Final answer:
The number of differences in nucleotide sequences being proportional to time elapsed since divergence from a common ancestor is a key concept of molecular clocks in biology, which helps estimate the time of divergence between species.
Step-by-step explanation:
The assumption that the number of positions that differ in the nucleotide sequence is proportional to the time elapsed since the two species formed their own lines of descent from a common predecessor is generally true. This concept is embedded in the idea of molecular clocks, which use DNA sequences (or the proteins they encode) to estimate relatedness among species and the time in geologic history when related species diverged from a common ancestor. Molecular clocks are based on the assumption that mutations accumulate through time at a steady average rate for a given region of DNA. Species that have accumulated greater differences in their DNA sequences are assumed to have diverged from their common ancestor in the more distant past. These molecular clocks can be refined by analyzing data from conserved proteins, understanding mutation rates, and cross-referencing fossil records. When evaluating the likelihood that negative selection pressures have been active, if two species show a single nucleotide replacement in genes coding for a particular feature, it suggests that this feature has been under strong selective pressure, maintaining its form because of its importance to the organism's survival.