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Jens Jens is a male
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Registration Date: 18.03.2007
Posts: 2,167
Herkunft: HGW, momentan Jura, Schweiz

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Halloa,

ich habe hier ein interessantes PDF über Auswirkungen erhöhter innerartlicher Varianz bei Trilobiten und ihrer Auswirkung auf makroevolutionäre Entwicklungen im Unterkambrium gefunden, ich weiß nicht ob es für alle downloadbar ist, aber stelle den Link mal rein. Ansonsten bitte die Summary lesen.

http://www3.interscience.wiley.com/cgi-b...330480/PDFSTART

Nigel C. Hughes 2007: Strength in numbers: high phenotypic variance in early Cambrian trilobites and its evolutionary implications. - BioEssays, 29 (11): p. 1081-1084.

Summary
Analysis of the degree of intraspecific morphological polymorphism during the evolutionary history of trilobites using an informatic approach(1) provides striking evidence of a long-suspected but previously unsubstantiated pattern: degrees of polymorphism are markedly higher in phylogenetically basal, stratigraphically early species. This unequivocal pattern prompts further exploration of the relationship between microevolutionary variance and macroevolutionary history. It demonstrates that the ‘traditional’ fossil record of skeletonized organisms can provide unique insight into questions of major evolutionary interest.

Ich habe mal ein paar lesenswerte Passagen rausgenommen und möchte sie vorstellen:

"This is because, as Darwin recognized, species boundaries based on morphology are commonly indistinct. Indeed, it’s always a challenge to distinguish intraspecific from interspecific differences, and especially so in fossils. Terms such as ‘‘lineage crossing’’ ‘‘developmental flexibility’’ and ‘‘plasticity’’ have been bandied about in studies of Cambrian trilobites to donate the observed large morphological variances, while those working with geologically younger, more evolutionarily derived, trilobites have found species definition easier. This frustration in classification of the early trilobites hints that there was something different about the way in which these species varied phenotypically when compared to later ones, and prompts the question of why this might be so. ...."

Die gewonnenen Erkenntnisse von Hughes beruhen auf statistischen Auswertungen zum Polymorphismus von 982 unterkambrischen Trilobitenarten von Webster 2007 ( A Cambrian peak in morphological variation within trilobite species. Science 317:499–502.)

"It would be interesting to know if the degree of polymorphism among the Cambrian species in Webster’s analysis markedly exceeds to that known among living taxa, which can show extremely high rates of intraspecific morphological evolution.(8) That more than70%of early and middle Cambrian species were polymorphic in at least one character may not be an astonishingly high proportion — what of cryptic sexual dimorphism, for example? ......."

"Furthermore, the extent to which the earlier Cambrian, in which Webster finds high intraspecific variance, shows morphological diversity at the level of the clade Trilobita, as a whole, depends on the characters examined. This interval, however, does appear to be the acme of variation in the overall number of trunk segments (13) but, in the early Cambrian, aspects of head shape show markedly less morphological variety than at later times.(14) It has long been have argued that, while derived clades were dominated by peramorphic heterochrony,(15) early trilobite evolution was characterised by the dominance of paedomorphosis,(16) hinting at a difference in evolutionary mode among earlier trilobites and those clades that rose to dominance later, in the Ordovician and thereafter. .... "


"Trilobite paleontology may be a specialist field, but it has special strengths. Webster’s paper provides the first convincing demonstration of a long-suspected general pattern at the taxonomic levels, one at which we witness and make inferences about evolution. It will stimulate research into dissecting the fossil record for patterns of morphological change, through a combination of detailed studies of individual taxa and synoptic views at higher taxonomic levels, thus encompassing both microevolution and macroevolution. ...."

Ich finde den letzten Abschnitt wichtig, da hierin auf die klassische, Morphologie basierte Arbeit der Paläontologen hingewiesen wird, die in Zeiten genetischer Stammbäume leider immer mehr in den Hintergrund gerät. Obwohl damit auch wichtige evolutionäre Fragestellungen beantwortet werden können. Leider ist der klassische Taxonom als Wissenschaftlertyp immer weniger gefragt.

lg,

Jens
03.10.2008 18:35 Jens is offline Send an Email to Jens Homepage of Jens Search for Posts by Jens Add Jens to your Buddy List
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