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Topics for February 20

Phylogeny and classification

Macroevolution

Fossils: What are they? What can we learn from fossils?

Group report #4: Constructing a phylogenetic tree for some familiar species.

Phylogeny: The evolutionary history of a group of species.

The classification of living species are based on evidence of common ancestry. The evidence is shared inherited traits.

Important caveat: similarities based on common ancestry (homologies) must be separated from similarities based on evolutionary convergence (analogies).

Similarities due to evolutionary convergence: analogy

Similar traits which have a separate evolutionary origin are called analogies.

Example: streamline form of shark, penguin, and porpoise (p255).

Homologies: traits with a common origin.

Homologies exist because of derivation from a common ancestor.

Example (p254): The array of bones in the forelimb of quadrapeds: humerus, radius, ulna, carpels, metacarpels, phalanges. (Notice that in this example the homologous structures no longer serve precisely the same function.)

How can we distinguish analogy from homology?

It’s not always easy, and some classifications have been undone by more complete information.

Clues: Development: homologies follow a common developmental pathway. Relationship among traits: the relationship among the bones of the forelimb remain the same. Neutral features: non-adaptive features are more reliable clues!

The basic rules for constructing phylogenies: shared traits.

Individual taxa (species, genera, families, etc.) are assigned to:

The same group if they share a homologous trait,

To separate groups if the traits are not shared.

Groups: large and small

Members of a large group may share an ancestral trait: e.g. mammals, reptiles, fish, birds share a conspicuous feature (vertebral column).

A smaller group is identified by a derived trait not shared by the large group. e.g. mammals are separated from other vertebrates based on milk for their young.

Ancestral traits and derived traits.

Ancestral traits are shared throughout the larger group.

Derived traits are present only in a smaller group. The smaller group is defined and identified by having the derived trait. The derived trait is a feature which was present in the ancestor of the members of the smaller group.

Construction of phylogenetic trees.

A phylogenetic tree is constructed based on the patterns of ancestral and derived traits.

The various branches are based on having or not having a particular trait or group of traits. (Derived traits are most useful!)

Derived traits are evidence of a shared evolutionary heritage.

The logic of using ancestral or derived traits for classification.

Ancestral traits already existed in the ancestral group. Such traits indicate affinity with a larger taxonomic unit, but don’t identify a species as part of a smaller group. Example: mammals are all vertebrates, along with many other species.

Derived traits are unique to a group, and identify a species as belonging to the smaller taxonomic unit. Only mammals nurse their young.

Newer data and newer methods reinforce many past decisions.

Molecular biology has introduced many new techniques.

Classification based on prior information (fossils, morphology, behavior, etc.) can be re-examined with molecular data.

Example: Cytochrome C data on page 256.

Molecular methods can help resolve old controversies.

Example: Are Pandas bears? Or Raccoons?

Yes. (see p257)

Molecular data indicates that Red Pandas are more closely related to raccoons.

Molecular data indicates that Giant Pandas are more closely related to other bears.

The similarities (between the two pandas) are analogies, due to natural selection.

Tracing evolutionary history: macroevolution.

Macroevolution is the study of what has happened over the long span of life on earth.

Important consideration: the duration of life on earth has been very long.

There have been several major extinctions and several major proliferations of various life forms.

Some general considerations.

Geology provides a sedimentary record of life (and geochemistry, etc.).

Sediment accumulates "bottom up": deeper sediments are older.

Old sediments have been uplifted by geological forces, exposing past profiles.

There is no complete sequence anywhere, but the exposed sequences can be overlapped and thus linked

Fossils: what is a fossil?

Fossils are any material left behind which can be used to infer the biology of a once living organism.

Examples include: preserved remains of organisms, mineralized remains of organisms, impressions left by organisms (footprints, leaf-prints), dung or other waste products, food fragments, eggshells, etc.

Do all organisms leave behind fossils?

Most individuals do not leave any trace.

Some types of organisms are much more likely to leave behind good fossils: clams that live in muddy sediments, microscopic aquatic organisms that form ocean and lake sediments, etc. (see p250).

Some types of organisms lived where it was less likely that fossils would be formed: e.g. desert animals and plants.

What can we learn from fossils?

We can observe important aspects of the morphology (shape) of once living organisms. We can then see whether or not a particular structure had developed. Examples: vertebrae, feathers, scales, teeth, etc.

We can compare similarities with still living forms.

What other inferences are possible?

Diet: The structure of teeth, mouth parts, dung, intestinal structure, etc. tell much about the diet.

Behavior: Structure of joints and bones indicate what motions were possible. Upright posture? Flight?

Habitat: Structures reveal habitat: Water? Aquatic organisms are "streamlined", etc.

The Geological Time Scale

The geologic time scale has been developed by studying exposed layers of rocks in many different locations.

Rock layers can be identified from one location to another by their mineralogy or the fossils they contain.

When they were originally formed, the layers were laid down from the bottom up.

The geological eras (p269)

The oldest time period: pre-Cambrian ("older than Cambrian") 4600 million years ago until 590 million years ago.

Paleozoic ("ancient life") 590 million years ago until 248 million years ago.

Mesozoic ("middle life") 248 million years ago until 65 million years ago

Cenozoic ("recent life") 65 million years ago to the present.

How were these intervals selected?

The boundaries between the eras were set because there were major changes seen in the fossil assemblages.

The chronological ages were later added based on radiometric dating.

The major changes seen in the fossil assemblages turned out to be the result of mass extinctions. e.g. 65 million years ago, the Dinosaurs disappeared.

Where did the names come from?

The names were coined by the geologists who did the original detective work.

Example: Silurian and Cambrian were based on the names of ancient tribes who lived in Wales where the geological work on those strata was first done.

The eras are divided into further intervals.

Eras are divided into Periods. The periods were assigned and named in the same fashion.

For the most recent era, the Cenozoic, the periods are further subdivided into Epochs

Continental Drift

Continents have moved about on the surface of the planet.

Current evidence of continental movements: volcanoes and earthquakes

The areas of greatest activity are around the Pacific ocean ( "the ring of fire"), the mid-ocean ridges, the East African rift zone, and other areas.

The past positions of the continents were different.

250 million years ago, all the continents were together in one large continent (Pangaea)

65 million years ago, the continents had broken apart into their present configuration. (India was then an island).

The present day distribution of species reflects the influence of continental drift.

Examples of the influence of continental drift

Marsupials are present on 2 continents: South America and Australia. (Fossil marsupials have been found on Antarctica as well.)

Flightless birds are present on the southern continents: Africa (Ostrich), South America (Rhea), and Australia (Emu)

Group excercise #4: What your result should look like.

Use the data table and your general knowledge about these species to construct a phylogenetic tree.

At each branch point in the diagram, indicate the character or characters used to define the branch point.

The final diagram should consist of clusters of species which are similar at the ends of the branches.

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