Evolution Lecture No. 10: Phylogenetics
Monday October 1 , 2012
The Oldest Phylogenetic Tree:
-All living organisms on Earth are descendent from one common ancestor.
A More Recent Phylogenetic Tree:
-Contains all the organisms for which the entire genome was sequenced. The Homo sapiens genome
contains both prokaryotic (mitochondrial) DNA and eukaryotic (nuclear) DNA.
Principles Of Phylogenetics:
-The phylogenetic structure is indicative of a hierarchy among species of the planet and in classifying
monophyletic (single ancestry) species, there is stability and permanence of classification. Anagenesis
describes how as species change through time, differences accumulate within species (a form of
microevolution). Cladogenesis describes the accumulated differences between species (a form of
macroevolution). Paraphyletic species is where a subset of lineage members is excluded; some lineages
are missing. Polyphyletic species are made up of selected taxa from diverse lineages; missing origins.
Phylogenetic Classification & Its Issues:
-Ancestors are hardly available for study since only existing or surviving species can be examined for
genome sequencing. For example, many of the earliest ancestors of planet Earth’s lineages
(prokaryotes) are microorganisms that don’t leave anything in the fossil record for studying purposes.
Therefore, phylogenetics is applied only to existing species that are present in the modern world.
-Methods of reconstructing phylogenetic trees are often based on the principle of parsimony (where the
simplest explanation is the most likely to be true). In terms of what to look for in terms of parsimony
and phylogenetic trees, always look for the least amount of evolutionary changes across a lineage
because this would be the most parsimonious.
-Synapomorphies are shared, derived, homologous characteristics. An example of this would be the S-
shaped necks of birds (only they possess this feature) as well as their feathers. They are known as
homologous simply because it is due to common ancestry (they share the same genetic basis).
Homoplasious, Analogous & Homologous:
-Phylogenies are constructed using patterns of similarity (the more similar things are the more recently
they shared a common ancestor). Homology is when similarity usually reflects recently shared ancestry.
Not all similarity reflects recent common ancestry (Not all similarities are synapomorphies). -Convergence: a misleading similarity despite distant evolutionary relationship. E.g. bug wings, bird
wings; whale sonar, bat sonar. Divergence: a misleading dissimilarity d