Body plan

Abody plan,Bauplan(pl.‹See Tfd›German:Baupläne), orground planis a set ofmorphological featurescommon to many members of aphylumofanimals.^[1]Thevertebratesshare one body plan, whileinvertebrateshave many.

This term, usually applied to animals, envisages a "blueprint" encompassing aspects such assymmetry,layers,segmentation,nerve,limb,andgutdisposition.Evolutionary developmental biologyseeks to explain the origins of diverse body plans.

Body plans have historically been considered to have evolved in a flash in theEdiacaran biota;filling theCambrian explosionwith the results, and a more nuanced understanding of animal evolution suggests gradual development of body plans throughout the earlyPalaeozoic.Recent studies inanimalsandplantsstarted to investigate whether evolutionary constraints on body plan structures can explain the presence of developmental constraints duringembryogenesissuch as the phenomenon referred to asphylotypic stage.

History

Among the pioneeringzoologists,Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognised by modern zoologists has risen to 36.^[1]

Linnaeus, 1735

In his 1735 bookSystema Naturæ,Swedishbotanist Linnaeusgrouped the animals intoquadrupeds,birds,"amphibians" (includingtortoises,lizardsandsnakes),fish,"insects" (Insecta, in which he includedarachnids,crustaceansandcentipedes) and "worms" (Vermes). Linnaeus's Vermes included effectively all other groups of animals, not onlytapeworms,earthwormsandleechesbutmolluscs,sea urchinsandstarfish,jellyfish,squidandcuttlefish.^[2]

Cuvier, 1817

In his 1817 work,Le Règne Animal,French zoologistGeorges Cuviercombined evidence from comparative anatomy andpalaeontology^[3]to divide the animal kingdom into four body plans. Taking thecentral nervous systemas the main organ system which controlled all the others, such as the circulatory and digestive systems, Cuvier distinguished four body plans orembranchements:^[4]

with a brain and a spinal cord (surrounded by skeletal elements)^[4]
with organs linked by nerve fibres^[4]
with two longitudinal, ventral nerve cords linked by a band with two ganglia below the oesophagus^[4]
with a diffuse nervous system, not clearly discernible^[4]

Grouping animals with these body plans resulted in four branches:vertebrates,molluscs,articulata(includinginsectsandannelids) and zoophytes orRadiata.

Haeckel, 1866

Ernst Haeckel,in his 1866Generelle Morphologie der Organismen,asserted that all living things weremonophyletic(had a single evolutionary origin), being divided into plants, protista, and animals. His protista were divided into moneres, protoplasts, flagellates, diatoms, myxomycetes, myxocystodes, rhizopods, and sponges. His animals were divided into groups with distinct body plans: he named thesephyla.Haeckel's animal phyla werecoelenterates,echinoderms,and (following Cuvier) articulates, molluscs, and vertebrates.^[5]

Gould, 1979

Stephen J. Gouldexplored the idea that the different phyla could be perceived in terms of a Bauplan, illustrating their fixity. However, he later abandoned this idea in favor ofpunctuated equilibrium.^[6]

Origin

20 out of the 36 body plans originated in theCambrianperiod,^[7]in the "Cambrian explosion".^[8]However, complete body plans of manyphylaemerged much later, in thePalaeozoicor beyond.^[9]

The current range of body plans is far from exhaustive of the possible patterns for life: thePrecambrian Ediacaran biotaincludes body plans that differ from any found in currently living organisms, even though the overall arrangement of unrelated modern taxa is quite similar.^[10]Thus the Cambrian explosion appears to have more or less completely replaced the earlier range of body plans.^[7]

Genetic basis

Genes,embryosand development together determine the form of an adult organism's body, through the complex switching processes involved inmorphogenesis.

Developmental biologists seek to understand how genes control the development of structural features through a cascade of processes in which key genes producemorphogens,chemicals that diffuse through the body to produce a gradient that acts as a position indicator for cells, turning on other genes, some of which in turn produce other morphogens. A key discovery was the existence of groups ofhomeobox genes,which function as switches responsible for laying down the basic body plan in animals. The homeobox genes are remarkably conserved between species as diverse as the fruit fly and humans, the basic segmented pattern of the worm or fruit fly being the origin of the segmented spine in humans. The field of animalevolutionary developmental biology('Evo Devo'), which studies the genetics ofmorphologyin detail, is rapidly expanding^[11]with many of the developmental genetic cascades, particularly in the fruit flyDrosophila,catalogued in considerable detail.^[12]

