Evolution & Phylogeny

Evidence Used to Construct History Comparative Morphology of Living forms: Comparative Embryology: Comparative Molecular Biology: The Fossil Record

Cladistics : the most widely used approach using morphology & embryology Advantages: 1) systematic – repeatable (as long as assumptions used are clear) 2) testable – same assumptions & organisms + different characters should yield same cladogram. Drawbacks: 1) Characters not always easy to define. 2) Choice of outgroup subjective and difficult. 3) Assumptions necessary to chose among possible cladograms – subjective element added. Result: many disputes over proposed phylogenies – not easily resolved (e.g. Brusca’s discussion)

Comparative Molecular Biology I phenetic – counting differences Comparing gene products – amino acid sequences of common proteins e.g. haemoglobins. Comparing genes : 1) DNA hybridization – single strands from related organisms should bind more firmly than unrelated – determined by temperature required to separate. 2) Sequencing nucleotides – using PCR to sequence particular genes

Comparative Molecular Biology II Molecules used for nucleotide sequencing: Ribosomal DNA – chromosomal DNA coding for ribosomal RNA especially the 18SrDNA – coding for the 18S subunit of ribosomal RNA. Highly conserved thus most valuable at higher taxonomic levels e.g. comparing classes within a phylum or even comparing phyla. Mitochondrial DNA – e.g. cytochrome oxidase gene – less highly conserved than rDNA’ s – useful at lower taxonomic levels – comparing genera within a family or even species within a genus. Regulatory genes – DNA sequences that regulate expression of other genes – those associated with regulating developmental processes are particularly important. E.g hox genes, pax-6 gene. Thought to be especially useful in identifying developmental changes resulting in significant morphological change.

Comparative Molecular Biology III Advantages: 1) Equivalence of data clear - units can be precisely defined 2) Enormous size of data set - potentially all genes could be used. 3) Highly conserved genes such - rDNA genes can be used to trace relationships far back in time. 4) Potential for development of statistical methods to judge validity of relationships (as yet there is no consensus about such methods). 5) Non heritable variation avoided

Disadvantages: 1) assumes constant mutation rate over time and across taxa i.e. a constant molecular clock ticking along for selectively neutral portions of each gene. 2) no record of past changes in characters – only four possible nucleotides for each site, mutations followed by ‘back mutations’ go unrecorded. 3) no intermediate condition between characters, thus no way to asses a ‘primitive’ condition. 4) functional correlates of character changes seldom can be traces. Advantages & Disadvantages suggest that molecular data needs to be used along side morphological and embryonic data.

Bryozoa provide evidence supporting Punctuated Equilibrium

Cambrian Explosion I Fossil sources: Burgess Shale, Canada (520 MYA) – Cambrian ( all major phyla present) Chengjian, SW. China (530 MYA) - Cambrian Sirius Passet, N. Greenland (540 MYA) - Cambrian Ediacaran Range, S. Australia (560 MYA) – Lower Cambrian or Precambrian ( only ‘soft bodied forms’ Cnidaria, worms, etc.) Yangtse Gorge, S. China (580 MYA) – Lower Cambrian or Precambrian (sponges and some multicellular embryos – cleavage stages?)

Cambrian Explosion II Various analyses using molecular data suggest that the appearance of triploblastic organisms and the divergence of protstomes and deuterostomes occurred 100s of millions of years before the Cambrian; yet the diversity of current phyla doesn’t appear until the Cambrian. Possible Causes: 1)Origin of large forms and thus a diversity of tissues and structures due to increased availability of Oxygen and thus more efficient metabolism. 2)Origin of ‘hard parts” capable of preservation in the fossil record. 3)Mass extinction of competing Ediacaran animals creating available niches for adaptive radiation of larger multicellular forms. 4)Some threshold of developmental regulation and stability reached prompting a radiation into diverse bauplans Perhaps all four 5) An illusion caused by paucity of Precambrian fossils (increasingly, a less attractive explanation)

Brusca’s based primarily on morphology & embryology “strict consensus” tree

From Moore, 2001 An Introduction to the Invertebrates. based primarily on Molecular data