How would you prevent paralogs from being mistaken for orthologs in a comparative analysis?

Why

The simplest method, reciprocal best hits, assumes the most similar gene in another species is the ortholog. The OrthoFinder authors show how that fails. When evolutionary rates differ after a duplication, a fast-evolving true ortholog can look less similar than a slowly evolving paralog, so the score-based method assigns the wrong partner.

Gene loss makes it worse: if the true ortholog was lost in one species, its paralog becomes the best hit. Tree-based inference avoids this by asking where the gene tree's branching shows a speciation and where it shows a duplication. OrthoFinder groups genes into orthogroups, builds and roots gene trees, identifies duplication events and infers orthologs and paralogs from the tree, and its authors report it as equal to or better than comparable methods on benchmarks.

Interpretation still needs care. One-to-many and many-to-many relationships are real outcomes after lineage-specific duplications, and forcing them into one-to-one pairs throws away biology. Gene trees from short or divergent genes are uncertain, so low-support splits should be treated as unresolved.

How to work it through

1. Collect complete proteomes from the species of interest, with one representative isoform per gene.
2. Run an orthology tool that infers gene trees, such as OrthoFinder.
3. Check that the inferred species tree matches the accepted one.
4. Read orthology from the trees, keeping one-to-many relationships where duplications occurred.
5. Flag families with low tree support or known lineage-specific expansions.
6. Confirm key assignments with synteny or a manual tree when they matter.

Where the answer stops holding

Tree inference fails when genes are short, highly divergent or shaped by horizontal transfer. Incomplete or poorly annotated proteomes create false gene losses. Orthology says nothing certain about function, which can diverge between orthologs too.