Cambrian Explosion

Stephen Jay Gould's in "Wonderful Life" stimulated a lot of new ideas about the Cambrian explosion of life. It soon became clear that there were a huge variety of organisms difficult to classify, such as those in the Burgess Shale. However, there are a number of Cambrian fossil beds, such as the lagerstatte in China, which contain evidence of important phyla such as Chordata.

Richard Fortey from the British Museum of Natural History asked, 'But what does all this diversity mean?'. "There are today 30 living phyla. In the Cambrian, some claim that there were as many as 100 phyla, but the evidence does not support this. We now believe that morphological diversity did not explode as much as Gould originally suggested, although the explosion in evolutionary experimentation was real. By the time we get to the Cambrian, like at the Burgess Shale, the systems are very complex, such as trilobite eyes. Evolution was experimenting with many wondrous varieties, such as all the armor on the heads of trilobites."
The Woodstock of Evolution -- The World Summit on Evolution (ScientificAmerican.com).


"Stratigraphic sections spanning the Vendian-Cambrian boundary show a broadly similar pattern whereby the key events are bracketed by the 600-million-year (Myr)-old Neoproterozoic glacial deposits (tillites) and in the succeeding Cambrian diverse metazoan assemblages, typified by abundant skeletons, diverse trace fossils, and Burgess Shale-type faunas (Fig. 1, large). One key development is a series of accurate radiometric determinations (1). The Vendian-Cambrian boundary is now placed at 543 Myr, and the duration (45 Myr) of the Cambrian is substantially shorter than once thought. The preceding Ediacaran faunas have an approximate age range of 565-545 Myr. Accordingly, the overall time-scale for discussion is a relatively protracted 65 Myr, although the principal events of evolutionary interest are probably more tightly bracketed (550-530 Myr) between the diverse Ediacaran faunas of latest Neoproterozoic age (2) and the Chengjiang Burgess Shale-type faunas (3). Correlations are also assisted by emerging schemes of chemostratigraphy (2, 4), notably with reference to strontium (87Sr) and carbon (13C)." Cambrian taxonony & Phylogeny
Simon Conway Morris The Cambrian "explosion": Slow-fuse or megatonnage? PNAS April 25, 2000 vol. 97 no. 9 4426-4429

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Blogger qtr said...

Glossary of terms: classification systems and population mechanisms in speciation:

Allopatric speciation occurs when a geographical barrier sub-divides a parent species, resulting in geographic and reproductive isolation such that the descendent species can no longer interbreed upon removal of the barrier.

Anagenesis differs from cladogenesis in that one species progressively transforms into a replacement species when sufficient gene mutations fix in the descendant population. At this point, the ancestral species has become extinct. This mechanism is distinct from the increase in numbers of species generated by cladogenetic branching events.

Cladogenesis is the mechanism of speciation in which one or more lineages (clades) arise from an ancestral line. Such speciation events increase the variety of plants or animals through branching of the phylogenetic tree. Cladogenesis is differentiated from anagenesis, which is the in toto replacement of one species by an anatomically distinct species.

Monophyletic taxon or clade: an accurate grouping of only (opp. polyphyletic) and all (opp. paraphyletic) descendents of a shared common ancestor. A monopyletic group is genetically homogeneous and reflects evolutionary relationships.

Paraphyletic taxon or clade: a monophyletic group that excludes one or more discrete groups descended from the most recent common ancestral species of the entire group. Other descendent species of the most recent common ancestor have been excluded from the paraphyletic taxon, usually because of morphologic distinctiveness.

Phenetic system: groupings of organisms based on mutual similarity of phenotypic (physical and chemical) characteristics. Phenetic groupings may or may not correlate with evolutionary relationships.

Phylogenetic system: groups organisms based on shared evolutionary heritage. DNA and RNA sequencing techniques are considered to give the most meaningful phylogenies.

Phylogenetic separation into evolutionary relationships (clades), based on comparison of genomes is likely to supplant phenotypical (phenetic) taxonomies of the prokaryotes.

Peripatry (paripatry) is a subset of allopatry in which an isolated group has a smaller population than the parent group. Ernst Mayr introduced the term. Peripatric speciation occurs when the smaller sub-group of a species enters a novel niche within the range of the parent species, becoming geographically and reproductively isolated. Peripatric speciation (paripatric) is distinguished from allopatric speciation by the smaller size of the isolate group, and from sympatric speciation, which involves no barrier to breeding.

Polyphyletic taxon: opposite to monophyletic taxon: A polyphyletic group is mistakenly or improperly erected on the basis of homoplasy — characteristics that have arisen despite not sharing a common ancestor. Homoplasy arises because of convergent evolution, parallelism, evolutionary reversals, horizontal gene transfer, or gene duplications. Polyphyletic taxa are genetically heterogeneous because members do not share a common ancestor.

Neontology is a branch of biology that emphasizes the study of modern biota (living or recent organisms) rather than fossilized organisms (paleontology).

Numerical Taxonomies are a common approach to phenetic taxonomy that employ a number of phenotypic characteristics to generate similarity coefficients that may be mapped in dendrograms. Groupings based on numerical taxonomy may or may not correlate with evolutionary relationships.

Taxonomies aim to group organisms according to shared characteristics against the background of biological diversity.

Sympatry involves no geographical separation of sub-populations of individuals. Sympatric speciation events occur most often in plants by the mechanism of polyploidy in which the number of chromosomes is doubled or tripled. John Maynard Smith proposed a model called disruptive speciation, in which homozygotes might have greater fitness than heterozygotes under some environmental conditions.

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