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?

Key Concepts

Chromosomal Rearrangement

Chromosomal rearrangement refers to alterations in the structure or number of chromosomes, which can include duplications, deletions, inversions, and translocations. In the context of eutherian mammals, the observation of minimal rearrangement over 100 million years suggests that large-scale structural changes are relatively rare, allowing for the conservation of chromosome organization across species.

Indels and SNPs

Indels (insertions and deletions) and SNPs (single nucleotide polymorphisms) are types of genetic variations that occur within the DNA sequence. While chromosomal structures may remain stable over long evolutionary periods, these small-scale variations can accumulate rapidly, leading to significant genetic diversity among individuals and between species, such as humans and chimpanzees.

Evolutionary Time Scale

The evolutionary time scale refers to the vast periods over which evolutionary changes occur. While large chromosomal rearrangements may take millions of years to manifest, smaller genetic variations like SNPs and indels can arise and spread through populations much more quickly, allowing for adaptation and diversity without altering the overall chromosomal architecture.