In molecular phylogenetics, evolutionary biologists often rely on mitochondrial DNA (mtDNA) mutations to calibrate 'molecular clocks' that estimate when species diverged from common ancestors. Because mtDNA is inherited maternally and assumed to mutate at a steady, neutral rate via genetic drift, greater sequence divergence between two lineages is traditionally interpreted as indicating a more ancient common ancestor. However, recent comparative genomic studies of high-altitude avian populations demonstrate that metabolic adaptation to hypoxic conditions can drive intense positive selection on mitochondrial genes encoding oxidative phosphorylation enzymes. This selective pressure accelerates substitution rates in these specific genes, artificially inflating genetic distance calculations between high-altitude populations and their lowland relatives.
To resolve these divergence anomalies without abandoning mtDNA-based dating, some researchers propose masking—excluding from analytical models—the specific coding loci identified as being under positive selection. Nevertheless, critics contend that this remedy is fundamentally insufficient because positive selection frequently induces 'genetic hitchhiking,' wherein beneficial mutations drag linked, neutrally evolving non-coding regions to genomic fixation, thereby altering sequence variability outside the targeted coding loci. Therefore, critics conclude, masking selected coding loci cannot restore the reliability of mtDNA molecular clocks in adaptively evolving populations.
Which of the following is an unstated assumption on which the critics' argument regarding the insufficiency of masking selected coding loci relies?
- ANuclear genomic markers provide a demonstrably more accurate timeline of species divergence than mitochondrial DNA markers in avian species adapted to high-altitude environments.
- The sequence alterations in linked non-coding regions produced by genetic hitchhiking distort genetic variation sufficiently to impair neutral molecular clock calibrations.Cevap
- CMetabolic adaptation to high-altitude hypoxic conditions occurs exclusively through non-synonymous mutations within oxidative phosphorylation mitochondrial genes.
- DResearchers who advocate for masking coding loci believe that genetic hitchhiking has no observable effect on non-coding regions of the mitochondrial genome.
- EPositive selective pressure on mitochondrial genes occurs with equal intensity across both high-altitude and lowland avian lineages.