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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.
1)
2)
3)
4)
5)
Possible Causes:
Origin of large forms and thus a diversity of tissues and structures due to
increased availability of Oxygen and thus more efficient metabolism.
Origin of ‘hard parts” capable of preservation in the fossil record.
Mass extinction of competing Ediacaran animals creating available
niches for adaptive radiation of larger multicellular forms.
Some threshold of developmental regulation and stability reached
prompting a radiation into diverse bauplans
Perhaps all four
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