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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