Classification and Phylogenetic Relationships
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Classification and Phylogenetic Relationships
• Early natural historians classified organisms based on shared features
• Now we seek to understand how organisms features are related to phylogenetic relationships, descent from a common ancestor
• Classifications and relationships change over time, as we learn more details and test hypotheses
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Fig. 11.1 pg 231
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Species Concepts
• How to recognize species– Morphological species
– Biological species
– Evolutionary species
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Morphological Species• Assemblages of individuals with morphological
features in common
• Separable from other assemblages by discontinuities in a number of features
• Numerical (phenetic) taxonomy– Incorporates statistical tools to identify taxonomic
distinctions
– High statistical correlation of large number of characters distinguishes groups
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Biological Species• From Buffon
– A sexually interbreeding (or potentially interbreeding) group, separated from other species by reproductive isolation
• Sibling species– Nearly identical but do not
normally cross-fertilize
• Polytipic species– Multiple geographic subspecies,
can exchange genes if come into contact
• Difficulties– Allopatric populations
– Subjective scale of success for interbreeding
– Cannot test fossils
– Does not apply to plants
– Horizontal gene transmission between different species
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Fig 11.2 pg 233Evolution in the Genus Geum, Evolution 13, 378-388 by W. Gajewski. Reprinted by permission.)
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Evolutionary Species• Evolutionary isolation
• Not dependent upon sexual isolation
• A species concept that incorporates change over time– Stage when group has attained complete
separation subjective
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Phylogeny• Origin of new species
• Phyletic versus Cladogenesis– Phyletic evolution
• Changes within a single lineage
– Phylogenetic branching, Cladogenesis
• Splits and divisions result in multiplication in number of species
• The more a group shares common inherited attributes, the more likely their descent from a common ancestor
Fig 11.4 pg 235
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Phylogeny• Shared attributes can occur by several ways
– Homology• Feature is shared because species derived from
common ancestor
– Homoplasy• Common feature but ancestor was different
• Similar adaptation from different starting points– Parallelism
» Immediate common ancestor was different
– Convergence
» Lineage has been separated for considerable time
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Homology• A phenotypic feature that is shared due to common
ancestry
• It is critical to specify the level at which homology is designated– Genetic > Development > Morphology > Function
– Serial homology
• Similarities among parts of the same organism
• E.g. vertebrae
– Homologous features, may arise by different developmental pathways
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Homology, Partial Homology and Percent Similarity
• Homology as applied to DNA sequence similarity– Orthologous genes
• Genes duplicated in different species
– Paralogous genes• Genes duplicated within a species
• Partial Homology, percent similarity – Early molecular workers comparing two genes might have
discussed partial homology, percent similarity between to genes
– No longer accepted– Homologous or not homologous
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Homology and Constructing Phylogenetic Trees
• Homology is the basis for constructing phylogenetic trees
• Monophyletic groups– Individuals share a
common ancestor
• Polyphyletic groups– Individuals in do all share
a common ancestor– Convergence
Fig 11.8 pg 240
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Phylogenetic Trees• Phylogeny is something that happened
• Classification is an arrangement of its results
• Ideal multi-limbed phylogenetic tree would include all ancestral populations– Impossible
• Most species are unknown (fossils)
• Unable to compute, to complex
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• Taxon (plural taxa)– Unit of classification,
whether species, genus, order,…
• Groups might not always be monophyletic in different classification schemes
Fig 11.9 pg 241
Phylogenetic Trees
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Cladistics, Phylogenetic Systematics
• Willi Hennig (1913-1976)
• Separation of plesiomorphic (ancestral) characters from apomorphic (derived) characters
• Monophyletic groups– Defined by synapomorphic
(shared derived) characters
– Evolutionary steps marked by dichotomous branching of sister taxa
• Paraphyletic groups– Groups that includes some
descendents of a common ancestor, but not all
• Parsimony method– Preferred phylogenetic tree
has least number of steps
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Cladistics, Phylogenetic Systematics
Fig 11.12 pg 243