Premed · Premed · General Biology 2
Lecture 5: Phylogeny and the Tree of Life
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Read and interpret phylogenetic trees, identifying clades, sister taxa, and outgroups
- Distinguish between homologous and analogous traits in phylogenetic analysis
- Explain the principles of cladistics and parsimony
- Describe how molecular data are used to construct phylogenies
- Explain the three-domain system of classification and the major branches of the tree of life
- Interpret horizontal gene transfer and its implications for phylogenetics
Lecture Content
I. Taxonomy and Systematics
Taxonomy is the science of naming, describing, and classifying organisms, while systematics is the broader discipline that seeks to understand evolutionary relationships among organisms. The foundations of modern taxonomy were laid by Carolus Linnaeus (1707-1778), who introduced binomial nomenclature -- the two-part naming system in which each species is identified by its genus and species name (for example, Homo sapiens). Linnaeus also established the hierarchical classification system still in use today: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species (often remembered by the mnemonic "Dear King Philip Came Over For Good Spaghetti"). Modern systematics is explicitly evolutionary in its approach -- the goal is to ensure that classification reflects actual phylogeny, the evolutionary history of lineages.
II. Phylogenetic Trees
A phylogenetic tree, or phylogeny, is a branching diagram that represents the evolutionary relationships among organisms. Reading these diagrams correctly requires understanding several key terms. A node represents a branching point where lineages diverge, corresponding to a hypothetical common ancestor. A branch is a lineage evolving through time. A tip (or terminal node) represents a living or extinct taxon at the end of a branch. The root is the common ancestor of all taxa on the tree. Sister taxa are two groups that share an immediate common ancestor and are therefore each other's closest relatives. A clade (or monophyletic group) consists of an ancestor and all of its descendants -- a complete branch of the tree. An outgroup is a taxon outside the clade of interest, used to root the tree and determine which character states are ancestral versus derived. The ingroup encompasses the set of taxa being studied.
Several common errors in reading phylogenetic trees stem from the same misunderstanding. Relatedness is determined by recency of common ancestry, not by physical proximity on the page or the order in which taxa appear at the tips. Branches can be rotated around any node without changing the relationships the tree depicts -- just as a mobile can spin around its suspension points without changing which objects are attached to which arms.
The way taxa are grouped on a tree matters enormously. A monophyletic group includes an ancestor and all its descendants and constitutes a valid clade. A paraphyletic group includes an ancestor and some but not all of its descendants -- "reptiles" as traditionally defined, for example, are paraphyletic because the group excludes birds, which are phylogenetically nested within the reptilian clade. A polyphyletic group lumps taxa from different ancestors based on convergent traits rather than shared ancestry, such as grouping mammals and birds together as "warm-blooded animals."
<image>A labeled phylogenetic tree illustrating key terms. The tree shows 6 terminal taxa (A through F) with an outgroup (taxon F). Nodes are labeled as common ancestors. A monophyletic clade (taxa A, B, C) is highlighted with a blue box. A paraphyletic group (taxa D, E — excluding C) is highlighted with a dashed red box. Sister taxa (A and B) are indicated with brackets. The root of the tree is labeled at the base. An inset shows that rotating branches around a node does not change relationships — two versions of the same tree with different branch rotations are shown to be equivalent.</image>
III. Building Phylogenies: Cladistics
Cladistics is the dominant method of phylogenetic classification, and it is built on the principle that clades should be defined by shared derived characters -- features that evolved in the common ancestor of the clade and were inherited by its descendants. A character is any heritable feature that can be compared across taxa, whether morphological, molecular, or behavioral. A character state is a specific variant of a character, such as the presence or absence of a backbone. An ancestral character (plesiomorphy) is a character state present in the common ancestor and shared broadly across lineages, while a derived character (apomorphy) is one that is new or modified from the ancestral state. The key to cladistic analysis is the shared derived character (synapomorphy) -- a derived character shared by members of a clade, which is used to define that clade. Shared ancestral characters (symplesiomorphies) do not define clades because they are inherited from deeper in the tree and are shared too broadly to be informative about recent relationships. Homoplasy -- similarity not due to common ancestry, arising from convergent evolution, evolutionary reversal, or parallelism -- is the primary source of error in phylogenetic reconstruction.
The principle of parsimony (Occam's razor) holds that the simplest explanation is preferred. In phylogenetics, this means that the tree requiring the fewest evolutionary changes -- the fewest character state transitions -- is chosen as the best hypothesis. While parsimony is not always correct (evolution is not always parsimonious), it provides a useful and rigorous starting point. Maximum likelihood and Bayesian methods have become the standard for molecular data. These statistical approaches evaluate the probability of the observed data given a particular tree topology and an explicit model of sequence evolution. They are computationally intensive but generally more accurate than parsimony for molecular datasets.
IV. Molecular Phylogenetics
DNA and protein sequences provide an enormous supply of objective, quantifiable characters for phylogenetic analysis, and they have revolutionized our understanding of evolutionary relationships. Molecular data offer several advantages over morphological data: they are applicable to all organisms, including those with few distinguishing morphological features; they are less subjective, since nucleotide sequences are unambiguous; the amount of available data is vast, extending to entire genomes; and molecular comparisons can reveal cryptic species invisible to morphological analysis.
