12-history of lineages
TRANSCRIPT
Systematics
• Study of evolutionary relationships between organisms
–Classify biodiversity (Taxonomy)
–Unravel ancestor-descendant relationships (Phylogenetics)
Phylogenetic systematics
How can we reconstruct the history of evolution without seeing speciation events?
Identification of key characters that represent evolved
(derived) traits
Construct a tree (cladogram) based on the observed similarities between species
Clade: Any monophyletic evolutionary branch in a phylogeny using derived characters
Paraphyletic taxon– group that includes an ancestral taxon and some, but not all of its descendent taxa (artificial)
Polyphyletic taxon– group of organisms that does not include their most recent common ancestor (artificial)
need to pass through one or more ancestors before arriving at a
common ancestor for the group.
Phylogenetic Systematics/Cladistics
• Based on Willi Hennig work (1950)
• Modern approach that investigates how evolution occurred to generate the observed similarities between species
• Objective – deduce ancestry through observations of the changes in characteristics • Method – construction of phylogenetic trees
Phylogenetic Systematics How can the history of evolution be reconstructed
without seeing speciation events?
1) Identification of key characters (heritable parts or
attributes of an organism)
2) Transform characters into transformation series
3) Compare these characters to an outgroup
4) Construct a cladogram based on the observed similarities between species
Outgroup: a related taxon used to infer primitive and derived character states
Characters
• Heritable traits with different states that can be used to infer evolutionary relationships
• Morphological
– Physical measurements
– General form
• Molecular
Characters
• Heritable traits with different states that can be used to infer evolutionary relationships
• Morphological
• Molecular
– DNA sequencing
Phylogenetic systematics
Why bother to know if the trait is primitive or ancestral?
• Structures and functions usually not created de novo (but are modified)
Phylogenetic systematics
• Homology
– Character state shared by two or more taxa due to inheritance from a common ancestor
• Homoplasy
– Character state shared by two or more taxa due to convergent evolution (independent origin)
• Can only use homologous characters to understand evolutionary relationships
Phylogenetic systematics
• Only homologies should be used to understand evolutionary relationships and create phylogenetic trees
Praying mantis
Lacewing
Phylogenetic systematics
What kind of characters are used to construct phylogenetic trees?
• Classes of Homologous traits
– Plesiomorphic (primitive)
– Apomorphic (derived)
Phylogenetic systematics
What kind of characters are used to construct phylogenetic trees?
• Classes of Homologous traits
– Plesiomorphic (primitive)
– Apomorphic (derived)
– Symplesiomorphic (primitive-shared) – More inclusive than focal clade
– Synapomorphic (derived-shared) – Includes only focal clade
Hennig’s Rules for Phylogenetic Systematics
1. Assume homology, not homoplasy in absence of evidence
2. Homologies found within a monophyetic group that are shared with members of sister groups are pleisiomorphic while those found only in the ingroup are apomorphic
3. Only synapomorphies provide evidence of common ancestral relationships (not symplesiomorphies or homoplasies)
4. Can combine multiple transformation series
Phylogenetic Systematics
Constructing a cladogram
1) Identification of key characters (heritable parts or attributes of an organism)
2) Transform characters into transformation series
3) Compare these characters to an outgroup
4) Construct a cladogram based on the observed similarities between species
Outgroup: a related taxon used to infer primitive and derived character states
Molecular Systematics (molecular phylogenetics)
• Can include hundreds/thousands of characters in a phylogenetic study
• Morphological characters still useful
• Partition characters’ rates of evolution
Molecular clocks
Can we know when speciation occurred?
• Some DNA and proteins evolve at known and constant rates used as a molecular clock to date evolutionary steps
• Ongoing debate on the reliability of molecular clocks
• How are molecular clocks calibrated?
Molecular clocks
How are molecular clocks calibrated?
• Comparing known
rates of molecular
change with
speciation in the
fossil record
Fossils
• Provides direct factual evidence of evolution
• Provide evidence of character evolution and ancestral traits
• Document divergence times
• Document localities of past occurrence (distribution)
Fossil limitations
• Incomplete
• Biased preservation
• Only remains
• Trace fossils
• Destroyed/inaccessible
• Less known further back in time
• Transported from original distribution
Why are fossils and phylogenies important to biogeography?
• Evolutionary histories
Influences of vicariance and dispersal events
• Biogeographic history of a place
Changes in dispersal barriers
How these barriers influenced the biotas
Taxon vs. Area approaches
• Taxon biogeography
– Reconstructs the history of lineages across areas at a single lineage at a time
• Area biogeography
– Reconstructs the relationships among the areas and the biotas living on those areas
Taxon biogeography
• The goal is to reconstruct the biogeographic history of just one taxon.
• However, it could be to compare several taxon and see if there is any congruence across them.
Area biogeography
• The goal is to reconstruct the biogeographic history of a set of areas based on the relationships of taxa distributed across those areas