The sister group relationship between Nephrozoa and
Xenacoelomorpha supported by our phylogenomic analyses implies that the last common ancestor of bilaterians was probably a benthic, ciliated acoelomate worm with a single opening into an epithelial gut, and that excretory organs, coelomic cavities, and nerve cords evolved after xenacoelomorphs separated from the stem lineage of Nephrozoa.
My problem arises with their placement of Ctenophora on their own phylogenetic tree as the “more primitive out-group” (for lack of better words on the spur of a rushed moment). Myself, I always considered Ctenophora as bilateral – in this case more primitively bilateral which IMHO should root the bilateran tree… which of course begs more than one question upon rereading their analysis.
Forget Ctenophores – what about Cnidarians!? Some taxonomists argue that Cnidarians are descendents of ancient bilateral coelomates and not the other way around. Biologists have known since the 1920s that Cnideria had a directive axis which gave them right and left-hand sides. Volker Schmidt goes on to argue that non-radially organized hydrozoan larvae have an anterior concentration of sensory and ganglionic nerve elements, suggesting that a fundamental genetic toolkit for the establishment of bilateral and polarized anatomies was already present before the Cnidaria-Bilateria divergence. Volker Schmidt goes so far as to suggest that diploblastic status of adult Cniderians is derived and that true mesoderm can be even be detected during Cniderian embryogenesis. OK – I concede that last argument is particularly contentious… but you get my drift.
I am partial to the notion the UrBilateran that subsequently gave rise to “Protostomes” & Deuterostomes and was itself coelomate with possessed a dorsal nerve chord. Any subsequent acoelomy and pseudocoelomy was derived… ditto ventral nerve chords. But hey… now I am being really contentious!
Throughout the history of evolutionary biology, as well as many other sciences, there has been a conflict between two styles of thinking. One is conventionally called functionalism, although in evolutionary biology the term “adaptationism” is more frequently used today because a trait’s “functional fit for it’s office” is produced through adaptation by natural selection (i.e., function is explained by adaptation through natural selection). The functionalist stance is one that explains organismal traits through their functional and adaptive values.
The alternative style of thinking does not have a generic name in biology, although in other areas of study it is called “structuralist.”
Michael Denton in Evolution: Still A Theory In Crisis or Gunter P. Wagner in The Intellectual Challenge of Morphological Evolution: A Case for Variational Structuralism?
There is an approximate 8% excess of Adenine and Thymine above random in the DNA of humans. This suggests mutational bias and/or non-random mutation. If 3 billion coins were found to be 58% heads vs. 42% tails, then the chance hypothesis of a random unbiased coin flip would be easily rejected. The odds of such an event happening are astronomical according to the binomial distribution.
I often encounter posters here at TSZ who claim that Genetic Algorithms (GAs) either model or simulate evolution. They are never quite clear which it is, nor do they say what it means to model or simulate evolution (what would be required) and how GAs qualify as either one or the other. My position is that GAs neither model nor simulate evolution. In addition to other reasons I’ve given in the past I’d like to present the following argument.
GAs are often used to demonstrate “the power of cumulative selection.” Given small population sizes drift ought to dominate yet in GAs drift does not dominate. Why not?
Typology is perfectly consonant therefore with descent with modification. Each cladogram is witness to descent with modification and the existence of distinct Types. The modifications are novel taxa-defining homologs, acquired during the process of descent along a phylogenetic lineage, each of which defines a new Type.
– Denton, Michael. Evolution: Still a Theory in Crisis
I repeat, Michael Denton accepts common descent is is not a Creationist. My original thread has been inundated with scoffing and mocking and young earth creationism, all of which are far removed from the subject matter of Denton’s latest book. Trying again.
Denton’s stance is for structuralism and against functionalism, especially as functionalism appears in it’s current form as the modern synthesis or neo-Darwinism (the cumulative selection of small adaptive changes).
Denton argues for the reality of the types, that “there are unique taxon-defining novelties not led up to gradually from some antecedent form” and that the lack of intermediates undermines the Darwinian account of evolution. He also argues that a great deal of organic order appears to be non-adaptive, including “a great number of the taxa-defining Bauplans,” and that this also undermines the Darwinian account of evolution. Evo-devo is also showing us that “Darwinian selection is not the only or even the main factor that determined the shape and main branches of the great tree of life.”
The late John Davison often remarked that science could only answer “how” questions, not “why”. It seems to me philosophers, perhaps I’m really thinking of philosophers of religion rather than in general, attempt to find answers to “why” questions without always having a firm grasp on how reality works. Perhaps this is why there is so much talking past each other when the explanatory power of science vs other ways of knowing enters a discussion. Continue reading →
How about some cool science as we head toward the weekend?
Let’s talk about long noncoding RNAs (lncRNA) – they are (somewhat arbitrarily) defined as stretches of DNA that are at least 200 base pairs in length that are transcribed into mRNA but have little potential to code for proteins. Determining the function (if one exists) of a particular lncRNA can often be difficult. In part, this may be due to the fact that lncRNA evolve much more quickly than protein-coding genes do and therefore exhibit a much smaller degree of sequence conservation, which can make identifying orthologs in other related organisms more difficult. Nevertheless, if a particular lncRNA is functionally important, we would usually expect to see copies of it in related organisms, so finding these homologs can be an important indicator of function.
A new paper in Genes and Development by Quinn et al. is a useful demonstration of this. The authors find evidence of 47 homologs of roX, an lncRNA involved in X chromosome dosage compensation, across 35 fruit fly species. The researchers identity roX homologs based on a combination of short regions of sequence conservation (“microhomology”), RNA secondary structure and synteny (i.e., similarity in location along a chromosome) Here is the abstract (I believe the paper itself is open access): Continue reading →