Saturday, March 04, 2006

 

[evomech] From symmetry to asymmetry: Phylogenetic patterns of asymmetry variation

[A. Richard Palmer, PNAS, Dec '96]

From symmetry to asymmetry: Phylogenetic patterns of asymmetry variation in animals and their evolutionary significance

Abstract:

Phylogenetic analyses of asymmetry variation offer a powerful tool for exploring the interplay between ontogeny and evolution because (i) conspicuous asymmetries exist in many higher metazoans with widely varying modes of development, (ii) patterns of bilateral variation within species may identify genetically and environmentally triggered asymmetries, and (iii) asymmetries arising at different times during development may be more sensitive to internal cytoplasmic inhomogeneities compared to external environmental stimuli. Using four broadly comparable asymmetry states (symmetry, antisymmetry, dextral, and sinistral), and two stages at which asymmetry appears developmentally (larval and postlarval), I evaluated relations between ontogenetic and phylogenetic patterns of asymmetry variation. Among 140 inferred phylogenetic transitions between asymmetry states, recorded from 11 classes in five phyla, directional asymmetry (dextral or sinistral) evolved directly from symmetrical ancestors proportionally more frequently among larval asymmetries. In contrast, antisymmetry, either as an end state or as a transitional stage preceding directional asymmetry, was confined primarily to postlarval asymmetries. The ontogenetic origin of asymmetry thus significantly influences its subsequent evolution. Furthermore, because antisymmetry typically signals an environmentally triggered asymmetry, the phylogenetic transition from antisymmetry to directional asymmetry suggests that many cases of laterally fixed asymmetries evolved via genetic assimilation.

Full text at:

http://www.pnas.org/cgi/content/full/93/25/14279

John Latter
-- 
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Friday, March 03, 2006

 

[evomech] Re: Antisymmetry (Chapter 16 of 'Variation')

--- In evomech@yahoogroups.com, "John Latter" <jorolat@...> wrote:
>
> [A. Richard Palmer. "Antisymmetry." In Variation, Editors B Hallgrimmson and BK Hall. Elsevier (2005): 359-397]
>
> Introduction:
>
> The notion of antisymmetry likely strikes most people as bizarre. How can any
> variation exist that is "anti-" something else? To dismiss antisymmetry as mere
> intellectual catnip of academic snoots would seem easy. To dismiss it too hastily
> would be a big mistake.
>
> Antisymmetry is a peculiar kind of variation whose evolutionary significance is
> surprisingly unappreciated, no doubt in part because the term seems odd and foreboding. However, the phenomenon, with its particularly apt moniker, is actually
> widespread and offers the promise of valuable insights into a century-old debate
> about the interplay between development and evolution.

Now available at:

http://www.bms.bc.ca/library/Palmer'05%20CH16-Antisym-proofs-ed-4.pdf

John Latter

Model of an Internal Evolutionary Mechanism

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Re: [evomech] Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss.

From: "John Latter" <jorolat@gmail.com>
> Subject: [evomech] Limbs in whales and limblessness in other
> vertebrates: mechanisms of evolutionary and developmental
> transformation and loss. [Bejder & Hall, Evolution &
> Development, '02]
> text; http://whitelab.biology.dal.ca/lb/Bejder%20and%20Hall.pdf

> We address the developmental and evolutionary mechanisms
> underlying fore- and hindlimb development and progressive
> hindlimb reduction and skeletal loss...

There's nothing in the article about the evolutionary origin of
limbs.
That little point is glossed over in this discussion of
development
and of the evolutionary reduction of limbs.

> Limblessness in most snakes is also associated with adoption
>of a new (burrowing) lifestyle...

Were early snakes tunnelers? Or does this new burowing include
merely burrowing through surface debis? Just wondering.

> An evolutionary change in Hox gene expression--as occurs
> in snakes--or in Hox gene regulation--as occurs in some limbless
> mutants--is unlikely to have initiated loss of the hindlimbs in
> cetaceans. Selective pressures acting on a wide range of
> developmental processes and adult traits other than the
> limbs are likely to have driven the loss of hindlimbs in whales.

Are they suggesting that a change occurring via Hox genes or
their regulation can occur more quickly? Selective pressure may
drive the loss of a feature, but how do we know whether or not
Hox genes are involved?

Cliff

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Thursday, March 02, 2006

 

[evomech] Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss

[Bejder & Hall, Evolution & Development, '02]

Summary:

We address the developmental and evolutionary mechanisms underlying fore- and hindlimb development and progressive hindlimb reduction and skeletal loss in whales and evaluate whether the genetic, developmental, and evolutionary mechanisms thought to be responsible for limb loss in snakes "explain" loss of the hindlimbs in whales. Limb loss and concurrent morphological and physiological changes associated with the transition from land to water are discussed within the context of the current whale phylogeny. Emphasis is placed on fore- and hindlimb development, how the forelimbs transformed into flippers, and how the hindlimbs regressed, leaving either no elements or vestigial skeletal elements. Hindlimbs likely began to regress only after the ancestors of whales entered the aquatic environment: Hindlimb function was co-opted by the undulatory vertical axial locomotion made possible by the newly evolved caudal flukes. Loss of the hindlimbs was associated with elongation of the body during the transition from land to water. Limblessness in most snakes is also associated with adoption of a new (burrowing) lifestyle and was driven by developmental changes associated with elongation of the body. Parallels between adaptation to burrowing or to the aquatic environment reflect structural and functional changes associated with the switch to axial locomotion. Because they are more fully studied and to determine whether hindlimb loss in lineages that are not closely related could result from similar genetically controlled developmental pathways, we discuss developmental (cellular and genetic) processes that may have driven limb loss in snakes and leg-less lizards and compare these processes to the loss of hindlimbs in whales. In neither group does ontogenetic or phylogenetic limb reduction result from failure to initiate limb development. In both groups limb loss results from arrested development at the limb bud stage, as a result of inability to maintain necessary inductive tissue interactions and enhanced cell death over that seen in limbed tetrapods. An evolutionary change in Hox gene expression--as occurs in snakes--or in Hox gene regulation--as occurs in some limbless mutants--is unlikely to have initiated loss of the hindlimbs in cetaceans. Selective pressures acting on a wide range of developmental processes and adult traits other than the limbs are likely to have driven the loss of hindlimbs in whales.

