Overview
This lecture explains how to construct phylogenetic trees and cladograms using different types of data, focusing on interpreting character tables and molecular data to determine evolutionary relationships.
Data Types for Phylogenetic Trees
- Morphological data refers to physical characteristics and structures, such as body shape or traits found in fossils.
- Biochemical data involves metabolic pathways present or absent in organisms.
- Molecular data includes DNA sequences or amino acid sequences, which provide the most accurate evolutionary evidence.
- Behavioral data can also inform tree construction.
- Morphological data may be misleading due to convergent evolution, where unrelated species evolve similar traits.
Building a Character Table
- Character tables list species and their traits, using "+" for present and "−" for absent.
- Shared traits indicate shared ancestry; unique traits differentiate species.
- Traits can be gained or lost over evolutionary time (e.g., loss of limbs in snakes).
Constructing a Cladogram
- Shared ancestral traits appear at the base of the tree; unique traits mark divergences.
- Species lacking derived traits are likely outgroups, diverging earlier than others.
- Shared derived traits (highlighted) help infer common ancestry and relationships.
Outgroup and Ingroup Concepts
- The outgroup is the least related species, used as a reference point for rooting the tree.
- The ingroup contains species with shared derived characteristics and closer evolutionary relationships.
Using Molecular Data
- Tables of amino acid or nucleotide differences between species quantify evolutionary distances.
- Fewer differences mean a more recent common ancestor; more differences indicate greater evolutionary distance.
- Molecular clocks estimate how long species have been separated.
Drawing and Interpreting Trees from Data
- Start tree construction with species pairs sharing the smallest differences (closest related).
- Identify outgroups as those with the largest differences.
- Branch points (nodes) can be rearranged without changing tree meaning, as long as relationships are preserved.
- Using molecular clocks helps calibrate tree timelines.
Key Terms & Definitions
- Phylogenetic tree — Diagram showing evolutionary relationships among species.
- Cladogram — Type of phylogenetic tree focusing on shared derived characteristics.
- Morphological data — Physical traits used for classification.
- Molecular data — DNA or amino acid sequences used to determine relationships.
- Outgroup — Species that diverged earliest, used as a comparison baseline.
- Ingroup — Group of species with closer evolutionary relationships.
- Convergent evolution — Unrelated species evolve similar traits independently.
- Molecular clock — Concept that mutation rates are relatively constant, used to estimate divergence times.
Action Items / Next Steps
- Practice drawing phylogenetic trees using character tables and molecular data tables.