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 major changes like inversions, translocations, or fusions of chromosome segments, while small-scale variations include single nucleotide polymorphisms (SNPs) and small insertions or deletions (indels).
2. Recognize that the observation of minimal chromosomal rearrangement over 100 million years refers to the overall structure and organization of chromosomes remaining largely conserved among eutherian mammals, indicating evolutionary stability at the macro level.
3. Acknowledge that within species (such as individual humans) or between closely related species (like humans and chimpanzees), genetic diversity arises primarily through small-scale mutations such as SNPs and indels, which accumulate more rapidly and contribute to genetic variation without altering chromosome structure significantly.
4. Explain that these small-scale mutations can occur frequently and are responsible for genetic differences observed within populations and between closely related species, while the large-scale chromosomal architecture remains stable over much longer evolutionary timescales.
5. Conclude that the reconciliation lies in the different scales and types of genetic changes: chromosome structure is conserved over millions of years, but small mutations accumulate continuously, leading to detectable genetic variation among individuals and 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 accumulate rapidly and contribute to genetic diversity without altering large chromosomal structures.

Evolutionary Timescales and Mutation Rates

Different types of genetic changes occur at different rates; large chromosomal rearrangements happen slowly over millions of years, while small mutations like SNPs and indels accumulate quickly within populations and species. This explains why chromosomes appear stable over long periods, yet individuals and closely related species show many small genetic differences.