Systematics and Phylogeny

Chapter 25

BCOR 012

March 19 and 21, 2007

I. The Fossil Record and Geologic Time

The fossil record is written in sedimentary rocksGeologic timetableRadiometric dating permits the absolute dating of fossils. Plate tectonics When the fossil and molecular evidence do not agree

II. Systematics: Connecting Classification to Phylogeny

Systematics, Taxonomy, and PhylogenyConstructing cladograms

Phylogenetic Classification Molecular Systematics Parsimony

Two kinds of knowledge inform our

study of evolutionary relationship:

• Direct inference: the fossil record

• Indirect inference: phylogeny reconstruction

The Fossil Record and Geologic Time

Sedimentary rocks contain a sequence of fossils that reveal much of thehistory of life on earth.

SEA CLIFF WITH STRATA AT JOGGINS, NOVA SCOTIA

WILLIAM SMITH (1769-1839)

and his map.

William Smith’s map - inferring a vertical sequence upward from older to younger rocks.

Jurassic beds near Lyme Regis on the south coast of Dorset, England, and ammonites from one of the beds. While studying these beds and others about 1800, William Smith developed the concept of "guide fossils."

Georges Cuvier (1769-1832) and Alexandre Brongniart (1770-1847) early recognized the phenomena of restriction of distinctive fossils to particular zones, formations or series — guide fossils — and applied this tool in their stratigraphical studies. Moreover, they observed a pattern or trend in the change from level to level. Of the shells found in the upper, more recent levels, he states that the "eye of the most expert naturalist cannot distinguish from those which at present inhabit the ocean." Forms of life recovered from successively more ancient strata were observed to become progressively more strange and "peculiar" (Cuvier 1817:13, 108-109).

The Geological Time Scale

The fossil record chroniclesthe relative age of fossilgroups.

Radiometric dating permits the absolute dating of fossils.

Systematics and Phylogeny

Systematics is the study of biological diversity in an evolutionary context. It includes (among other aspects) taxonomy, classification, and phylogeny.

Taxonomy is the branch of systematics concerned with naming and classification.

•Scientific names are binomials •Example: Acer saccharum

- Acer is the genus name - it is a Latin noun

- saccharum is the specific epithet - it is a Latin adjective

Biological classifications are hierarchical: each taxonomic group is nested within a more inclusive higher order group.

(Note that only the genus name and specific epithet are italicized.)

“Our classificationswill come to be, asfar as they can beso made, genealogies.”

- Charles Darwin, 1859

Phylogeny is the study of evolutionary relationships among organisms.

Willi Hennig, 1913 - 1976

The Father of PhylogeneticSystematics

A cladogram summarizesinformation aboutancestor-descendentrelationships.

Branch points represent inferredcommon ancestor. on the diagram,the red star denotes the common ancestor of leopard and turtle, whilethe yellow star indicates the commonancestor of tuna, salamander, turtle,and leopard.

A monophyletic group (also called a clade) includes an ancestor and all of its descendents. (Note that clades nest within larger clades, as, say, species nest within genera.)

How to construct a cladogram:

• Choose a study group• Choose an appropriate outgroup• Compile data matrix• Polarize characters using the outgroup• Use shared, derived characters to associate

study group taxa and construct the cladogram

The outgroup is the group used to polarize characterstates in the study group. It should be the group most closely related (on the basis of other lines ofevidence) to the study group that is not actually part of the study group.

The nested relationship of cladesclades is reflected in the nested relationship of taxataxa in the resultant classification.

Convergent evolution ofStem succulence in Euphorbiaceae and Cactaceae

Incorrect interpretation of the four-chambered heart as a homology would

lead to incorrect assessment of the relationship of birds and mammals.

It it critical to distinguish homology from analogy in phylogenyreconstruction

The four-chambered heart evolved at least twice in vertebrate history.

G

In molecular phylogenetic studies, individual nucleotidepositions are the characters, while the particularnucleotide occurring at that position is the characterstate.

Molecular Phylogenetics

Using parsimony analysis to choose among competing cladograms.

5

5 5 55

A B C

Under the principle of parsimony, tree A would bepreferred over B and C as it is one or two stepsshorter.

A cladogram is an evolutionary hypothesis. It can be revised as new evidence becomes available.

A molecular clock canbe used to estimatethe time that HIV firstjumped from non- human primates to man.

Sequence

Differences

%

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

It’s rather remarkable,wouldn’t you say?

Molecular systematicsis shedding new lighton the evolution of life onearth.

EXTRAS

The Situation in the Late Precambrian

Precambrian origin of integrated organisms and basic body plans

Parazoa (Porifera) spongesRadiata (Cnidaria) sea anemones

and sea pensBilateralia

Protostomesprimitive

molluscs unidentified

worms

Charnia masoni - a sea pen Dickinsonia costata - annelid?

Glossopteris flora

The geographic distribution of particular fossils is explainablein terms of continental drift.

The landmasses ofearth drift about thesurface of the globe,floating on the hot, underlying mantle.

Glossopteris leaf fossilPermian periodca. 260 mybp

At present, Glossopteris fossils are found in :

• South America• Antarctica• Africa• India• Australia

How do we account for this disjunct distribution pattern?

Distribution of Late Paleozoic Fossils in the Southern Landmasses

Lystrosaurus, a mammal-like reptile

Pangaea, the late Paleozoic supercontinent

The distinctive biota of Australia is understandablein terms of the long isolationof that continent from othersouthern landmasses.

The marsupial wolf,extinct in Australia since the 1950s

Sometimes fossiland molecularevidence do notagree.