Tree Thinking and Phylogenetic Concepts in General Biology

Tree Thinking in Biology

Introduction to Phylogenies

Phylogenies are diagrams that represent the evolutionary relationships among species. They focus on extant species (those currently living) and help avoid common misconceptions about ancestry and evolutionary progress.

• Phylogeny: A diagram (tree) showing evolutionary relationships.

• Extant species: Species that are alive today.

• Misconceptions: Phylogenies do not show which species are 'older', 'more advanced', or 'primitive'.

Understanding Relationships on Trees

Phylogenetic trees illustrate how closely related different species (or individuals) are, based on their most recent common ancestor.

• Closeness: Two species are more closely related if they share a more recent common ancestor.

• Example: Your first cousin is more closely related to you than your second cousin, because you share a grandparent with your first cousin and a great-grandparent with your second cousin.

Example Tree: Human Family Relationships

• You and Sibling share parents (most recent common ancestor).

• First Cousin shares a grandparent with you.

• Second Cousin shares a great-grandparent with you.

Determining Relatedness

Being 'next to' on a tree does not necessarily mean closely related. The key is the node (branch point) representing the most recent common ancestor.

• Example: Species C is more closely related to A than to D if the C-A ancestor is closer to the present than the C-D ancestor.

Types of Phylogenetic Trees

Tree Formats

Phylogenetic trees can be drawn in different styles, but the relationships they show remain the same.

• Rectangular trees: Branches are drawn horizontally and vertically.

• Slanted trees: Branches are drawn at angles.

• Scaled trees: Branch lengths can represent genetic change or time.

Table: Types of Phylogenetic Trees

Type	Branch Lengths	Purpose
Cladogram	Unscaled	Shows relationships only
Phylogram	Scaled to genetic change	Shows amount of genetic difference
Chronogram	Scaled to time	Shows timing of divergence

Interpreting Phylogenetic Trees

Common Misconceptions

Phylogenies do not indicate which extant species are older, more advanced, or more primitive. All extant species are equally evolved and have been evolving for the same amount of time since their last common ancestor.

• No species is 'older' or 'younger' than another extant species.

• No species is 'more advanced' or 'primitive'.

• No extant species is ancestral to another extant species.

Application: Fungi, Humans, and Bacteria

Phylogenetic trees can be used to determine which species are more closely related. For example, fungi are more closely related to humans than to bacteria because fungi and humans share a more recent common ancestor (~800 million years ago) compared to the ancestor shared with bacteria (~3 billion years ago).

• Example: Fungi and humans share a common ancestor more recently than fungi and bacteria.

Phylogenetic Classification

Monophyletic, Paraphyletic, and Polyphyletic Groups

Groups in phylogenetics are defined by their ancestry.

• Monophyletic group (clade): Includes a common ancestor and all its descendants.

• Paraphyletic group: Includes a common ancestor and some, but not all, descendants.

• Polyphyletic group: Includes species with different ancestors, grouped by similarity rather than ancestry.

Table: Phylogenetic Group Types

Group Type	Definition	Example
Monophyletic	All descendants of a common ancestor	Mammals
Paraphyletic	Some descendants of a common ancestor	Scaly reptiles (excluding birds)
Polyphyletic	Species grouped by similarity, not ancestry	Flying animals (bats & birds)

Ancestral and Derived Traits

Ancestral State Reconstruction

Traits can be classified as ancestral (present in the common ancestor) or derived (evolved more recently). Parsimony is used to infer the simplest evolutionary scenario.

• Ancestral trait: Trait present at the deepest node (oldest trait).

• Derived trait: Trait present at the shallowest node (most recent trait).

• Parsimony: The simplest explanation (fewest changes) is preferred.

Example: If a DNA base pair is 'C' in gorilla and orangutan, but 'T' in human and chimp, the ancestral state is likely 'C', with a single change to 'T' in the human-chimp lineage.

Evolutionary Success and Species Age

Extant Species and Lineages

All extant species are equally 'successful' in evolutionary terms, as they have survived to the present. The concept of 'new species' refers to newly described species, not newly evolved lineages.

• Extant species: Present-day species, none are older or younger than others.

• New species: Newly named or discovered, not newly evolved.

Analogies: Tree of Life and Universe

No Center or Pinnacle

Just as the Earth is not the center of the universe, no species is at the 'center' or 'top' of the tree of life. Evolution has no goal or endpoint.

• No main branch or trunk: All branches end in extant species.

• No endpoint: Homo sapiens is not the pinnacle of evolution.

Summary Table: What Phylogenies Can and Cannot Tell Us

Can Tell	Cannot Tell
Which species share a recent common ancestor	Which extant species are older or younger
Which traits are ancestral or derived	Which species are more advanced or primitive
When lineages diverged	Which extant species are ancestral to others

Key Equations and Concepts

• Genetic Change: Branch lengths in phylograms can represent percent genetic change.

• Time: Branch lengths in chronograms can represent millions of years since divergence.

• Parsimony Principle: $\text{Best tree} = \text{Tree with fewest evolutionary changes}$

Conclusion

Tree thinking is essential for understanding evolutionary relationships. Phylogenetic trees help clarify which species are closely related, the nature of evolutionary change, and the classification of life, while avoiding misconceptions about progress, age, and complexity.