Unit 5: Phylogenetic Trees
1. Concept of Phylogeny and Phylogenetic Trees
Phylogeny is the evolutionary history of a species or group of related species. It is represented graphically through phylogenetic trees.
Phylogenetic Trees
A diagrammatic representation of the evolutionary relationships among various biological species based on similarities and differences in their physical or genetic characteristics.
- Root: The common ancestor of all organisms in the tree.
- Node: Represents a branching point where a lineage splits into two (speciation).
- Taxon: A group of organisms at the tip of the branches.
- Sister Taxa: Groups that share an immediate common ancestor.
2. Construction of Trees and Interpretation
Building a phylogenetic tree involves analyzing characters (morphological or molecular) to determine relationships.
Key Principles
- Homology vs. Analogy: Trees are built using homologous structures (derived from a common ancestor) rather than analogous ones (derived from convergent evolution).
- Parsimony: The principle of choosing the simplest explanation that fits the data (the tree with the fewest evolutionary changes).
- Dating: Trees can be calibrated using the fossil record or molecular clocks (the constant rate of mutation in certain genes).
3. Cladistics: Cladograms and Types of Groups
Cladistics is an approach to biological classification in which organisms are categorized based on shared derived characteristics (synapomorphies).
The Cladogram
A type of phylogenetic tree that only shows the branching order (patterns of shared ancestry) but does not represent time or the amount of change.
Types of Evolutionary Groups
4. Molecular Evolution and Neutral Theory
Evolution can also be studied at the level of DNA and protein sequences.
Neutral Theory of Molecular Evolution
Proposed by Motoo Kimura. It suggests that most evolutionary changes at the molecular level are caused by genetic drift of neutral mutations rather than by natural selection.
- Most mutations do not affect an organism's fitness (they are neither "good" nor "bad").
- These mutations accumulate at a relatively constant rate, allowing them to be used as a "molecular clock" to estimate when two species diverged.
5. Exam Focus Enhancements
Exam Tips
- Monophyletic Rule: In cladistics, only monophyletic groups are considered valid taxonomic units.
- Cladogram vs. Phylogram: Remember that in a cladogram, branch lengths are arbitrary; in a phylogram, branch lengths represent genetic change or time.
- Synapomorphy: This is a "shared derived trait." It is the most important type of character used to define a clade.
Frequently Asked Questions
- Define phylogeny. What are the components of a phylogenetic tree?
- Differentiate between monophyletic, paraphyletic, and polyphyletic groups with diagrams.
- Explain the Neutral Theory of Molecular Evolution as proposed by Kimura.
- What is the significance of the principle of parsimony in tree construction?
Common Mistakes
- Assuming that organisms at the tips of a tree evolved from one another. Instead, they share a common ancestor.
- Confusing Plesiomorphy (primitive trait) with Apomorphy (derived trait). Only derived traits help in grouping organisms.