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Big Idea 1

Ch. 25 - Phylogeny

Overview: Investigating the Tree of Life

• Phylogeny is the evolutionary history of a species or group of related species

• To get information about ancient organisms and their relationship to current species, biologists draw on the fossil record

• Systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time.– used to understand the

evolutionary history of life on Earth

• Systematists use morphological, biochemical, and molecular comparisons to infer evolutionary relationships (they also examine fossils)

The Fossil Record

• Sedimentary rocks are the richest source of fossils– Soft– Haven’t been heated or

melted like igneous and metamorphic

• Sedimentary rocks are deposited into layers called strata

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Rivers carry sediment to the ocean. Sedimentary rock layers containing fossils form on the ocean floor.

Over time, new strata are deposited, containing fossils from each time period.

As sea levels change and the seafloor is pushed upward, sedimentary rocks are exposed. Erosion reveals strata and fossils.

Younger stratumwith more recentfossils

Older stratum witholder fossils

• The fossil record is based on the sequence in which fossils have accumulated in such strata

• Fossils reveal ancestral characteristics that may have been lost over time

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Dinosaur bones beingexcavated from sandstone

Casts of ammonites, about375 million years old

Boy standing in a 150-million-year-olddinosaur track in Colorado

Tusks of a 23,000-year-old mammoth, frozen wholein Siberian ice

Petrified trees in Arizona, about 190million years old

Insects preserved whole in amber

Leaf fossil, about 40 millionyears ago

Morphological and Molecular Homologies

• In addition to fossils, phylogenetic history can be inferred from morphological and molecular similarities in living organisms

• Organisms with very similar morphologies or similar DNA sequences are likely to be more closely related than organisms with vastly different structures or sequences

Sorting Homology from Analogy

• In constructing a phylogeny, systematists need to distinguish whether a similarity is the result of homology or analogy

• Homology is similarity due to shared ancestry• Analogy is similarity due to convergent evolution (the

evolution of similar traits in unrelated lineages)– Ex – opossums have opposable thumbs like primates, even though

they aren’t related

• Convergent evolution occurs when similar environmental pressures and natural selection produce similar (analogous) adaptations in organisms from different evolutionary lineages

Evaluating Molecular Homologies

• Systematists use computer programs and mathematical tools when analyzing comparable DNA segments from different organisms

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Deletion

Insertion

Phylogenetic systematics connects classification with evolutionary history

• Taxonomy is the ordered division of organisms into categories based on characteristics used to assess similarities and differences (putting things into groups based on similarities)

• In 1748, 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

Binomial Nomenclature Cont.• Rules:

– All living things are given a two-part name.• The first part of the name

identifies the genus• The second part of the

name identifies the species

*Both parts together name the species (not the specific epithet alone)

– The genus is ALWAYS capitalized and the species is ALWAYS lowercase

– Both words are ALWAYS either italicized or underlined

– Latin or Greek

Hierarchical Classification

• Linnaeus introduced a system for grouping species in increasingly broad categories

• *the more levels shared, the more organisms will have in common and the more closely related they will be

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SpeciesPantherapardus

PantheraGenus

FamilyFelidae

CarnivoraOrder

MammaliaClass

PhylumChordata

KingdomAnimalia

EukaryaDomain

Linking Classification and Phylogeny

• Systematists depict evolutionary relationships in branching phylogenetic trees

• Each branch point represents the divergence of two species

• “Deeper” branch points represent progressively greater amounts of divergence

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Carnivora

Pantherapardus

(leopard)

Mephitismephitis

(striped skunk)

Lutra lutra(European

otter)

Canisfamiliaris

(domestic dog)

Canislupus(wolf)Sp

Felidae Mustelidae Canidae

Panthera Mephitis Lutra Canis

Phylogenetic systematics informs the construction of phylogenetic trees based on shared characteristics

• A cladogram depicts patterns of shared characteristics among taxa– Takes characteristics of certain

organisms and putting together a hyposthesis of how you think they may be related

• A clade is a group of species that includes an ancestral species and all its descendants– Clades can be nested in larger

clades, but not all groupings or organisms qualify as clades

• Cladistics studies resemblances among clades

• Primative to derived characteristics

• Types of clades:– monophyletic, signifying

that it consists of the ancestor species and all its descendants

– A paraphyletic grouping consists of an ancestral species and some, but not all, of the descendants

– A polyphyletic grouping consists of various species that lack a common ancestor

Shared Primitive and Shared Derived Characteristics

• In cladistic analysis, clades are defined by their evolutionary novelties

• A shared primitive character is a character that is shared beyond the taxon we are trying to define

• A shared derived character is an evolutionary novelty unique to a particular clade– derived is when a trait shows up that was

NOT present in a common ancestor. ancestral/primitive characters WERE present in a common ancestor.

– E.G. Among primates, the relatively shortened and straight spine of a gorilla is a derived trait (it was not present in the common ancestor of all primates).

However, the shortened spine of a gorilla is ancestral to apes (it is present in all apes and in the common ancestor of apes).

Outgroups• An outgroup is a species or group of

species that is closely related to the ingroup, the various species being studied– “one of these things is not like the other”

• Systematists compare each ingroup species with the outgroup to differentiate between shared derived and shared primitive characteristics

• Outgroup comparison assumes that homologies shared by the outgroup and ingroup must be primitive characters that predate the divergence of both groups from a common ancestor

• It enables us to focus on characters derived at various branch points in the evolution of a clade

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Amniotic (shelled) egg

Four walking legs

Hinged jaws

Vertebral column(backbone)

Character table

Turtle Leopard

Amniotic egg

Four walking legs

Hinged jaws

Vertebral column

Salamander

Lamprey

Lancelet (outgroup)

Cladogram

Phylograms

• A phylogram is a phylogenetic tree that indicates the amount of evolution in addition to the branching order.– the length of a branch in a

cladogram reflects the number of genetic changes that have taken place in a particular DNA or RNA sequence in that lineage

Phylogenetic Trees as Hypotheses

• The best hypotheses for phylogenetic trees fit the most data: morphological, molecular, and fossil

• Sometimes the best hypothesis is not the most parsimonious (simplest, fewest changes)

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Lizard Bird Mammal

Four-chamberedheart

Mammal-bird clade

Lizard Bird Mammal

Four-chamberedheart

Lizard-bird clade

The Universal Tree of Life

• The tree of life is divided into three great clades called domains: Bacteria, Archaea, and Eukarya

• The early history of these domains is not yet clear

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