Am I missing something here? Here’s an article with the grand title of “Researchers propose expanded evolutionary concept”, which declares that we’re going to have to expand and rethink our understanding of evolutionary theory. “Oh really?” I thought, but I read it with an open mind, expecting some dramatic new phenomenon to be explained. I was a little disappointed to find it was about ramets.
Don’t get me wrong, ramets are interesting, and the paper’s content is fine, if maybe a little overhyped. Ramets are an asexual kind of reproduction, fairly common in plants, where runners sprout shoots that develop into individuals — a familiar example is aspen trees, where a clump of trees, even a whole forest, may consist of clones of a single source individual, each tree born of the original source root system, spawning more roots that generate more ramets that expand the clone.
The paper makes the good point that these organisms aren’t constrained by Weisman’s barrier, which postulates that there is no exchange of heritable variation from the soma to the gonads, which we animals take for granted. If your fingers were irradiated, producing a new collection of mutations in those tissues, it won’t matter to future offspring because those cells don’t contribute to sperm and egg. That would all change if you were able to sprout clonal copies of yourself from your fingertips, like these ramets, because then those irradiated digits would have a way to reproduce independent individuals. It would also be hard to type with all these finger-fetuses growing from my hands.
So in organisms with the ability to propagate from somatic tissues, somatic mutations are a mechanism for generating new variations. That’s the story here.
Evolutionary consequences of somatic mutations when they enter the germline.
(A) In most animals, the Weisman barrier between soma and germline prevents transfer. However, germline determination occurs late in plants, fungi, and some basal metazoans. In the hydrozoa, for example, stem cells differentiate into germ cells throughout the life of the colony. In others, trans-differentiation of soma into germ cells may occur. Thus, as somatic mutations accumulate, some may enter the germ line. Once in the germline, somatic mutations are recombined into different genetic backgrounds during meiosis similar to germline mutations. This reduces linkage between potentially deleterious mutations, which otherwise would lead to increasing genetic load. (B) Multilevel selection may also speed up adaptive evolution by providing a first filter of negative selection at the level of cell populations. However, the success of adaptive somatic genetic variation (SoGV) depends on whether they occur in stem cells and the specifics of how new modules arise. Homogeneous modules each arising from single mutated stem cells may compete with each other at the within-genet level and be subject to selection.
Yes, fine, this is an important phenomenon, but is it really new? Does it require bold new changes to evolutionary theory? In my head, I’m quite aware that asexual species can still evolve and acquire new traits, and that is perfectly compatible with the evolutionary principles I understand. There is often an erroneous bias in humans to assume that all populations reproduce sexually, like us, and that somatic tissues can’t propagate from cuttings, like us animals, but evolutionary theory isn’t blinkered in that way. Why is phys.org announcing we need an “expanded evolutionary concept” to account for this? We don’t. This could just be phys.org over-hyping something, as they tend to do.
But no, it’s actually in the paper itself. Oh no, it’s the dreaded paradigm shift.
Evolutionary biology has made tremendous progress in explaining the emergence and maintenance of sexual reproduction despite the two-fold costs of sex. Here, we addressed the flip side of the coin, namely, how do a large number of species cope with extended phases of asexual reproduction that, according to conventional wisdom, precludes the emergence of genetic and phenotypic diversity and hence, adaptive evolution? With empirical data increasingly confirming earlier conceptual work, it is now timely to suggest a paradigm shift that acknowledges the evolution of modular species at multiple levels. Cell lineages evolve within ramets, which in turn are forming asexual populations featuring a mix of mosaic and fixed SoGV. Both of these levels of variation and selection are, in turn, nested within sexually reproducing populations of genets that are corresponding to the ‘classical’ level of individuality in population genetics of unitary species, leading to potentially complex pathways of adaptation that merit further study.
Great, yes, I agree, multiple levels of selection, somatic mutation can contribute to genetic diversity, there’s nothing wrong or surprising about that. But where’s the “paradigm shift”? What’s the part that can’t be accommodated by our current understanding of genes and phenotypes and populations? Come on, people, tone down the exaggeration.
Unless there’s something I am missing here, which does happen. I may be a bit of an animal-chauvinist, so it feels awkward to have to remind a plant-person that evolutionary theory can handle bacteria quite well, so the peculiarities of our multicellular models aren’t necessarily going to require radical renovation of the whole idea.
News for profit are a major root of problems in our society.
They earn on hype, so they must hype.
Interspecific swaps of genetic material among microbes was a lot more mind-blowing for me than this was.
What was that exchange vehicle? Were the “chromatids”? I feel like they might have been called chromatids, but now I’m way into the deep end of my biological ignorance.
@2 The usual exchange vehicle for interspecies exchanges in prokaryotes are plasmids. Small circular rings of DNA. Many are self transmissable. Phages, bacterial viruses can also work. Some types of bacterial will take up exogenous DNA, transformation. Three different kinds of genetic exchange in prokaryotes.
