Difficulty: Intermediate | Prerequisites: Basic understanding of evolution by natural selection, DNA, and heredity.
Phylogenetics is the study of evolutionary relationships among organisms. It sits at the intersection of taxonomy (classifying organisms) and evolutionary biology (understanding how species change over time). If you can read a phylogenetic tree, you can trace how any group of organisms is related, when lineages diverged, and which traits are shared because of common ancestry versus independent evolution.
This material builds directly on what you already know about natural selection and genetic variation. The key shift here is from thinking about evolution within a single population to thinking about evolution across many species over deep time. You will need these concepts again when studying speciation, the history of life, and comparative genomics.
Phylogenetic trees are diagrams that map the evolutionary relationships among organisms. Scientists build them by identifying shared derived traits (synapomorphies) and applying the principle of parsimony to choose the simplest explanation. The main pitfall is confusing traits that look alike because of shared ancestry (homology) with traits that look alike for other reasons (homoplasy, such as convergent evolution).
Phylogeny
The evolutionary history of a group of organisms. Think of it as the family tree for a set of species.
Taxon (pl. taxa)
A group of organisms that scientists classify together as a unit. Taxa exist at every level of the hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
Tree of life
The ultimate evolutionary phylogeny that maps the relationships of all living things on Earth.
Node
A point on a phylogenetic tree representing the origin of a common ancestor. In simple terms, each node is a branching point where one lineage split into two (or more).
Branch
A line extending from a node, representing a lineage evolving through time.
Sister groups
Two taxa that share the same immediate node on a tree. They are each other's closest relatives.
Polytomy
A node that splits into more than two branches. This usually signals that scientists have not yet resolved which taxa diverged first.
Character / trait
Any genetic, morphological, physiological, developmental, or behavioural characteristic used for study in phylogenetics.
Outgroup
A taxon related to the group under study but not part of it. It serves as a reference point for determining which traits are ancestral and which are derived.
Ancestral trait
A characteristic that existed in the ancestor of a group. Think of it as the "original" version of a feature.
Derived trait
A modified form of an ancestral trait. In simple terms, it is the newer version that evolved after the ancestor.
Cladistic approach (cladistics)
The method of classifying organisms by grouping them according to shared derived traits inherited from a common ancestor.
Synapomorphy
A derived trait shared by two or more taxa that is present in their recent common ancestor but absent in more distant ancestors. All synapomorphies are homologies, but not all homologies are synapomorphies.
Homology
Similarity between species due to shared ancestry. Think of it as "same origin, possibly different function."
Homoplasy
Similarity between species that is not due to shared ancestry. This includes convergent evolution, trait reversal, and parallel evolution.
Convergent evolution
The independent evolution of similar traits in distantly related organisms because they adapted to similar environments or lifestyles. Think of it as "different origin, similar result."
Parsimony
The principle of choosing the phylogenetic tree that requires the fewest evolutionary changes. In simple terms, the simplest explanation wins.
Branches extend from nodes. Each node represents a common ancestor.
Sister groups are the two taxa that share the same node, meaning they share a most recent common ancestor.
A polytomy is a node with more than two branches, indicating the order of divergence is unresolved.
The number of nodes on a tree is not informative on its own, and neither is the physical distance between taxa on the page. What matters is the branching pattern.
Cladistics groups organisms by shared derived characters (traits inherited from a common ancestor that have been modified).
An outgroup is included to help determine which traits are ancestral (already present before the group diversified) and which are derived (evolved more recently).
A synapomorphy is the key unit of evidence in cladistics: a derived trait found in two or more taxa and their recent ancestor, but not in more distant ancestors.
All synapomorphies are homologies (traits from shared ancestry), but not all homologies are synapomorphies (some homologies are ancestral traits shared by a much wider group).
Homoplasy (convergent evolution): Two species can develop the same trait independently, without inheriting it from a shared ancestor. Wings in birds and bats are a classic example. This can make unrelated species appear more closely related than they are.
Trait gain followed by loss: A trait may appear on one branch and then be lost on a descendant branch, creating the false impression that no evolutionary change occurred.
Conflicting signals: A single species may have one trait that groups it with one clade and another trait that groups it with a different clade, making placement ambiguous.
Because of these complications, scientists rely on parsimony: they prefer the tree that requires the fewest total evolutionary changes (gains, losses, and modifications of traits).
Homology and homoplasy both describe trait similarity between species, but they have completely different explanations.
Homology = similarity due to shared ancestry. The bones in a human arm, a whale flipper, and a bat wing are homologous: same underlying skeletal structure, inherited from a common ancestor, repurposed for different functions.
Homoplasy = similarity due to something other than shared ancestry. The most common cause is convergent evolution, where unrelated organisms independently evolve similar traits because they face similar environmental pressures.