References

^^a ^bValentine, James W. (2004).On the Origin of Phyla.University of Chicago Press. p. 33.ISBN 978-0226845487.
^Linnaeus, Carolus (1735).Systema naturae, sive regna tria naturae systematice proposita per classes, ordines, genera, & species.Leiden: Haak. pp. 1–12.
^Reiss, John (2009).Not by Design: Retiring Darwin's Watchmaker.University of California Press. p. 108.ISBN 978-0-520-94440-4.
^^a ^b ^c ^d ^eDe Wit, Hendrik Cornelius Dirk De Wit.Histoire du Développement de la Biologie,Volume III, Presses Polytechniques et Universitaires Romandes, Lausanne, 1994, p. 94-96.ISBN 2-88074-264-1
^Haeckel, Ernst.Generelle Morphologie der Organismen: allgemeine Grundzüge der organischen Formen-Wissenschaft, mechanisch begründet durch die von Charles Darwin reformirte Descendenz-Theorie.(1866) Berlin
^Bowler, Peter J. (2009).Evolution: the History of an Idea.California, p. 364.
^^a ^bErwin, Douglas; Valentine, James; Jablonski, David (1997)."The origin of animal body plans".American Scientist(March–April).
^Erwin, D. H. (1999)."The Origin of Bodyplans".Integrative and Comparative Biology (American Zoologist).39(3): 617–629.doi:10.1093/icb/39.3.617.
^Budd, G. E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla".Biological Reviews of the Cambridge Philosophical Society.75(2): 253–95.doi:10.1111/j.1469-185X.1999.tb00046.x.PMID 10881389.S2CID 39772232.
^Antcliffe, J. B.; Brasier, M. D. (2007)."Charnia and sea pens are poles apart".Journal of the Geological Society.164(1): 49–51.Bibcode:2007JGSoc.164...49A.doi:10.1144/0016-76492006-080.S2CID 130602154.
^Hall, Brian K. (28 March 2005)."Evo Devo is the New Buzzword..."Scientific American.292(4): 102–104.Bibcode:2005SciAm.292d.102H.doi:10.1038/scientificamerican0405-102.Retrieved13 September2014.
^Arthur, Wallace.(1997).Animal Body Plans.Cambridge England: Cambridge University Press.ISBN 0-521-77928-6.

External links

Developmental Biology 8e Online: Patterning of the Mesoderm by Activin

Videos

The Science of Evolution: Sean B. Carroll explains the genetics of the fruit fly body plan.

[Valentine2004-1] Valentine, James W. (2004).On the Origin of Phyla.University of Chicago Press. p. 33.ISBN 978-0226845487.

[2] Linnaeus, Carolus (1735).Systema naturae, sive regna tria naturae systematice proposita per classes, ordines, genera, & species.Leiden: Haak. pp. 1–12.

[Reiss2009-3] Reiss, John (2009).Not by Design: Retiring Darwin's Watchmaker.University of California Press. p. 108.ISBN 978-0-520-94440-4.

[DeWit-4] De Wit, Hendrik Cornelius Dirk De Wit.Histoire du Développement de la Biologie,Volume III, Presses Polytechniques et Universitaires Romandes, Lausanne, 1994, p. 94-96.ISBN 2-88074-264-1

[5] Haeckel, Ernst.Generelle Morphologie der Organismen: allgemeine Grundzüge der organischen Formen-Wissenschaft, mechanisch begründet durch die von Charles Darwin reformirte Descendenz-Theorie.(1866) Berlin

[6] Bowler, Peter J. (2009).Evolution: the History of an Idea.California, p. 364.

[AmSci-7] Erwin, Douglas; Valentine, James; Jablonski, David (1997)."The origin of animal body plans".American Scientist(March–April).

[Erwin1999-8] Erwin, D. H. (1999)."The Origin of Bodyplans".Integrative and Comparative Biology (American Zoologist).39(3): 617–629.doi:10.1093/icb/39.3.617.

[Budd2000-9] Budd, G. E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla".Biological Reviews of the Cambridge Philosophical Society.75(2): 253–95.doi:10.1111/j.1469-185X.1999.tb00046.x.PMID 10881389.S2CID 39772232.

[10] Antcliffe, J. B.; Brasier, M. D. (2007)."Charnia and sea pens are poles apart".Journal of the Geological Society.164(1): 49–51.Bibcode:2007JGSoc.164...49A.doi:10.1144/0016-76492006-080.S2CID 130602154.

[11] Hall, Brian K. (28 March 2005)."Evo Devo is the New Buzzword..."Scientific American.292(4): 102–104.Bibcode:2005SciAm.292d.102H.doi:10.1038/scientificamerican0405-102.Retrieved13 September2014.

[Wallace-12] Arthur, Wallace.(1997).Animal Body Plans.Cambridge England: Cambridge University Press.ISBN 0-521-77928-6.

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