Bio Phylo Tree Manipulation

Version Compatibility

Reference examples tested with: BioPython 1.83+. Alternatives: ape 5.8+ / phangorn / phytools (R), DendroPy 5+ and ete3 (Python), Newick Utilities 1.6+, MAD and RootDigger as standalone CLIs.

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show biopython then help(module.function) to check signatures
• R: packageVersion('ape') then ?drop.tip to verify parameters
• CLI: <tool> --version then <tool> --help to confirm flags

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

ape has NO midpoint function of its own (use phangorn::midpoint or phytools::midpoint.root); ete3 prune drops branch lengths unless preserve_branch_length=True; ape di2multi filters by branch LENGTH (tol), not support.

Tree Manipulation -- Rooting Is an Inference, Not a Cosmetic Operation

"Root and prune my tree" -> Edit an existing tree object, treating the root placement as a separate statistical inference and every structural edit as potentially distance-corrupting.

• Python: tree.root_with_outgroup(...), tree.root_at_midpoint(), tree.prune(...), tree.collapse_all(...) (Bio.Phylo)
• R: root(), phangorn::midpoint(), drop.tip(), di2multi() (ape/phangorn); CLI: nw_reroot, nw_prune (Newick Utilities); outgroup-free rooters: mad, FastRoot.py, rootdigger

Scope: editing an existing tree -- rooting, pruning/subsetting, extracting clades or induced subtrees, collapsing branches into polytomies, resolving polytomies, ladderizing. Reading/writing/converting tree files -> tree-io. Plotting and mapping annotations -> tree-visualization. Inferring the tree in the first place -> modern-tree-inference. Clock-based or Bayesian rooting co-estimated with dates -> divergence-dating.

The Single Most Important Modern Insight

Rooting is a separate statistical inference layered on top of the topology, and it is the highest-error decision in the entire tree. Nearly every inference method (ML under a time-reversible model, neighbor-joining, most Bayesian runs) returns an UNROOTED tree: a reversible model is blind to the direction of time, so the likelihood is identical wherever the root sits. The unrooted tree states which taxa are neighbors; it is silent about who is ancestor and who is descendant. Placing the root is what converts a neighbor-graph into an evolutionary narrative, and it CREATES every "X is basal", every "the common ancestor had trait T", every character-polarity, every divergence ordering. Three load-bearing facts:

1. Rooting needs its own justification and its own uncertainty statement, distinct from branch support. Bootstrap 95 on an ingroup clade says nothing about whether the root is in the right place. The relationships can be robust while the root -- the thing every downstream story hangs on -- is the least reliable part of the figure.
2. The deep nodes near an outgroup-defined root are the LEAST trustworthy. A distant outgroup sits on a long branch, and long branches attract each other (long-branch attraction): the outgroup pulls the fastest ingroup taxon toward the root and makes it look artifactually "basal". Accuracy degrades monotonically as outgroup distance grows.
3. The choice of rooting METHOD is a modeling choice with assumptions (clock? outgroup monophyly? non-reversible model?), and the wrong method silently fabricates basal lineages. Pruning, collapsing, and ladderizing are comparatively mechanical -- but each has its own silent traps (distance corruption, soft/hard polytomy conflation, perceived trends).

Rooting Method Decision

Rule: never report a root from a single method without a sanity check. If a close, monophyletic outgroup is available, use it. If not, run MAD and MinVar and prefer agreement; disagreement means the root is poorly determined and all "basal" claims must be hedged. With an alignment and a need for a confidence value, use RootDigger (Bettisworth and Stamatakis 2021) or non-reversible IQ-TREE. MAD = Tria et al. 2017; MinVar = Mai et al. 2017; outgroup-free CLIs and Newick Utilities = Junier and Zdobnov 2010.

Tool Taxonomy

Root with an Outgroup (Multiple, Monophyletic)

Goal: Root the tree using known sister-group taxa, preferring multiple close outgroups and verifying ingroup monophyly first.