Yeah, I don’t see anything either new or nonobvious in what PZ references above.
I took some cuttings from a 50 year old rose bush this spring, an example of asexual reproduction. They rooted well but I’m not going to call this a paradigm shift that expands the Theory of Evolution.
Even this isn’t quite right.
The human genome is 8% defective retroviruses.
It is clear that occasionally retroviruses invade the human germline and become fixed in the species lineage. It is likely they infect somatic cells first and then rarely spread to germline cells.
@Raven
PLASMID! Yes! Thank you! I knew it ended in “-id”.
The way “paradigm” has been used in software engineering–to refer to superficial and often faddish differences in methodology–has been a peeve of mine so long that the edge has worn off and I don’t care anymore.
Fine. Different fields use words to mean different things, and “paradigm” originally just referred to an “example or pattern”. I will take it to mean that. But I believe that when someone describes, say, OO or functional as “paradigms” what they really mean is something so mind-altering that it will rock your world and change the way you do things forever. (And I am not knocking functional or OO programming, but a competent software developer should have different approaches in their toolkit and know when to use them.)
I’m not a biologist, but I’m familiar enough with runners and other forms of asexual plant propagation just from long experience growing up in the suburbs and occasional interest in propagating them myself. I don’t even think this is the first time I ever wondered about the possibility of mutations in these clones. Regardless, I’m certain countless others have. My reasoning goes “Could that result in adaptation? Sure, I don’t see why not.”
Having this confirmed experimentally is a great (is this really new?), but it is “science as usual” and not the challenge suggested by “paradigm shift.”
I think a lot of this again shows the tendency of people to establish rules and categories, when reality is better described as statistical clusters. I thought evolutionary biologists at the very least were able to avoid this tendency (endemic among creationists, needless to say).
After some guy in the early eightes used the word paradigm in a book, it became a catchword used by every Tom, Dick and Harry none of which knew the definition if the word. Add the concept of clickbait, and we get the present situation of BS.
(advice to Kent Hovind)
“something really surprising” but not the kind of surprise that gets you an electronic ankle bracelet.
This is very interesting, but not paradigm shifting. Clearly there are multiple forms of gene transfer that occur in species other than quadrupeds.
For an actual paradigm shift, the latest archeology at White Sands has shown that humans were living in North America far earlier than ten to twelve thousand years ago. I’ve never believed the ice corridor hypothesis, so it’s very cool that it’s being disproven with radiocarbon dating of seeds found within the layers of fossilized footprints of humans and various extinct megafauna.
*from the National Park Service webpage on White Sands
I never had enough knowledge or expertise to say that the ice corridor hypothesis was implausible (or even less than likely), but I’ve always thought that we should be cautious about putting money on that when the coastal route followed land that is no submerged and thus not only not yet investigated, but not even possible to thoroughly investigate.
If that date holds up, it’s very, very interesting.
There are entire forests growing on top of glaciers in Alaska. I see no reason why people immigrating from Siberia could not have navigated over them through the center of the continent, in addition to traveling along the shoreline. Nomadic hunting tribes had boats and dogs pulled their sledges. It must have been an amazing sight, camels, wooly rhinos, herds of mammoths and ground sloths all grazing around a huge lake.
When I was working on the evolution of outcrossing-with-recombination (often called the “evolution of sex” for dramatic effect) there were declarations that the inability to totally account for the prevalence of “sex” amounted to a crisis in evolutionary biology. Funny thing, the “crisis” didn’t seem to prevent all the progress in the field. Could the word “crisis” have been a self-promoting exaggeration?
In the 1970s and 1980s graduate students in evolutionary biology had all heard of Thomas Kuhn’s conclusion that really important science all was introducing new paradigms, while “normal science” was just dull unimportant stuff. The air became filled with claims of new paradigms, which was a real nuisance. It soon became clear that the way to be innovative was to do “normal science” since nobody else was doing it.
The idea that somatic mutations can lead to organisms with new traits has been familiar to gardeners for some time: https://en.m.wikipedia.org/wiki/Sport_(botany)
“Paradigm shift” is so 1990’s (followed by “creative destruction”).
Hard to beat the once popular “end of history” for pure arrogant simplicity, but If you’re at a loss for words nowadays and want to sound portentous, “disruptive” is where it’s at, baby– until of course, Republicans decide to make a “roiling, fuming slag field” of the soon to be past.
“Paradigm shift” is a perfectly good term ruined by misuse.
stroppy@15–
Joseph Schumpeter, the economist most responsible for bringing “creative destruction” into modern usage, argued that it would eventually destroy capitalism. So when today’s parasitic business leaders crow about their industry disruption and creative destruction, they’re indirectly calling for their own economic demise (and I suspect there’s a good correlation between CEOs who flaunt these terms and companies bankrupted by wildly speculative high-risk decisions).
Crip Dyke–
The two biggest mind-blowing things I ever learned from biology have been endosymbiosis and prion diseases (and I’m old enough to remember when both concepts were considered highly controversial).