Three lines of evidence are used to determine whether a similarity is a true homology:
Phylogenetic evidence: Does the trait map onto a phylogenetic tree in a pattern consistent with inheritance from a common ancestor?
Structural evidence: Do the structures share the same underlying anatomy, even if their outward appearance differs?
Genetic and developmental evidence: Are the same genes and developmental pathways responsible for the trait in both species?
Students often think that the physical distance between two taxa on a printed tree indicates how closely related they are. It does not. Only the branching pattern (which nodes they share) tells you about relatedness.
Students often assume that more nodes on a tree means more evolutionary change. The number of nodes is not informative about the amount of change; it simply reflects how many branching events are shown.
Students frequently confuse homology with homoplasy. If two species share a trait, the reflex is to assume common ancestry. Always check whether convergent evolution could explain the similarity instead.
Students sometimes think parsimony means the simplest tree is always correct. Parsimony is a methodological preference, not a guarantee. The real evolutionary history may have involved more steps than the most parsimonious tree suggests.
⚠️ Be prepared to read a phylogenetic tree and identify sister groups, outgroups, and monophyletic groups from its branching pattern.
⚠️ Know the difference between ancestral and derived traits, and be able to identify synapomorphies on a given tree.
⚠️ Expect a question asking you to distinguish homology from homoplasy given a scenario (two species share trait X, are they related or is it convergent evolution?).
⚠️ Parsimony problems are common: given a set of traits and taxa, choose or construct the tree requiring the fewest changes.
⚠️ The relationship "all synapomorphies are homologies, but not all homologies are synapomorphies" is a frequently tested distinction.
True or False: Sister groups share the same node on a phylogenetic tree. (True)
True or False: The more nodes on a tree, the more evolutionary change has occurred. (False, the number of nodes is not informative about amount of change)
Fill in the blank: A trait found in two or more taxa and their recent common ancestor, but not in distant ancestors, is called a __________. (synapomorphy)
True or False: Convergent evolution produces homologous traits. (False, it produces homoplastic traits)
Fill in the blank: The principle of choosing the tree with the fewest evolutionary steps is called __________. (parsimony)
Q: Two species of fish live in different oceans but both have streamlined bodies and forked tails. Is this similarity more likely homology or homoplasy? Explain.
A: It depends on how closely related the two species are. If they share a recent common ancestor that also had these traits, it is homology. If they are distantly related and evolved these features independently due to similar aquatic environments, it is homoplasy (convergent evolution). You would check phylogenetic, structural, and genetic evidence to decide.
Q: On a phylogenetic tree, species A and species B share a node. Species C branches off from a node one level deeper. Which two species are sister groups?
A: Species A and species B are sister groups, because they share the most recent common ancestor (the same immediate node).
Q: What is the difference between an ancestral trait and a derived trait, and why does the distinction matter for building phylogenetic trees?
A: An ancestral trait is a characteristic present in the ancestor of a group. A derived trait is a modified version of that ancestral trait. Only shared derived traits (synapomorphies) are useful for grouping organisms in cladistics, because ancestral traits are shared too widely to reveal specific relationships.
Q: A researcher builds two possible phylogenetic trees for the same set of organisms. Tree 1 requires 12 evolutionary changes and Tree 2 requires 8. Which tree would be preferred under parsimony, and why?
A: Tree 2 would be preferred. Parsimony favours the tree that requires the fewest evolutionary steps, so the 8-step tree is the more parsimonious explanation.
Q: Explain why all synapomorphies are homologies, but not all homologies are synapomorphies.
A: A synapomorphy is a shared derived trait inherited from a recent common ancestor, which by definition makes it a homology (a trait shared due to common ancestry). However, homology is a broader category: it also includes ancestral traits shared by many groups. An ancestral trait like having a backbone is homologous across all vertebrates but is not a synapomorphy for any smaller subgroup within vertebrates.
This material connects directly to speciation (Ch. 480-490), because phylogenetic trees are the tools used to visualise how new species arise and diverge. Understanding monophyletic groups is essential for the phylogenetic species concept covered there.
It also links to the history of life material (Ch. 439-444, 503-506), where phylogenetic trees are used to trace major innovations like the transition from fish to tetrapods and the evolution of whales from land mammals. The fossil record provides the physical evidence that populates and tests these trees.
Phylogenetics, phylogenetic tree, cladogram, cladistics, clade, taxon, taxa, taxonomy, tree of life, node, branch, sister group, sister taxa, polytomy, outgroup, ingroup, ancestral trait, derived trait, character, synapomorphy, shared derived character, homology, homologous structures, homoplasy, convergent evolution, analogous structures, parsimony, maximum parsimony, evolutionary tree, branching diagram, common ancestor, most recent common ancestor (MRCA), monophyletic group, BIO I, Introduction to Biology, University of Washington, Dobkowski, Ch. 47-48, pp. 496-503