Early Earth and the Origin of Life (Ch
NOTES: CH 26 – Phylogeny and the Tree of Life
Phylogeny:
● Systematics: discipline focused on classifying organisms and determining their evolutionary relationships
● The fossil record: , within layers, or strata, of sedimentary rock
● Paleontologists:
● Taxonomy –
● Systematics – naming & classifying organisms
● Phylogenetics – reconstructing the evolutionary relationships among organisms
Binomial Nomenclature
● In the 18th century, Carolus Linnaeus published a system of taxonomy based on resemblances
● Two key features of his system remain useful today: two-part names for species and hierarchical classification
● The two-part scientific name of a species is called a binomial
● The first part of the name is the
● The second part is within the genus
● The first letter of the genus is capitalized, and the entire species name is italicized
● Both parts together name the species
Hierarchical Classification
● The taxonomic groups from broad to narrow are , , , , , , , and
● A taxonomic unit at any level of hierarchy is called a
Linking Classification and Phylogeny – PHYLOGENETIC TREES!
● Systematists depict evolutionary relationships in branching
● Phylogenetic tree – hypothesized genealogy (i.e., the most recent) through hierarchical, dichotomous branching
● A phylogenetic tree represents a hypothesis about evolutionary relationships
● Each branch point represents the
● Sister taxa are groups that
● A rooted tree includes a branch to represent the last common ancestor of all taxa in the tree
● A basal taxon diverges early in the history of a group and originates near the common ancestor of the group
● A polytomy is a branch from which
Constructing Phylogenetic Trees
● Sorting homology vs. analogy...
● Homology:
● Analogy: likenesses attributed to similar ecological roles and natural selection
● Convergent evolution: species from different evolutionary branches that
HOMOLOGIES: Similar characters (e.g., , etc. traits or features) suggest relatedness…
● Homologous characters
● As a general rule, the more homologous characters shared by two species,
● Sequences of DNA & RNA (nucleotides) and proteins (amino acids) are used as characters; as a general rule, the more recently two species shared a common ancestor, the
CONVERGENT EVOLUTION: can produce superficially similar traits that lack homology with one another
● Analogous characters
Evaluating Molecular Homologies:
● Systematists use computer programs and mathematical tools when analyzing comparable DNA segments from different organisms
● Molecular systematics uses and other molecular data (i.e. ) to
Shared characters are used to construct phylogenetic trees
● Once homologous characters have been identified, they can be used to infer a phylogeny
CLADISTICS:
● Cladistics groups organisms by
● A clade is a group of species that includes an
● Clades can be nested in larger clades, but not all groupings of organisms
qualify as clades
● A valid clade is , signifying that it consists of the
ancestor species and all its descendants
● A grouping consists of an ancestral species and some,
but not all, of the descendants
● A grouping consists of various species with different ancestors
Shared Ancestral and Shared Derived Characters
● In comparison with its ancestor, an organism has both and characteristics
Ancestral vs. Derived Characters
● A shared ancestral character is a character that of the taxon
● A shared derived character is (“ ”) to a particular clade
● A character can be both ancestral and derived, depending on the context
● When inferring evolutionary relationships, it is useful to know in which clade a shared derived character first appeared
**The sequence of branching in a cladogram then represents the sequence in which ( ) evolved
Ingroup vs. Outgroup
● An OUTGROUP is a species or group of species that is closely related to the INGROUP, the various species being studied
● The outgroup is a group that has the ingroup
● Systematists compare each ingroup species with the outgroup to differentiate between shared derived and shared ancestral characteristics
● Characters shared by the outgroup and ingroup are that of both groups from a common ancestor
An outgroup helps identify shared ancestral and shared DERIVED CHARACTERS ( )
Phylogenetic Trees with Proportional Branch Lengths
● In some trees, the length of a branch can reflect the that have taken place in a particular DNA sequence in that lineage
● In other trees, branch length can represent , and branching points can be determined from the
Phylogenetic Trees as Hypotheses
● The best hypotheses for phylogenetic trees fit the most data: , , and
● Phylogenetic bracketing allows us to predict features of an ancestor from features of its descendants
-For example, phylogenetic bracketing allows us to infer characteristics of dinosaurs
● Birds and crocodiles share several features: four-chambered hearts, song, nest building, and brooding
● These characteristics likely evolved in a common ancestor and were shared by all of its descendants, including
● The fossil record supports nest building and brooding in dinosaurs
An organism’s evolutionary history is documented in its genome
● Comparing nucleic acids or other molecules to infer relatedness is a valuable approach for tracing organisms’ evolutionary history
● DNA that codes for rRNA and is useful for investigating branching points
● mtDNA evolves rapidly and can be used to explore recent evolutionary events (i.e. )
Molecular clocks help track evolutionary time
● To extend molecular phylogenies beyond the fossil record, we must make an assumption about how change occurs over time
Molecular Clocks
● A molecular clock uses constant rates of evolution in some genes to estimate the absolute time of evolutionary change
● In orthologous genes (genes found in different species; e.g. ), nucleotide substitutions are proportional to the time since they last shared a common ancestor
● In paralogous genes (similar genes found within one species; e.g. ), nucleotide substitutions are proportional to the time since the genes became duplicated
● Molecular clocks are calibrated against branches whose dates are known from the fossil record
● Individual genes vary in how clocklike they are
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