= Solution
A <point mutation> changes a nucleotide at one genomic position. Replication errors and chemical damage can generate <Nucleotide substitution>[substitutions]; repair and other proofreading processes influence the retained <mutation rate>. A coding change may be synonymous or alter an <amino acid>, while a noncoding change can affect regulation. Its evolutionary effect depends on context, so <mutation>, functional consequence and eventual <Fixation probability>[fixation] are distinct events.
<Mutation> supplies new variants, while <genetic drift>, <Natural selection>[selection] and population history determine their persistence. Under a simple neutral model, increasing population size raises the number of new variants but lowers each one's <Fixation probability>[fixation] <probability>; the neutral <Nucleotide substitution>[substitution] rate is approximately the <mutation rate>. Selective constraint, generation time and lineage-specific processes can therefore complicate a universal <molecular clock>. Molecular evolution is not directed towards anticipated needs.
To infer a <Phylogenetic tree>[phylogeny], compare <Biological homology>[homologous] positions in appropriately chosen <Orthology>[orthologous] sequences and estimate the tree and branch lengths under a <Nucleotide substitution>[substitution] model. Shared derived characters support <Common descent>[common ancestry], but reversals and parallel <Nucleotide substitution>[substitutions] mean raw mismatches underestimate evolutionary distance. In a simple equal-rates four-state illustration, a mismatch fraction $p$ gives corrected pairwise distance $d=-(3/4)\log(1-4p/3)$. The formula's strong divergence as $p\to3/4$ reflects saturation and loss of reliable historical information, not arbitrarily precise evidence for an ancient split.
The <molecular-clock calibration and rate-time identifiability> issue is fundamental: branch lengths first measure expected <Nucleotide substitution>[substitutions] per site. A clock relates them to time by $d_i=r_it_i$; for two equal-rate lineages separated for time $t$, pairwise distance is $d=2rt$, so $t=d/(2r)$. Absolute times require external information, such as independently dated fossils or geological constraints, or informative sampling dates when the data contain a temporal signal. Sequence differences alone generally identify only rate-times-time, not both quantities separately.
Time-calibrated inference should propagate uncertainty in topology, <Nucleotide substitution>[substitution] rates, ancestral-state reconstruction and calibration bounds. Relaxed clocks permit different lineage rates. Fossil minima are not automatically exact split dates; orthology mistakes, <incomplete lineage sorting>, <Genetic recombination>[recombination], <Horizontal gene transfer>[horizontal transfer] and <Natural selection>[selection] can cause discordance between one <Phylogenetic tree>[gene tree] and <species> history. Multiple independent loci and well-justified calibrations make a stronger historical argument than a single uncalibrated sequence comparison. No unique numerical tree or date can be constructed here without actual sequence observations and calibration data.
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