Determining Evolutionary Relationships

Core Concepts

Homologous versus analogous structures

Homologous structures are inherited from a common ancestor, even if they now serve different functions — the forelimbs of a human, a bat, and a whale share the same underlying bone pattern because they descend from a common tetrapod ancestor. Analogous structures look or function similarly but evolved independently, because similar environments favor similar solutions. The wings of birds and insects are analogous; so are the streamlined bodies of sharks (fish), dolphins (mammals), and ichthyosaurs (extinct reptiles). This independent evolution of similar traits is convergent evolution. Only homologous traits carry information about ancestry; analogous traits can mislead you into grouping unrelated organisms.

Molecular evidence: DNA, RNA, and proteins

Since the 1960s–70s, molecular comparisons have become the most powerful source of evidence. Genes and their protein products accumulate mutations over time, so the more similar two species' DNA or amino-acid sequences are, the more recently they are inferred to share a common ancestor. Comparing ribosomal RNA (rRNA) sequences famously split prokaryotes into Bacteria and Archaea; mitochondrial DNA comparisons trace recent events such as human migrations. The molecular clock idea — that a given gene accumulates changes at a roughly steady rate — lets researchers estimate when lineages diverged, although rates vary across genes and lineages, so estimates are approximate and need fossil calibration.

Shared ancestral versus shared derived characters

Not every similarity is equally informative. A shared ancestral character (plesiomorphy) is a trait inherited from a distant ancestor and shared by many organisms — hair is ancestral for all mammals, so it tells you little about which mammals are most closely related. A shared derived character (synapomorphy) is a trait that evolved in a particular lineage's common ancestor and is shared only by its descendants — for example, mammary glands or a four-chambered heart. Cladistics builds trees by identifying synapomorphies, because only these mark the branching events that define groups.

Outgroups and parsimony

To recognize which traits are derived, systematists compare the study group (the ingroup) with a closely related species outside it (the outgroup). A character found in both the ingroup and the outgroup is likely ancestral; a character found only in part of the ingroup is likely derived. When multiple trees are possible, maximum parsimony selects the tree requiring the fewest evolutionary changes — the simplest explanation consistent with the data, following Occam's razor. Maximum likelihood (and its Bayesian cousins) instead evaluates the probability of the data given a model of sequence evolution and picks the tree with the highest likelihood. Parsimony is intuitive for small datasets; likelihood methods dominate modern molecular phylogenetics.

Trees are hypotheses

Every phylogenetic tree is a hypothesis about ancestry that can be strengthened or overturned by new data. Different genes can yield slightly different trees, so systematists combine many genes (phylogenomics) for more robust results. When evidence conflicts, the uncertainty is sometimes shown as a polytomy — a node with more than two branches, meaning the order of divergence is unresolved.

How It Works / Step-by-Step Process

1. Collect characters: record morphological traits and/or molecular sequences for the species of interest.
2. Sort homologies from analogies: keep similarities that reflect shared ancestry; set aside convergent ones.
3. Choose an outgroup: add a related species outside the group to identify which characters are ancestral.
4. Score derived characters: mark which species share each synapomorphy; shared novelties cluster species into clades.
5. Build and select the tree: apply parsimony (fewest changes) or likelihood (highest probability).
6. Test and revise: incorporate additional genes or fossils; unresolved nodes may be shown as polytomies.

Common Confusions