|
Royal ferns haven’t undergone any significant changes since the Jurassic, according to a report by Pete Spotts in the Christian Science Monitor, titled, Fossil fern: not much to show for at least 180 million years of evolution (21 March 2014):
A newly described bit of fossil fern, reported in the current issue of the journal Science, marks the first time researchers can make direct comparisons at the level of individual plant cells and the chromosomes they contain.
It verifies that the genome of royal ferns has remained unchanged for at least the past 180 million years.…
Royal ferns first emerged in the Southern Hemisphere more than 250 million years ago, researchers say. Fossil specimens dating to 220 million years ago show structural similarities with today’s species. In addition, information gleaned from today’s plants suggests that their genetic makeup has remained remarkably stable over geologic time.
Using three types of microscopes to analyze the newly described rhizome, Dr. McLoughlin and colleagues Benjamin Bomfluer and Vivi Vajda were able to image cells in a variety of developmental stages, as well as the fossil remains of the gel-like cytoplasm that filled the cells and the chromosomes that emerged in them. Features in the cells are virtually identical to those in living royal ferns, the team found, leading to the conclusion that they have remained evolutionarily dormant for most of their history on Earth.
A report from Lund University, titled, Unique Chromosomes Preserved in Swedish Fossil, describes the techniques that were used by researchers to identify cell nuclei, cell membranes and even chromosomes from ferns living 180 million years ago. The result? No observable change:
Researchers from Lund University and the Swedish Museum of Natural History have made a unique discovery in a well-preserved fern that lived 180 million years ago. Both undestroyed cell nuclei and individual chromosomes have been found in the plant fossil, thanks to its sudden burial in a volcanic eruption…
“The preservation happened so quickly that some cells have even been preserved during different stages of cell division”, said Vivi Vajda, Professor of Geology at Lund University.
Thanks to the circumstances of the fern’s sudden death, the sensitive components of the cells have been preserved. The researchers have found cell nuclei, cell membranes and even individual chromosomes. Such structures are extremely rare finds in fossils, observed Vivi Vajda…
Living representatives of this family are very similar in appearance to the Jurassic fossil, which suggests that only limited evolutionary change has taken place over the millennia. By comparing the size of the cell nuclei in the fossilised plant with its living relatives, the researchers have been able to show that the royal ferns have outstanding evolutionary stability.
An IFLScience report by Janet Fang, titled, Royal ferns are “living fossils,” unchanged since the Jurassic (March 21, 2014) spells out what this means at the genetic level:
Let’s step back for a minute. You might remember the various stages of mitosis (prophase, metaphase, and so on), but what we often forget is that about 90 percent of the time, cells are in interphase. That’s when it copies its chromosomes (which are kept within the nucleus) in preparation for cell division, or reproduction.
Using at least three different types of microscopy, a team led by Benjamin Bomfleur from the Swedish Museum of Natural History analyzed the fossil stem, measuring the dimensions of interphase nuclei in the fern’s fossilized pith (the core) and the cortical parenchyma cells (a type of unspecified cell that differentiates into other plant cell types).
They found that they match those of its living relative — O. cinnamomeum — very closely. The ancient Korsaröd fern has essentially the same number of chromosomes and DNA content in the Early Jurassic period as Osmundaceae ferns do today. Talk about evolutionary stasis.
This interpretation is confirmed by the authors of the study in Science, who described the genomes of the fossil ferns in an article titled, Fossilized Nuclei and Chromosomes Reveal 180 Million Years of Genomic Stasis in Royal Ferns by Benjamin Bomfleur et al. (Science, 21 March 2014: Vol. 343 no. 6177 pp. 1376-1377, DOI: 10.1126/science.1249884):
Rapidly permineralized fossils can provide exceptional insights into the evolution of life over geological time. Here, we present an exquisitely preserved, calcified stem of a royal fern (Osmundaceae) from Early Jurassic lahar deposits of Sweden in which authigenic mineral precipitation from hydrothermal brines occurred so rapidly that it preserved cytoplasm, cytosol granules, nuclei, and even chromosomes in various stages of cell division. Morphometric parameters of interphase nuclei match those of extant Osmundaceae, indicating that the genome size of these reputed “living fossils” has remained unchanged over at least 180 million years — a paramount example of evolutionary stasis.
What are the implications for macroevolution?
 |
In a recent post of mine, I drew readers’ attention to paleontologist Professor Donald Prothero (pictured above, courtesy of Wikipedia) who candidly admitted that he had absolutely no idea what makes living things stop evolving for millions of years, in an article titled, Stephen Jay Gould: Did He Bring Paleontology to the “High Table”? (Philosophy in Theory and Biology, Volume 1, December 2009). Writing as a paleontologist, Prothero expresses his frustration that “neontologists” – biologists who only study living things which are alive today – remain blissfully unaware of the fact that ecosystems can undergo drastic changes over the course of geological time, while the species comprising those ecosystems don’t undergo any evolutionary change at all. Prothero was utterly baffled as to why macroevolution grinds to a halt in these cases:
By the mid-1980s, a consensus had emerged within the paleontological community that nearly all metazoans [i.e. animals – VJT] (vertebrate and invertebrate, marine and terrestrial) show stasis and punctuated speciation through millions of years of geologic time and strata, with only minor possible examples of gradual anagenetic change in size (Geary 2009; Princehouse 2009; Hallam 2009; Jablonski 2000, 2008). That has been the accepted view of paleontologists for more than 20 years now.
