Saturday, March 04, 2006
Why research an 'Internal Evolutionary Mechanism'?
Why research an 'Internal Evolutionary Mechanism'?
Part 1: Background and Perspective: 'Life isn't a rehearsal'
Part 2: Aims
have been posted to the Personal Posts category.
technorati tags: internal+evolutionary+mechanism
Friday, March 03, 2006
Modularity and sense organs in the blind cavefish, Astyanax mexicanus.
Evol Dev. 2006 Jan-Feb;8(1):94-100.
Mexican tetra (Astyanax mexicanus) exist as two morphs: a sighted (surface) form and a blind (cavefish) form. In the cavefish, some modules are lost, such as the eye and pigment modules, whereas others are expanded, such as the taste bud and cranial neuromast modules. We suggest that modularity can be viewed as being nested in a manner similar to Bauplane so that modules express unique sets of genes, cells, and processes. In terms of evolution, we conclude that natural selection can act on any of these hierarchical levels within modules or on all the sensory modules as a whole. We discuss interactions within and between modules with reference to the blind cavefish from both genetic and developmental perspectives. The cavefish represents an illuminating example of module interaction, uncoupling of modules, and module expansion.Reprints of Modularity and sense organs in the blind cavefish can be requested from Tamara Franz-Odentaal at the email address here
technorati tags: modularity, cavefish, bauplane, genes, cells, modules, eye, pigment, organs
Thursday, March 02, 2006
If the proposal of an internal evolutionary mechanism is seen as a Copernican expansion upon the Ptolemaic modern synthesis, then in terms of explanatory power alone, obvious areas to zero in on would be any evolutionary equivalents to retrograde motion.Vestigial Organs/Structures are equally interesting - hence the previous post of To See or Not to See: Evolution of Eye Degeneration in Mexican Blind Cavefish and a request in Wanted Papers for Modularity and sense organs in the blind cavefish*).
Evolutionary atavisms are among such suitable candidates and have been brought to the fore (for a day or two at least!) by John F. Fallon's kind response to a request for a copy of The Development of Archosaurian First-Generation Teeth in a Chicken Mutant.
The TalkOrigins webpage (title link) gives a useful overview of both Atavisms & Vestigial Structures and has the following outline:
1. Anatomical vestigial structuresJohn Latter
* Answers to criticisms of vestigial structures
1. Vestiges can have functions
2. Positive evidence demonstrates lack of functionality
3. Negative evidence is scientific when controlled
* Living whales with hindlimbs
* Newborn babies with tails
3. Molecular vestigial structures
4. Ontogeny and developmental biology
* Mammalian ear bones and reptile jaws
* Pharyngeal pouches and branchial arches
* Snake and whale embryos and with legs
* Embryonic human tail
* Marsupial eggshell and caruncle
5. Present biogeography
6. Past biogeography
* Apes and humans
Model of an Internal Evolutionary Mechanism:
*This paper now appears on the Main Blog (here) and contains details on how to obtain reprints
Wednesday, March 01, 2006
The validity of this rule for behavioral ontogeny has never been proven. Few developmental psychologists or ethologists have committed themselves on the issue. A clear position, based on empirical data, is needed. The present interdisciplinary article offers cogent reasons why the biogenetic rule has no relevance for behavioral ontogeny.The biogenetic rule states that ontogeny repeats phylogeny. The validity of this rule for behavioral ontogeny has never been proven. Few developmental psychologists or ethologists have committed themselves on the issue. A clear position, based on empirical data, is needed. The present interdisciplinary article offers cogent reasons why the biogenetic rule has no relevance for behavioral ontogeny.
Tuesday, February 28, 2006
Common objections to 'Internal Evolutionary Mechanisms' (1)
In their paper "The Spandrels of San Marco and the Panglossian Paradigm" Gould and Lewontin briefly described the European concept of Bauplan ('bodyplan') which, in its 'strong' form, speculates:
"But the important steps of evolution, the construction of the Bauplan itself and the transition between Bauplane, must involve some other unknown, and perhaps 'internal,' mechanism."
An internal mechanism cannot be 'mystical' because if one exists then it would be testable. This suggests the concept ought to evoke no greater uncertainty than that which would be appropriate to the words of Einstein: "If we knew what we were doing, we wouldn't call it research, would we?".
[The following also appears as "An error in associating Lamarck with 'Adaptive Mutations?"]
In 1640 Galileo Galilei wrote a letter to Fortunio Liceti in which he said:
"If Aristotle were to see the new discoveries recently [made] in the heavens, whose immobility he had asserted, because no alteration had previously been seen in them, he would now without doubt state the contrary." ['Galileo Galilei - Towards a Resolution of 350 Years of Debate', Paul Cardinal Poupard].
The above statement highlights the danger of placing dependence on words frozen in time without taking into account how different those words might be if their author had had access to the discoveries that have since been made.
Lamarck, for example, published his "Zoological Philosophy" in 1809 and is today popularly associated with "the inheritance of acquired characteristics" whereby organisms somehow direct their own evolution. On the basis of Galileo's words, however, it could be argued that had Lamarck been alive in the 1890s, over thirty years after publication of Darwin's "On the Origin of Species", his views would have progressed from the moment in time in which they had been caught.
