Notes

• Our web site is www.life.umd/classroom/zool210/jensen
• Lectures will be put on the web in a timely manner.
• Reading for Lecture 1 should be pages 4 - 33 in your textbook.
• Reading for Lecture 2 should be pages 53 - 62 in your textbook.

Phylogeny, Systematics, and
"Tree Thinking"

What is a classification?

• A means for organizing information about Life's diversity
• Scala Naturae - Aristotle - grouped organisms by structural similarities
• Carolus Linnaeus, Swedish botanist (1700s) - Systema Naturae - first modern classification
  • Binomial nomenclature
  • Groups within groups, a nested hierarchy based on structural similarities
  • Did not have idea of common descent

Darwin's Insight

• After the Renaiisance, pre-Darwinian biologists, such as Jean Baptiste de Lamarck (late 1700s), thought there may have been multiple independent origins of life, each of which changed over time without much branching
• Darwin (mid-1800s) proposed the key ideas of common descent and species multiplication
• The major evidence for common descent was HOMOLOGOUS traits
• New (derived) traits that arise become homologous in all subsequent descendants (common ancestry even though they may become modified over time as new species form), giving rise to nested hierarchies of lineages

How can we reconstruct history in order to understand ancestral relationships?

• Analysis of fossils
  • Body fossils
  • Trace fossils

How can we reconstruct history in order to understand ancestral relationships?

• Analysis of fossils
  • Body fossils
  • Trace fossils
• Phylogeny reconstruction
  • If we know how animals are related, we can reconstruct their evolution

What is a phylogeny?

• The branches and connections on Darwin's tree of life.
• Produced by two evolutionary events

What is a phylogeny?

• The branches and connections on Darwin's tree of life.
• Produced by two evolutionary events
  • Cladogenesis - splitting of lineages, or species multiplication
  • Anagenesis - acquisition of new characteristics within a lineage over time

How are these animals related?

What is a cladogram?

• A branching diagram depicting an estimate of the phylogeny.

Reading cladograms

• Read vertically, not horizontally!
• These three cladograms are the same!

How do we recognize
monophyletic groups?

• Overall similarity?
• Two major problems
  • Convergence
  • Unequal rates of evolution

Convergent Evolution

• Polyphyletic groups: Independent origin of characteristics that are similar in 2 different groups of organisms
• Analogous (not homologous) characters

Unequal rates of evolution

• Some groups look similar due to recency of shared ancestry - Monophyletic
• Some groups look similar due to retention of primitive similarity - Paraphyletic

Review of terms

• Monophyletic groups (e.g. mammals)
  • Ancestor and all descendants
  • United by derived similarity
• Polyphyletic groups (e.g. birds+bats)
  • Don't share a unique common ancestor
  • United by convergent similarity
• Paraphyletic groups (e.g. different groups of "reptiles")
  • Don't share a unique common ancestor
  • United by primitive similarity

Approaches to Classification

• 1. Traditional evolutionary taxonomy
• George Gaylord Simpson (1902-1984)
  • Branch may be a higher taxon if it represents a distinct adaptive zone, or "grade" of organization
  • Groups may be either monophyletic or paraphyletic
  • Eliminates polyphyletic groups

More Recent Approaches
to Classification

• 2. Phenetics
  • Don't bother trying to reconstruct phylogeny -- it is too subjective
  • Overall similarity only; lump all characters you can find and average them without subjective judgments that some are more important than others
  • Not in vogue now, we now have more and better tools to assess relatedness
• 3. Cladistics - phylogenetic systematics
  • Monophyletic groups only - clades
  • All groups in classification must be monophyletic
  • Only derived similarity should be used in classification
  • Primitive similarity and convergent similarity are uninformative

Cladistic Methods

• Willi Hennig (1913-1976)
• Two principle techniques for reconstructing phylogeny
  • Outgroup analysis
  • Parsimony

Outgroup Analysis
Distinguishing derived from primitive similarity

• Outgroups allow us to identify sister taxa
• Sister taxa share more recent common ancestors with each other than either one does with any other taxon
  • birds and the different groups of reptiles are sister groups, while amphibians are the sister group to birds + "reptiles"

Parsimony

• The cladogram requiring the fewest evolutionary changes is (usually) preferred
• Why? The simplest explanation for the distribution of characters
• Not necessarily the true phylogeny
• A reasonable and explicit goal

What have we learned?

• Phylogeny vs. Cladogram
• Derived vs. Convergent vs. Primitive similarity
• Monophyly vs. Polyphyly vs. Paraphyly
• Three approaches to classification
• Cladistic classification - Monophyly only
• Monophyletic groups recognized using
  • Outgroup analysis
  • Parsimony

Why is this useful?

• Understanding phylogeny and classification helps you:
  • Organize and summarize material
  • Interpret the evolutionary stories of this course
  • Contend with the conflicting terminology in your book (and elsewhere) (e.g. "Synapsids")
  • Comprehend the process of diversification and extinction over geological time -- essential information for conserving and managing biodiversity for future generations
• In combination, this allows you to pose specific, testable hypotheses about evolution - "Tree thinking"
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