How does the gradual accumulation of sequence differences relate to the time since two species diverged from a common ancestor?

Sourav PanNovember 8, 2024

Answered

The gradual accumulation of sequence differences in DNA or protein sequences is closely related to the time since two species diverged from a common ancestor. This relationship is primarily explained by the molecular clock hypothesis, which posits that genetic changes occur at a relatively constant rate over time. Here’s how this process works:

1. Molecular Clock Hypothesis

• The molecular clock hypothesis suggests that mutations in DNA and proteins accumulate at a predictable rate. This rate can vary among different organisms and genes but tends to be consistent within a specific lineage under neutral evolutionary conditions. By measuring the number of mutations or sequence differences between two species, scientists can estimate the time since their divergence from a common ancestor.

2. Rate of Mutation

• Each gene or protein has an associated mutation rate, which is the frequency at which changes occur in the genetic material. For example, if a particular gene accumulates mutations at a rate of one mutation per 1 million years, and two species differ by four mutations in that gene, it can be inferred that they diverged approximately 4 million years ago. This method relies on knowing the mutation rate for the specific gene being analyzed .

3. Calibration with Fossil Records

• To effectively use molecular clocks for dating divergences, researchers often calibrate them using known divergence times derived from fossil records or geological events. For instance, if the divergence between two groups of mammals is known to have occurred 50 million years ago based on fossil evidence, this information can help establish a baseline for estimating divergence times in other lineages .

4. Genetic Divergence and Time

• As species evolve and adapt to different environments, genetic divergence occurs due to factors like genetic drift, natural selection, and mutation. The longer two species have been separated, the more pronounced their genetic differences are likely to be. This accumulation of differences serves as a molecular signature of their evolutionary history .

5. Statistical Models and Analysis

• Various statistical models are employed to analyze sequence data and estimate divergence times accurately. These models take into account factors such as population size, generation time, and variations in mutation rates across different lineages . By applying these models to genetic data, researchers can infer how long ago two species diverged based on the extent of their genetic differences.