With access to the discoveries and discussions that occured throughout the 19th Century it is conceivable that Lamarck might even have reached broad agreement with J. Mark Baldwin over the latter's proposal of an indirect factor in evolution, known today as the "Baldwin Effect", and described in the 1896 paper "A New Factor in Evolution" [American Naturalist].
Pure speculation ,of course, but if sufficient to illustrate a general principle (that "words frozen in time should be differentiated from those carved in stone") then the inappropriateness of interpreting new discoveries or proposals in 'Lamarckian terms' is readily apparent.
"Evolution is a process that results in heritable changes in a population spread over many generations."
This is a good working scientific definition of evolution; one that can be used to distinguish between evolution and similar changes that are not evolution. Another common short definition of evolution can be found in many textbooks:
"In fact, evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next."
- Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974
One can quibble about the accuracy of such a definition (and we have often quibbled on these newsgroups) but it also conveys the essence of what evolution really is. When biologists say that they have observed evolution, they mean that they have detected a change in the frequency of genes in a population.
Individuals do not evolve, but if shared circumstances 'triggered' individual internal evolutionary mechanisms in a subset of a population then this could cause similar genetic changes to appear in their progeny. Thereby causing a "change in the frequency of alleles within a gene pool from one generation to the next."
Extract from the transcript (click here) of a radio program in which Robyn Williams (ABC: Australian Broadcasting Corporation) talked to Ted Steele (author of "Lamarck's Signature"):
Steele: ...First, in 1885, three years after Darwin's death, a German biologist, August Weismann, responding to the challenge of Darwin's Theory of Pangenesis, erected his now famous 'barrier' between the somatic cells and germ cells. 'Weismann's Barrier' was assumed to protect the germ cells from any type of genetic change within the body.
The bulk of Weismann's experimental refutation focused on testing whether acquired parental mutilations could be inherited.
He is most famous for his work on chopping off the tails of rats shortly after birth. He then showed in breeding experiments extending over many generations, that such tail chopping at birth never produced a tailless offspring.
Critics of this experiment have pointed out that such experiments did not test Lamarck's idea. A short tail caused by chopping is a modification that was not produced by the rat. In contrast, Lamarck believed that only modifications produced by a response of the rat to the environment would be inherited.
Although the criticism is valid the most important point is that Weismann wasn't testing for a specific internal evolutionary mechanism whereby it had been proposed that 'acquired characteristics' could be inherited.
Monday, February 27, 2006
[Badyaev, Proceedings The Royal Society Biological Sciences, May '05]
Stress-induced variation in evolution from behavioural plasticity to genetic assimilation
Extreme environments are closely associated with phenotypic evolution, yet the mechanisms behind this relationship are poorly understood. Several themes and approaches in recent studies significantly further our understanding of the importance that stress-induced variation plays in evolution. First, stressful environments modify (and often reduce) the integration of neuroendocrinological, morphological and behavioural regulatory systems. Second, such reduced integration and subsequent accommodation of stress-induced variation by developmental systems enables organismal `memory' of a stressful event as well as phenotypic and genetic assimilation of the response to a stressor. Third, in complex functional systems, a stress-induced increase in phenotypic and genetic variance is often directional, channelled by existing ontogenetic pathways. This accounts for similarity among individuals in stress-induced changes and thus significantly facilitates the rate of adaptive evolution. Fourth, accumulation of phenotypically neutral genetic variation might be a common property of locally adapted and complex organismal systems, and extreme environments facilitate the phenotypic expression of this variance. Finally, stress-induced effects and stress-resistance strategies often persist for several generations through maternal, ecological and cultural inheritance. These transgenerational effects, along with both the complexity of developmental systems and stressor recurrence, might facilitate genetic assimilation of stress-induced effects. Accumulation of phenotypically neutral genetic variance by developmental systems and phenotypic accommodation of stress-induced effects, together with the inheritance of stress-induced modifications, ensure the evolutionary persistence of stress-response strategies and provide a link between individual adaptability and evolutionary adaptation.
Sunday, February 26, 2006
[Newman & Muller/Müller, J. Experimental Zoology, '00]
"The close mapping between genotype and morphological phenotype in
many contemporary metazoans has led to the general notion that the
evolution of organismal form is a direct consequence of evolving
genetic programs. In contrast to this view, we propose that the
present relationship between genes and form is a highly derived
condition, a product of evolution rather than its precondition.
Prior to the biochemical canalization of developmental pathways, and
the stabilization of phenotypes, interaction of multicellular
organisms with their physico-chemical environments dictated a many-to-
many mapping between genomes and forms. These forms would have been
generated by epigenetic mechanisms: initially physical processes
characteristic of condensed, chemically active materials, and later
conditional, inductive interactions among the organism's constituent
tissues. This concept, that epigenetic mechanisms are the generative
agents of morphological character origination, helps to explain
findings that are difficult to reconcile with the standard neo-
Darwinian model, e.g. the burst of body plans in the early Cambrian,
the origins of morphological innovation, homology, and rapid change
of form. Our concept entails a new interpretation of the relationship
between genes and biological form."