[Taxacom] Cyanobacteria and the Origin of Life

Mary Barkworth Mary.Barkworth at usu.edu
Sun Oct 28 06:35:20 CDT 2018


Thank you for sharing that link
-----Original Message-----
From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> On Behalf Of Kenneth Kinman
Sent: Saturday, October 27, 2018 7:56 PM
To: taxacom <taxacom at mailman.nhm.ku.edu>
Subject: [Taxacom] Cyanobacteria and the Origin of Life

Hi All,
         I have long believed that Cyanobacteria are the most primitive forms of cellular life to have survived to the present day.  And that the earliest cyanobacteria might not have been capable of oxygenic photosynthesis (at least one that required both Photosystems I and II).   The question is whether a recent October 2018 research paper might help support such an hypothesis.  Opinions (pro or con) are welcome.  Here is a weblink to the paper:            http://www.pnas.org/content/early/2018/09/26/1808176115

                               ----------------Ken Kinman

________________________________
From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of John Grehan <calabar.john at gmail.com>
Sent: Wednesday, October 24, 2018 5:55 AM
To: taxacom
Subject: [Taxacom] Orthogeneisis

For anyone interested in such esoterica, below is the text of an article published in The International Encyclopedia of Biological Anthropology (2018). If anyone wants a pdf please contact me.

John Grehan

Like many controversial concepts, orthogenesis will mean different things to different people.
For most evolutionary biologists the term is likely to embody an obsolete view of evolution as teleological or predetermined, or that it involves some kind of “straight line” or linear evolution that is incompatible with the reality of evolutionary divergence. As emphasized by Darwin (see Darwin, Charles R), evolution was (and still
is) largely seen as the result of random variation subject to natural selection. But even Darwin referred to “laws of growth” and other early evolutionists saw that evolution was not entirely “random” as novel variations failed to obfuscate the existence of preexisting ancestral architecture (To put this issue in simplistic terms: A penguin in the water remains a bird and does not become a fish.) There was also widespread acceptance that evolution was generally irreversible—that while individual characters may sometimes revert, overall structural organization did not. Darwin later expressed regret over his initial emphasis on natural selection (see evolution).

The idea of orthogenesis was proposed in 1893 by the German evolutionist William Haacke, who argued that if any kind of variation was possible, one would expect reversals to be as frequent as novelties. The rarity of such reversals was seen as evidence that the evolutionary origin of variation was not only limited, but constrained or biased by preexisting biological organization. Orthogenesis was subsequently adopted by a minority of evolutionists, including some who applied it to theories of vital forces or goal-driven evolution.The fossil record was sometimes regarded as demonstrating a linear direction to evolution, such as the classic reduction of horse digits from five to one.This linear concept is widely seen as having been refuted by evidence that evolution was not linear, but involved a process of diversification or branching (e.g., Ernst Mayr (see Mayr, Ernst), George Gaylord Simpson (see Simpson, George Gaylord), Stephen Jay Gould (see Gould, Stephen J.)—see Grehan and Ainsworth 1985). This critique mistakenly conflated linear evolution with orthogenesis. Haacke cited the reduction of horse toes from five to one as an illustration of a biologically driven trend, not a linear or purposeful evolution. Whether or not horse evolution was “linear” or a radiating “bush,” there was a reduction of toes that took place over space and time resulting in the single-toed horse of today.
This reduction may be attributed to “random”
variations that imparted increased fitness (see fitness), or it may be attributed to a biological process where new variants resulted in reduction of toes. Horses survived because these reductions were compatible with the requirements of ecological survival. The apparent contradiction between an orthogenetic trend and evolutionary divergence was addressed by Leon Croizat (1958,
1964) in the panbiogeographic evolutionary synthesis.
He regarded differentiation as the result of allopatric divergence across a widespread ancestral range. The heterogeneous distribution of genetic characters across this range provides spatially different starting points that biased subsequent differentiation and speciation.

In natural selection theory, a random variation is spread through a population by differential reproductive success (fitness). In orthogenetic models, a new variant does not require differential reproduction to become established. While observations of biological organization and biogeographic distribution led some evolutionists to adopt orthogenetic models of evolution, there remained the widespread objection that the concept was “mysterious” because there was no mechanism to explain the spread of novel variants without requiring natural selection.
But in recent decades evolutionary biologists have widely recognized that aspects of genomic architecture, gene structure, and developmental pathways are difficult to explain without involving nonadaptive processes of genetic drift and mutation. Gene expression is controlled by complex pathways and networks of interacting regulatory genes, and there has been no formal demonstration of an adaptive origin for any genetic network.Many features of transcriptional networks can readily arise through nonadaptive drift, mutation, and recombination. This raises the question as to whether natural selection is sufficient or even necessary for the origin of gene-network topologies. Recombination events are accompanied by unidirectional exchanges known as gene conversion. There is evidence, for example, that many eukaryotes, including humans, are GC-biased—that is, their total DNA complement contains a higher-than-expected proportion of guanine–cytosine base pairs. In humans, there is a genome-wide positive correlation between crossover rate and GC content.
Other biological processes capable of producing evolutionary trends without environmental selection include the emergence of error minimization in the genetic code, expansion of genome size by mobile element proliferation, and widespread reductions in genome size in multiple lineages of mammals. Complementary to genetic and developmental studies is the widespread existence of trends in morphology involving reduction, suppression, and fusion in complex multipartite structures such as the angiosperm flower and vertebrate skull.

The recognition of orthogenesis does not imply a rejection of natural selection. Orthogenesis implies only that biological mechanisms can result in mutations and the spread of mutations without requiring increased fitness. As natural selection can also result in the spread of mutations (variants), there are two kinds of dynamic processes involved with evolution: one internal and biological and the other external and environmental.
These are not two separable entities.
Earlier formulations of orthogenesis emphasized a contrast between the roles of “internal” and “external” forces in evolution, but no biological process exists without an environment; and conversely, the effects produced by environmental forces are contingent upon the organisms involved. How these biological and environmental processes interact to “co-construct” organisms and their ecological relationships through development represents a new and largely unexplored aspect of evolutionary biology—one that involves intrinsic biological mechanisms, regardless of whether they are labeled orthogenesis or something else (Heads 2016).
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Nurturing Nuance while Assaulting Ambiguity for 31 Some Years, 1987-2018.
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You can reach the person managing the list at: taxacom-owner at mailman.nhm.ku.edu

Nurturing Nuance while Assaulting Ambiguity for 31 Some Years, 1987-2018.


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