Full text at;

http://whitelab.biology.dal.ca/lb/Bejder%20and%20Hall.pdf

John Latter
-- 
Model of an Internal Evolutionary Mechanism:
http://members.aol.com/jorolat/index.html

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Wednesday, March 01, 2006

 

[evomech] Conrad Hal Waddington: the last Renaissance biologist? (Nature)

[Slack, Nature, Nov '02]

Perspective (Timeline):

Conrad Hal Waddington was a leading embryologist and geneticist from the 1930s to the 1950s. He is remembered mainly for his concepts of the 'epigenetic landscape' and 'genetic assimilation'. This article reviews his life and work, and enquires to what extent his ideas are relevant tools for understanding the biological problems of today.

Full text at:

http://wwworm.biology.uh.edu/evodevo/lecture3/slack02.pdf

John Latter
-- 
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Tuesday, February 28, 2006

 

[evomech] The morphogenesis of evolutionary developmental biology (Int. J, Dev. Biol.)

[Gilbert, Int. J. Dev. Biol. 47: 467-477 (2003)]

Abstract:

The early studies of evolutionary developmental biology (Evo-Devo) come from several sources. Tributaries flowing into Evo-Devo came from such disciplines as embryology, developmental genetics, evolutionary biology, ecology, paleontology, systematics, medical embryology and mathematical modeling. This essay will trace one of the major pathways, that from evolutionary embryology to Evo-Devo and it will show the interactions of this pathway with two other sources of Evo-Devo: ecological developmental biology and medical developmental biology. Together, these three fields are forming a more inclusive evolutionary developmental biology that is revitalizing and providing answers to old and important questions involving the formation of biodiversity on Earth. The phenotype of Evo-Devo is limited by internal constraints on what could be known given the methods and equipment of the time and it has been framed by external factors that include both academic and global politics. [Evolution]

Full text at:

http://www.ijdb.ehu.es/web/contents.php?vol=47&issue=7-8&doi=14756322

John Latter
-- 
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Monday, February 27, 2006

 

[evomech] Re: Symmetry Breaking and the Evolution of Development (Science)

--- In evomech@yahoogroups.com, "John Latter" <jorolat@...> wrote:
>
> [Palmer, Science, Oct '04]
>
> Abstract:
>
> " Because of its simplicity, the binary-switch nature of left-right asymmetry permits meaningful comparisons among many different organisms. Phylogenetic analyses of asymmetry variation, inheritance, and molecular mechanisms reveal unexpected insights into how development evolves. First, directional asymmetry, an evolutionary novelty, arose from nonheritable origins almost as often as from mutations, implying that genetic assimilation ("phenotype precedes genotype") is a common mode of evolution. Second, the molecular pathway directing hearts leftward—the nodal cascade—varies considerably among vertebrates (homology of form does not require homology of development) and was possibly co-opted from a preexisting asymmetrical chordate organ system. Finally, declining frequencies of spontaneous asymmetry reversal throughout vertebrate evolution suggest that heart development has become more canalized."
>
> Copies of the full article can be obtained either by emailing Rich direct or from myself here
>

This paper is now available at:

http://www.biology.duke.edu/nijhout/PDFs/Palmer04.pdf

John Latter

Model of an Internal Evolutionary Mechanism
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Sunday, February 26, 2006

 

[evomech] Putting Genes in Perspective (Book Review)

[Pfennig, American Scientist Book Review, Jan '04]

Developmental Plasticity and Evolution. Mary Jane West-Eberhard.

An unfortunate outgrowth of the modern revolution in genetics is the widespread belief that the genes of an individual organism determine its appearance, physiology and behavior. The genome does not, of course, completely determine how an organism is constructed: The environment is an essential partner. Nowhere is this point more clearly illustrated than by the principle of developmental plasticity—the tendency for genetically identical organisms to differ in response to various environmental stimuli, or for individuals to vary over time as the result of changing conditions in their surroundings...

... In Developmental Plasticity and Evolution, Mary Jane West-Eberhard, an evolutionary biologist at the Smithsonian Tropical Research Institute and a member of the National Academy of Sciences, undertakes to explain how developmental plasticity fits within a genetic theory of evolution. She believes (with considerable justification) that evolutionary and developmental biologists have failed to incorporate developmental plasticity into their framework for understanding the living world.

Full text at:

http://www.americanscientist.org/template/BookReviewTypeDetail/assetid/29782

Amazon.com link:

http://www.amazon.com/gp/product/0195122356/qid=1140942822/103-2980949-5514221

John Latter
-- 
Model of an Internal Evolutionary Mechanism:
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