When the human genome is examined, the chromosomes appear to have undergone only minimal rearrangement in the 100 million years since the last common ancestor of eutherian mammals. However, when individual humans are examined or when the human genome is compared with that of chimpanzees, a large number of small indels and SNPs can be detected. How are these observations reconciled?

Verified step by step guidance

1. Understand the difference between large-scale chromosomal rearrangements and small-scale genetic variations. Large-scale rearrangements involve changes like inversions, translocations, or fusions of whole chromosome segments, while small-scale variations include single nucleotide polymorphisms (SNPs) and small insertions or deletions (indels).
2. Recognize that over long evolutionary timescales (e.g., 100 million years), the overall structure and number of chromosomes in eutherian mammals have remained relatively stable, indicating minimal large-scale rearrangements.
3. Acknowledge that within species or between closely related species (like humans and chimpanzees), genetic diversity is primarily due to numerous small-scale mutations such as SNPs and indels, which accumulate more rapidly and contribute to individual and species differences.
4. Explain that the stability of chromosome structure over millions of years does not contradict the presence of many small genetic changes; these small changes do not necessarily alter chromosome structure but can affect genetic sequences and traits.
5. Conclude that the observations are reconciled by understanding that chromosome structure is conserved over long evolutionary periods, while small-scale genetic variations accumulate more quickly and are responsible for differences observed within and between closely related species.

Key Concepts

Chromosomal Rearrangements and Genome Stability

Chromosomal rearrangements involve large-scale changes like inversions, translocations, or fusions that alter chromosome structure. Over long evolutionary timescales, such as 100 million years, these events are relatively rare in eutherian mammals, leading to overall genome stability at the chromosomal level despite species divergence.

Small-scale Genetic Variations: SNPs and Indels

Single nucleotide polymorphisms (SNPs) and small insertions/deletions (indels) are minor genetic changes occurring frequently within populations and between closely related species. These variations contribute to genetic diversity and can accumulate rapidly without altering large chromosomal structures.

Evolutionary Timescales and Genetic Variation

Large chromosomal changes accumulate slowly over millions of years, while small-scale mutations like SNPs and indels arise more rapidly within populations and species. This explains why chromosomes appear stable over long periods, yet individual genomes show abundant small variations when compared within or between species.