Approach: Confirm the outgroup taxa form a monophyletic group, root on the branch separating them from the ingroup, then check the ingroup is recovered as monophyletic -- if not, the rooting is suspect.

from Bio import Phylo

tree = Phylo.read('tree.nwk', 'newick')
outgroup = [{'name': 'OutA'}, {'name': 'OutB'}]      # multiple close outgroups beat a single long branch

if tree.is_monophyletic([tree.find_any(name='OutA'), tree.find_any(name='OutB')]):
    tree.root_with_outgroup(*outgroup)               # root_with_outgroup, NOT root_with_midpoint
else:
    print('outgroup not monophyletic: root placement is unreliable, re-check taxon choice')

Root at the Midpoint (Clock-Limited Fallback)

Goal: Root an outgroup-free tree where evolution is approximately clock-like (shallow/viral data).

Approach: Place the root at the midpoint of the longest tip-to-tip path; trust it only when no single long branch can hijack that path. For deep trees with rate variation prefer MAD/MinVar.

tree = Phylo.read('tree.nwk', 'newick')
tree.root_at_midpoint()                              # assumes a clock; a long branch slides the root onto the fast lineage
# Outgroup-free and clock-relaxed (deep trees): standalone CLIs run on the Newick file
# MAD:    mad tree.nwk           -> tree.nwk.rooted   (per-branch root ambiguity index)
# MinVar: FastRoot.py -i tree.nwk -m MV -o rooted.nwk

Prune Taxa With Branch-Length Preservation

Goal: Remove tips and keep every surviving patristic distance unchanged.

Approach: Drop the tip and SUPPRESS the resulting degree-2 ("knee") node, ADDING its branch length to the child so path lengths are conserved. Bio.Phylo prune and ape drop.tip do this by default; ete3 prune needs preserve_branch_length=True.

tree = Phylo.read('tree.nwk', 'newick')
keep = {'Human', 'Chimp', 'Mouse'}

for term in list(tree.get_terminals()):
    if term.name not in keep:
        tree.prune(term)                             # collapses the degree-2 parent and sums branch lengths
# ape (R):   drop.tip(phy, c('X','Y'))               # sums suppressed branch lengths by default
# ete3 (Py): tree.prune(list(keep), preserve_branch_length=True)   # the flag is mandatory, else distances shrink

Non-monophyletic targets cannot be "extracted as a clade" -- there is no node whose descendants are exactly those taxa. Prune to the taxon set to get the induced subtree instead; common_ancestor of non-monophyletic taxa returns an MRCA whose clade contains EXTRA taxa.

Collapse Low-Support Branches Into SOFT Polytomies

Goal: Replace poorly-supported resolved nodes with multifurcations that honestly say "unresolved".

Approach: Collapse any internal branch whose support is below a stated cutoff. The result is a SOFT (uncertainty) polytomy, never a HARD (simultaneous-radiation) one -- label it as such.

tree = Phylo.read('tree.nwk', 'newick')              # support parsed into clade.confidence

tree.collapse_all(lambda c: c.confidence is not None and c.confidence < 70)   # 70 for std bootstrap; use 95 for UFBoot2
tree.collapse_all(lambda c: c.branch_length is not None and c.branch_length < 1e-8)   # collapse genuinely-zero branches
# ape (R): di2multi filters by LENGTH (tol), not support -> zero out low-support branch lengths first, THEN di2multi(phy)

Resolving the inverse direction (multi2di / resolve_polytomy) invents an arbitrary order with zero-length branches; analyzing one random resolution treats an arbitrary choice as fact. If a binary tree is required, integrate over many random resolutions and treat the inserted zero-length branches as "no information", not instantaneous divergence.

Per-Method Failure Modes

Distant Outgroup Roots Inside the Ingroup

Trigger: A single distant outgroup, or many outgroups all far from the ingroup, used to root. Mechanism: The long outgroup branch attracts the fastest ingroup taxon (LBA), pulling the root into the ingroup; far outgroups attach at essentially random positions (DeSalle et al. 2023). Symptom: Ingroup not recovered as monophyletic; a fast taxon appears "basal"; the root jumps as the outgroup set changes. Fix: Use multiple CLOSELY-related, monophyletic, roughly-equidistant outgroups; verify ingroup monophyly; treat a lonely distant outgroup as a red flag.