Chapter 20: Phylogeny

Introduction to Phylogeny

Phylogeny is the study of the evolutionary history and relationships among species or groups of organisms. It helps biologists understand how different organisms are related through common ancestry and how traits have evolved over time.

• Phylogeny: The evolutionary history of a species or group of related species.

• Systematics: The scientific discipline focused on classifying organisms and determining their evolutionary relationships.

• Phylogenetic tree: A branching diagram that represents the evolutionary relationships among organisms.

Limbless Lizards and Convergent Evolution

Some organisms, such as snakes and limbless lizards, appear similar but have evolved independently. This is an example of convergent evolution, where unrelated groups develop similar traits due to adaptation to similar environments.

• Limbless lizards have evolved multiple times in different lineages, not all are closely related to snakes.

• For example, the European glass lizard lacks limbs but does not share all traits with snakes.

• Convergent evolution leads to analogous structures, which are similar due to function, not common ancestry.

Phylogenetic Trees: Structure and Interpretation

Phylogenetic trees are visual representations of evolutionary relationships. They can be drawn in various forms but always show patterns of descent.

• Branch point (node): Represents the divergence of two evolutionary lineages from a common ancestor.

• Root: The most recent common ancestor of all taxa in the tree.

• Terminal taxa: The species or groups at the tips of the branches.

• Phylogenetic trees can be rooted (showing a common ancestor) or unrooted (not showing the direction of ancestry).

Cladograms and Phylograms

Different types of phylogenetic trees are used to represent evolutionary relationships.

• Cladogram: Displays the pattern of evolutionary relationships (topology) but does not indicate the amount of evolutionary change.

• Phylogram: Shows both the relationships and the amount of evolutionary divergence, with branch lengths proportional to genetic change.

Homology vs. Analogy

Biologists must distinguish between similarities due to shared ancestry (homology) and those due to convergent evolution (analogy).

• Homology: Similarity due to shared ancestry (e.g., vertebrate forelimbs).

• Analogy: Similarity due to convergent evolution (e.g., wings of birds and insects).

• Homologous structures are used to infer phylogenetic relationships.

Monophyletic, Paraphyletic, and Polyphyletic Groups

Classification of groups in phylogenetic trees depends on their ancestry and descendants.

Ancestral and Derived Characters

Characters used in phylogenetic analysis can be ancestral or derived, depending on the context.

• Ancestral character: A trait inherited from the common ancestor of a group.

• Derived character: An evolutionary novelty unique to a particular clade.

• Example: The backbone is a derived character for vertebrates but ancestral for mammals.

• Loss of a trait (e.g., limbs in snakes) can also be a derived character.

Using Derived Characters to Infer Phylogeny

Shared derived characters help biologists construct phylogenetic trees and infer evolutionary relationships.

• Groups are compared using a character table, where presence (1) or absence (0) of traits is recorded.

• An outgroup (closely related but not part of the group being studied) helps determine which traits are ancestral.

Example: The character table above can be used to infer a phylogenetic tree showing the evolutionary relationships among these taxa.

Branch Lengths in Phylogenetic Trees

Branch lengths can represent the amount of genetic change or the passage of time.

• Longer branches indicate more genetic divergence or a longer time since divergence.

• Some trees use branch lengths to show evolutionary time, while others show only relationships.

Principle of Parsimony

When constructing phylogenetic trees, biologists use the principle of parsimony to choose the simplest explanation that fits the data.

• Parsimony: The tree that requires the fewest evolutionary changes is preferred.

• For DNA data, the most parsimonious tree has the fewest base changes.

Phylogenetic Trees from DNA Sequences

Molecular data, such as DNA sequences, are used to infer evolutionary relationships among organisms.

• Closely related organisms share more similar DNA sequences.

• DNA sequence alignment helps identify homologous regions and infer phylogenies.

• Example: DNA analysis can identify the species origin of whale meat, aiding conservation efforts.

Summary Table: Key Terms in Phylogeny

Key Equations and Concepts

• Molecular clock: Estimates time of evolutionary divergence based on genetic mutations.

• DNA sequence alignment: Used to compare homologous regions between species.

Additional info: Some context and definitions were expanded for clarity and completeness.