Phylogenomics: Gene Trees, Species Trees and Why They Disagree
Different genes give different trees for the same set of species, through incomplete lineage sorting, introgression, duplication and plain estimation error. This stop separates those causes and uses methods that expect the conflict instead of hiding it.
What a learner can do afterwards
- Distinguish incomplete lineage sorting from introgression using tree topology counts and site patterns
- Explain when concatenating genes is misleading and what a coalescent summary method does instead
- Read a set of gene trees and locate the nodes where the conflict is real rather than noise
1 · Read
You keep the species tree apart from gene trees in your head. The species tree records how populations split. Each gene has its own history, and it can differ for purely biological reasons.
You learn the classic cause first. When ancestral populations are large and splits come quickly, gene variants fail to sort neatly into the descendant species. This incomplete lineage sorting clusters at short internal branches, and you model it rather than average it away.
You string all your genes into one giant alignment and get full support for one tree. But the genes genuinely disagree, so concatenation has converged on the wrong answer with full confidence. You switch to a coalescent summary method: you infer each gene tree, then find the species tree most consistent with the distribution.
You separate the two causes with topology counts plus site patterns. Sorting spreads discordance symmetrically, while gene flow enriches exactly the topology grouping the exchanging partners, with linked sites sharing the pattern. Estimation error shrinks with longer sequences, but true conflict persists with structure.
You expect gene conflict at short branches, you distrust concatenation under discordance, and you separate sorting from gene flow with counts plus site patterns.
More to read
18.2 Formation of New Species
New species form when populations diverge and reproductive barriers accumulate, through geographic isolation, ecological shifts, or chromosomal change. The species tree records this splitting history. But individual genes have their own histories, and a gene tree can differ from the species tree for purely biological reasons. The classic cause is incomplete lineage sorting. When ancestral populations are large and splits come quickly, gene variants fail to sort neatly into the descendant species. The result is genuine disagreement among gene trees, concentrated at short internal branches, and it is a signal to be modeled rather than an error to be averaged away.
20.2 Determining Evolutionary Relationships
Sequence comparison promises a flood of characters for tree building, and genomes deliver millions of them. The temptation is to string all genes together into one giant alignment and infer a single tree with overwhelming support. When genes genuinely disagree, this concatenation approach can converge on the wrong answer with complete confidence. Coalescent summary methods take the opposite route. They infer each gene tree separately, accept that the trees differ, and then find the species tree most consistent with the observed distribution of gene trees. This two stage logic respects the biology of recombination and independent assortment.
20.3 Perspectives on the Phylogenetic Tree
No single tree captures every aspect of life history once hybridization and gene transfer enter the picture. Networks and collections of trees represent these events more honestly than one bifurcating diagram. Reading such summaries means looking for nodes where many independent genes agree versus nodes where support splits several ways. Introgression leaves a distinctive signature. Unlike lineage sorting, which spreads discordance symmetrically, gene flow enriches exactly the topology that groups the exchanging partners, and linked sites share the pattern. Topology counts together with site patterns therefore separate the two causes in a way that neither shows alone.
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2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.