported that claim about how multicellularity involved. All you have shown, if anything, is that one molecule used in intercellular signalling and its appropriate receptors are common to two different groups of multicellular organisms. Thatâs two out of 5 or so, not even counting various colonial and facultatively multicellular taxa.
More importantly, whatâs this about ânon-essential in unicellular organismsâ? Do you know the distribution of this feature in eukaryotes? If, as you claim, itâs only useful in multicellular organisms, how could it have been maintained in the billion or more years before it became useful? What keeps what would have been junk DNA from decaying with time?
We have already seen how genetic, developmental, or organismal characteristics of an organism can provide evolutionary bias. Hymenoptera and Rodentia presumable experienced the same kinds of mutations, yet in one eusociality evolved 8 times, while in the other it evolved only once.
Now, letâs take this insight and apply it to something more relevant to front-loading: The origin of multicellularity. Multicellularity has evolved more than 25 times in eukaryotes, with âtrue multicellularityâ (division and specialization from a single cell) having evolved independently in animals, plants, animals, and fungi, as well as several times in algaea (Parfrey & Lahr, 2013).
The fact that natural selection has seemingly âdiscoveredâ multicellularity so many times indicate that itâs a succesful strategy and is something that can be expected to evolve, regardless of the specific mutations that occur.
Yet this is an incomplete picture. After all, there are no bacterial analogues to animals, plants, fungi or algaea. Multicellularity may be a succesful strategy, but not every type of organism is equally equipped to acces it.
Indeed, it turns out that multicellularity arose multiple times, each time by co-opting the same ancient homeobox âgenetic toolkitâ, which was present in the last common ancestor of all eukaryotes:
Multicellular organization arose several times by convergence during the evolution of eukaryotes (e.g., in terrestrial plants, several lineages of âalgae,â fungi, and metazoans). To reconstruct the evolutionary transitions between unicellularity and multicellularity, we need a proper understanding of the origin and diversification of regulatory molecules governing the construction of a multicellular organism in these various lineages. Homeodomain (HD) proteins offer a paradigm for studying such issues, because in multicellular eukaryotes, like animals, fungi and plants, these transcription factors are extensively used in fundamental developmental processes and are highly diversified. A number of large eukaryote lineages are exclusively unicellular, however, and it remains unclear to what extent this condition reflects their primitive lack of âgood building blocksâ such as the HD proteins. Taking advantage from the recent burst of sequence data from a wide variety of eukaryote taxa, we show here that HDâcontaining transcription factors were already existing and diversified (in at least two main classes) in the last common eukaryote ancestor. Although the family was retained and independently expanded in the multicellular taxa, it was lost in several lineages of unicellular parasites or intracellular symbionts. Our findings are consistent with the idea that the common ancestor of eukaryotes was complex in molecular terms, and already possessed many of the regulatory molecules, which later favored the multiple convergent acquisition of multicellularity. (Derelle et al., 2007)
Furthermore, this genetic toolkit was independently lost four times in unicellular lineages, indicating that it plays a non-essential role on unicellular eukaryotes. Indeed, Derelle et al. conlude that âeukaryotes as a whole are preadapted for multicellularityâ:
As a corollary of ancestral molecular complexity, Ur-eukaryota probably possessed many of the good building blocks, which were subsequently recruited, by convergence in several lineages, to perform the functions required for development of multicellular organisms. In other terms, we suggest that the eukaryotes as a whole are preadapted for multicellularity, which only means that the ancestral complexity of the eukaryote genome and cell biology facilitated multiple acquisitions of multicellularity.
