Genomes and ancestry · quick reference
Genomes rewrote the family album
In 1995, scientists usually compared a few genes. Whole genomes and ancient DNA now reveal histories that are branching overall but mosaic in detail.
Short version: ancestry is mostly branching, but genes can also cross branches. Whole and ancient genomes let researchers see both the backbone and the cross-links.
Four observations to add to the book
- Archaic admixture is measurable. Many people with ancestry outside Africa carry some Neanderthal-derived DNA; Denisovan-related ancestry is especially substantial in parts of Oceania and also appears elsewhere. These are statistical ancestry patterns, not fixed modern racial categories. Ancient DNA turned an inference into direct historical tests.
- Introgression can contribute useful variants. A Denisovan-related EPAS1 DNA region shows strong evidence of positive selection associated with Tibetan high-altitude adaptation. Gene flow supplied variation; selection changed its frequency.
- One reference is not the species. Pangenomes represent sequence and structural variation missed by a single linear reference. The 2023 human draft pangenome reduced reference bias.
- Genes can have different histories. Horizontal gene transfer is especially important in microbes; hybridization and incomplete lineage sorting also make gene trees disagree with species trees.
Keep the tree; lose the ladder. Common descent is strongly supported. Trees remain excellent summaries of predominantly vertical ancestry. But a species tree need not equal every gene tree, and some histories need network edges. “The tree of life is dead” is as misleading as “every gene follows one tree.”
Explore the model
Three bacterial lineages inherit most core genes vertically, but a plasmid carrying antibiotic resistance moves from lineage C to A. What representation preserves both facts?
Regulation moved center stage
Evolution does not require inventing a new protein every time. Changes in promoters, enhancers, splicing, gene copy number, and regulatory networks alter where, when, and how much genes are used. Comparative conservation can indicate purifying selection—but biochemical activity alone does not prove that a sequence has an organism-level function maintained by selection.
Headline filter: ENCODE reported biochemical activity across much of the human genome. That is not equivalent to “80% is functional” under an evolutionary definition of function. Always ask which definition and evidence are being used.
Contingency and convergence both survived experiment
Long-term microbial experiments produce repeated solutions under shared conditions and history-dependent surprises. Predictability depends on resolution: a broad phenotype may recur while the exact mutation does not. Evolution is neither wholly inevitable nor arbitrary.
Try the case
Two species share a DNA segment. Which conclusion is licensed before further tests?
Landmark evidence
- Green et al. (2010), draft Neanderthal genomePrimary ancient-DNA study.
- Huerta-Sánchez et al. (2014), adaptive Denisovan-like introgression at EPAS1Concrete selection + gene-flow example.
- Human Pangenome Reference Consortium (2023)A draft human pangenome reference.
- Soucy, Huang & Gogarten (2015), HGT and the web of lifeAuthoritative review, especially for microbes.
- Good et al. (2017), 60,000 generations of bacterial evolutionPrimary experimental evidence on dynamics and contingency.