Different molecules evolve at different rates, making them suitable for different phylogenetic questions. Ribosomal RNA genes (such as 16S rRNA for prokaryotes and 18S rRNA for eukaryotes) are highly conserved and useful for reconstructing deep phylogenies spanning billions of years. Mitochondrial DNA evolves relatively rapidly and is ideal for studies of closely related species and population-level relationships. Protein sequences such as cytochrome c and cytochrome oxidase are conserved across wide taxonomic ranges. Whole-genome comparisons, increasingly feasible with modern sequencing technology, provide the most comprehensive data.
Molecular clocks rest on the observation that if mutations accumulate at a roughly constant rate in a given lineage, the degree of sequence divergence between two taxa can be used to estimate the time since they last shared a common ancestor. Molecular clocks are calibrated using fossil dates or known geological events, such as the separation of continents. However, their accuracy is limited by the fact that mutation rates vary among lineages, among genes, and among different types of mutations. Accurate sequence alignment is a critical prerequisite for any molecular phylogenetic analysis, since homologous positions in the sequences must be correctly identified before meaningful comparisons can be made.
<image>A step-by-step diagram of constructing a molecular phylogeny. Panel A: DNA sequences from 4 species aligned, with variable sites highlighted in color (substitutions marked). Panel B: A distance matrix showing the number of differences between each pair of species. Panel C: The resulting phylogenetic tree constructed from the matrix, with branch lengths proportional to genetic distance. Panel D: A molecular clock calibration — the tree is overlaid on a geological timescale, with a fossil calibration point (star) used to convert genetic distance into absolute time (millions of years).</image>
V. The Three-Domain System
In 1977, Carl Woese used comparisons of 16S and 18S ribosomal RNA sequences to propose a radical reorganization of the tree of life into three domains. Bacteria are prokaryotes with peptidoglycan cell walls. Archaea are prokaryotes that lack peptidoglycan, have membranes composed of ether-linked lipids, and include many extremophiles -- though they are now known to be common in moderate environments as well. Eukarya encompasses all eukaryotes: protists, fungi, plants, and animals.
Several findings from this work were profound. Archaea are more closely related to Eukarya than to Bacteria, despite the superficial similarities between all prokaryotes. This means that the traditional prokaryote-eukaryote division does not reflect true phylogeny. Some researchers now advocate a two-domain model in which eukaryotes arose from within Archaea -- specifically from the Asgard archaea -- making Archaea and Bacteria the only two primary domains. Within Eukarya, the traditional kingdom Protista is recognized as paraphyletic, representing a diverse collection of eukaryotic lineages rather than a coherent evolutionary group. The kingdoms Plantae, Fungi, and Animalia, by contrast, are each monophyletic. Increasingly, biologists describe the tree of life as a "web of life," reflecting the profound impact of horizontal gene transfer on evolutionary history.
VI. Horizontal Gene Transfer and Complications
Horizontal (lateral) gene transfer (HGT) is the transfer of genes between organisms that are not parent and offspring. HGT is common in prokaryotes, occurring through transformation, transduction, and conjugation, but it also occurs in eukaryotes through endosymbiotic gene transfer and viral-mediated mechanisms. HGT complicates phylogenetic reconstruction because different genes in the same organism may have different evolutionary histories, producing different gene trees. For prokaryotes in particular, the "tree of life" may be better represented as a network or web than as a strictly bifurcating tree.
Other complications include incomplete lineage sorting, in which ancestral polymorphisms are sorted differently in descendant lineages, producing gene trees that disagree with the species tree. Gene duplication and loss can create paralogs (genes related by duplication within a genome) that are mistakenly compared with orthologs (genes related by speciation) in different species. Long-branch attraction is an analytical artifact in which rapidly evolving lineages are incorrectly grouped together because their high rates of change produce superficial similarity.
Solutions to these challenges include using multiple genes or entire genomes (phylogenomics) rather than relying on any single gene, employing network-based methods instead of strictly tree-based approaches, and performing concordance analyses that assess the degree of agreement among gene trees. These approaches have made modern phylogenetics more robust, though the fundamental challenge of reconstructing events that occurred billions of years ago remains formidable.
<image>A comparison of a traditional tree of life and a web of life. Panel A: A simple bifurcating tree showing the three domains — Bacteria, Archaea, and Eukarya — branching from LUCA (Last Universal Common Ancestor) at the root. Major lineages within each domain are labeled (e.g., Proteobacteria, Cyanobacteria within Bacteria; Euryarchaeota, Crenarchaeota, Asgard archaea within Archaea; Animals, Plants, Fungi within Eukarya). Panel B: The same tree but with horizontal arrows between branches representing horizontal gene transfer events — arrows from Bacteria to Archaea, from an alpha-proteobacterium to the eukaryotic lineage (endosymbiosis/mitochondria), from cyanobacteria to the plant lineage (endosymbiosis/chloroplasts), and various cross-domain transfers. The resulting diagram looks more like a web or network than a simple tree.</image>