Yet one would never know this by looking at the popular accounts of the debate written by non-paleontologists, who still think it is a controversial and unsettled question. Even more surprising is the lack of response, or complete misinterpretation of its implications, by evolutionary biologists. Since the famous battle at the 1980 macroevolution conference in Chicago, neontologists have persisted in misunderstanding the fundamental reasons why paleontologists regard punctuated equilibrium as important. Many have claimed that people like Simpson (1944) and others were thinking along the same lines, or that gradual change on the neontological time scale would look punctuated on a geologic time scale. They miss the point of the most important insight to emerge from the debate: the prevalence of stasis. Before 1972, paleontologists did try to overemphasize examples of gradual evolution, and they expected organisms to gradually change through geologic time, as they do on neontological time scales. But the overwhelming conclusion of the data collected since 1972 shows that gradual, slow, adaptive change to environments almost never occurs in the fossil record. The prevalence of stasis is still, in my mind, the biggest conundrum that paleontology has posed for evolutionary biology, especially when we can document whole faunas that show absolutely no change despite major changes in their environments (Prothero and Heaton 1996; Prothero 1999; Prothero et al. 2009). That fact alone rules out the “stabilizing selection” cop-out, such as the effort by Estes and Arnold (2007) to invoke stabilizing selection to explain stasis, with no mention of the fact that the fossil record shows much stasis in the face of climatic change. For years now paleontologists and neontologists alike have struggled to find (unsuccessfully, in my opinion) a good explanation for why virtually all organisms are static over millions of years despite huge differences in their adaptive regime. be good examples of short-term microevolutionary change, but they simply do not address what the fossil record has shown for over a century.
Professor Prothero proposed species sorting as a possible explanation for why one species will suddenly appear in a small population. Wikipedia defines the term as follows:
Species sorting is a theory which states that each species will eventually have its own ecological niche as two species cannot occupy the same niche for an unlimited amount of time without extinction. One will be more competitive than the other species. Therefore, if two species with the same niche were left in the same area, eventually one of the species would evolve and participate in resource partitioning.
Let’s go back to fossil ferns. As we’ve seen, they haven’t evolved for 180 million years. What was happening 180 million years ago? The supercontinent of Pangaea was breaking up into Laurasia and Gondwana. Volcanic activity was common. The first birds, lizards and therian mammals were appearing, and coralline algae appeared in the oceans for the first time. Clearly there were changes occurring. Which prompts me to ask: why don’t we see a diversification of niches, with new kinds of ferns evolving in those niches? Why do we observe stasis instead?
In my previous post, I concluded: if scientists currently lack an understanding of what makes macroevolution stop, then how can they possibly claim to understand what makes it go?
Professor Moran responds
That kind of argument didn’t impress Professor Larry Moran, over at the Sandwalk. In a post titled, What do Intelligent Design Creationists really think about macroevolution? (March 20, 2014), he replied that evolution still continues at the genetic level in a population, even during long periods of apparent stasis when evolution ceases to occur at the morphological level:
You can’t stop evolution. The rate at which large populations change from one morphological form to another can be very slow but that does not mean they aren’t changing in diversity as new alleles increase in frequency and old ones are lost. From time to time, new morphological variants may become fixed in the population and evolution becomes visible in the fossil record. These types of change are more likely to occur during speciation events when the new daughter population (species) is quite small and rapid fixation of rare alleles is more likely. That’s what punctuated equilbirium is all about.
There’s no great mystery here. I think I understand what’s going on. Evolutionary biologists argue about whether punctuated equilibria describes a very common mode of macroevolutionary change or one that’s very rare but none of them think that changes in the allele frequencies of a population comes to a grinding halt during periods of stasis.
By now, I think we can see why this reply won’t wash. As we’ve seen, the evidence available indicates that not only the morphology, but also the genome of royal ferns has remained unchanged (as far as scientists can tell) for a period of 180 million years. Let’s look at the relevant quotes from the science reports again:
…the genome size of these reputed “living fossils” has remained unchanged over at least 180 million years — a paramount example of evolutionary stasis.
The ancient Korsaröd fern has essentially the same number of chromosomes and DNA content in the Early Jurassic period as Osmundaceae ferns do today. Talk about evolutionary stasis.
It verifies that the genome of royal ferns has remained unchanged for at least the past 180 million years…
Features in the cells are virtually identical to those in living royal ferns, the team found, leading to the conclusion that they have remained evolutionarily dormant for most of their history on Earth.
So again I ask: why the 180-million-year genetic stasis?
I’ll let Professor Moran have the last word:
I hope I’m not being unkind if I suggest that Vincent Torely (sic) is the one who doesn’t understand what’s going on. He is an IDiot, after all.
Someone appears to believe in an awful lot of invisible evolution.