Phylogenetic Trees: Evidence for Relatedness Between Species

What is a Phylogenetic Tree?

• A phylogenetic tree (also known as an evolutionary tree) is a diagrammatic representation illustrating the evolutionary relationships between different species or groups of organisms.
• It depicts the history of descent from a common ancestor.
• These trees are hypotheses, not definitive facts, and are subject to change with new evidence.
• Phylogenetic trees are constructed using various lines of evidence including:
  • Fossil Record: Provides information about extinct species and their chronological appearance.
  • Structural Morphology: Comparison of anatomical features (homologous and analogous structures).
  • Molecular Homology: Comparison of DNA, RNA, and protein sequences.
  • Comparative Genomics: Genome-wide comparisons to identify regions of similarity and difference.

KEY TAKEAWAY: Phylogenetic trees are visual representations of evolutionary relationships, based on available evidence, and are subject to revision.

Components of a Phylogenetic Tree

• Root: Represents the most recent common ancestor (MRCA) of all taxa in the tree.
• Branches: Lines connecting the nodes and tips, representing evolutionary lineages changing over time. Branch length can sometimes indicate the amount of evolutionary change or time.
• Nodes: Points where branches split, representing a speciation event (the divergence of two or more lineages from a common ancestor). Also called a furcation.
• Taxa (Tips): Represent the groups of organisms being studied. These can be species, populations, genes, etc.
• Clade: A group of organisms consisting of a common ancestor and all of its descendants. Also known as a monophyletic group.
• Sister Taxa: Two taxa that are each other’s closest relatives.

REMEMBER: Root = MRCA, Nodes = Speciation, Tips = Current Organisms

Interpreting Phylogenetic Trees

• Reading the Tree: Time typically flows from the root to the tips. Organisms at the tips are present-day species.
• Relatedness: Organisms are more closely related if they share a more recent common ancestor.
• Rotation at Nodes: The order of taxa along the tips does not necessarily indicate relatedness. The branching pattern is what matters. Trees can be rotated at the nodes without changing the evolutionary relationships.
• Monophyletic, Paraphyletic, and Polyphyletic Groups:
  • Monophyletic: A valid clade; includes a common ancestor and all its descendants.
  • Paraphyletic: Contains a common ancestor and some, but not all, of its descendants. (e.g., reptiles traditionally excluding birds). These are considered invalid groupings.
  • Polyphyletic: Groups taxa from multiple evolutionary origins without including a common ancestor. These are considered invalid groupings.
• Outgroup: A distantly related group used as a reference point when constructing a phylogenetic tree. The outgroup helps to determine the root of the tree.

EXAM TIP: When interpreting phylogenetic trees, focus on identifying the most recent common ancestor to determine relatedness.

Using Phylogenetic Trees as Evidence of Relatedness

• Evolutionary History: Phylogenetic trees provide a visual representation of the evolutionary history of a group of organisms.
• Testing Hypotheses: Trees can be used to test hypotheses about evolutionary relationships. For example, comparing a phylogeny based on morphological data to one based on molecular data can reveal inconsistencies and suggest areas for further research.
• Classification: Phylogenetic trees inform taxonomic classification. Modern classification aims to reflect evolutionary relationships.
• Conservation: Understanding the evolutionary relationships between endangered species can help prioritize conservation efforts.
• Disease Tracking: Phylogenetic analysis can be used to trace the origins and spread of infectious diseases, such as viruses (e.g., tracking the evolution of influenza strains or COVID-19 variants).
• Predictive Power: Phylogenetic relationships can be used to predict the characteristics of poorly studied organisms based on their relationship to well-studied ones.

APPLICATION: Phylogenetic analysis of viral genomes is crucial for tracking the emergence and spread of new variants, informing public health strategies.

Molecular Clocks

• A molecular clock uses the mutation rate of biomolecules (DNA, RNA, proteins) to estimate the time since two species diverged.
• The assumption is that mutations accumulate at a relatively constant rate.
• By calibrating the molecular clock with fossil data or known geological events, we can estimate divergence times for lineages with no fossil record.
• Different genes or regions of the genome evolve at different rates, so it is important to choose the appropriate molecule for the timescale being investigated.
• Molecular clocks are not perfect and can be affected by factors such as:
  • Generation time
  • Selection pressure
  • Mutation rate variation

COMMON MISTAKE: Assuming that molecular clocks are perfectly accurate. They provide estimates, not absolute dates.

Limitations and Considerations

• Incomplete Data: Phylogenetic trees are based on available data, which is often incomplete. New fossil discoveries or molecular data can change the tree.
• Horizontal Gene Transfer: The transfer of genetic material between organisms that are not parent and offspring (common in bacteria) can complicate phylogenetic analysis, as it creates “web-like” relationships rather than strictly branching ones.
• Convergent Evolution: Analogous structures (structures with similar function but different evolutionary origin) can lead to inaccurate inferences about relatedness if not carefully considered.
• Subjectivity: The methods used to construct phylogenetic trees can influence the resulting tree. Different algorithms or data sets may produce slightly different trees.

VCAA FOCUS: VCAA often asks about how new evidence can lead to revisions in phylogenetic trees and our understanding of evolutionary relationships.