It is worth noting that conventional evolutionary biology did not predict any of this. Indeed, the conventional view is fine with genes arising de novo as needed. As architect of the modern synthesis, Ernst Mayr wrote, long before the discovery of homeobox genes:
âMuch that has been learned about gene physiology makes it evident that the search for homologous genes is quite futile except in very close relatives. If there is only one efficient solution for a certain functional demand, very different gene complexes will come up with the same solution, no matter how different the pathway by which it is achieved. The saying âMany roads lead to Romeâ is as true in evolution as in daily affairs.â
Ernst Mayr, Animal Species and Evolution (Harvard University Press, 1963), p. 609
On the other hand, the discovery that homeobox genes were present in the last common eukaryotic ancestor is exactly what front-loading would have us expect. Indeed, two years before the Derelle et al. article, I used front-loading to predict this very finding:
If we assume that eukaryotes were designed with the purpose of giving rise to multicellular organisms, we can make certain predictions. For one, we would expect the first eukaryotes to have contained a predecessor to the modern tool kit, and itâs possible that some unicellular eukaryotes still possess it. It will probably not be the full set possessed by modern organisms (or rather, full sets, as several organisms differ in the number of genes they have), as some genes may have been generated through gene duplications, but I definitely expect genes that are clear precursors to modern tool kit genes to be found in unicellular eukaryotes.
âNon-essentialâ isnât the same as âuselessâ. Derelle et al. write:
The independent diversification of HDs in the various multicellular lineages, as well as their crucial developmental role in these phylogenetically distant organisms, despite convergent acquisition of multicellularity, raise the fascinating question of the function of HD-containing transcription factors in the (unicellular) common ancestor of eukaryotes. We lay the bet that HD proteins were not primitively involved in domestic cellular regulations, but rather in higher order processes, e.g., communication between individual cells, or cell modifications along the life cycle, as already suggested by data on unicellular fungi (Johnson 1995).
Indeed, it turns out that populations of Dictyostelium transition from single cells to multicellularity physiologically, when they are starved. Itâs just not that hard.
No, that shows that it played a non-essential role in those lineages in which it was lost, at the time it was lost. You canât generalize that to all unicellular eukaryotes. And in fact non-essential genes are lost much more quickly than your claim would imply.
âŚsomething irrelevant to the question of non-essential vs. useless. I truly do not think you have thought this through very much.
Homeobox genes arenât essential to the unicellular condition in the way they are to the multicellular condition. While homeobox genes have been lost indepently four times in unicellular organisms, they have been lost in no multicellular organisms, even in those having undergone streamlining selection.
Sure, if every unicellular eukaryote species had undergone streamlining selection. But thereâs no reason to think that.
Another subjective opinion.
None of that follows. Until you know what homeodomain genes (not quite the same as homeobox genes) do in various unicellular eukaryotes, you canât say anything useful.
Selection isnât necessary. Absence of selection will work. As you should know but perhaps donât, unnecessary genes undergo a process called pseudogenization. They accumulate out-of-frame indels, extra stop codons, extra or missing splice signals, and other sorts of deactivating changes.
Iâm afraid we have a terminological barrier to understanding. To clarify, by a non-essential gene I mean a gene that, while it may contribute positively to an organismâs fitness, is not required for the development of a viable organism.
Take, for example, the GLO gene responsible for the last step in the synthesis of vitamin C. While the ability to synthesize vitamin C is presumably a selective advantage (at least for species without ready acces to fruit), it is not essential for the development of a viable organism, and the GLO gene has been lost independently several times (including in primates). But non-essential doesnât mean useless, and thereâs no evidence that the GLO gene is undergoing pseudogenization in those species which rely on it for the synthesis of vitamin C.
I think your distinction is pointless. The real question is whether a gene is under selection, i.e. whether its absence is significantly deleterious. I would also suggest that GLO is essential, by your definition, in those species that donât get adequate vitamin C in their diets. Obviously whatâs essential and whatâs preserved by selection depend on the environment in which the organism finds itself.
I would presume that they experienced different mutations since eukaryotes fall into a nested hierarchy. If they acquired the same mutations then we wouldnât see this pattern.
Right. So multicellularity would evolve without any need for front loading.
Meteorology canât predict what the weather will be like on April 29th, 2021 in Miami, FL. Does this mean that weather is front loaded?
From 2001: