A scientist sequences the genomes of several species and analyzes their evolutionary relationships to a single common ancestor. The percentage of similarity between the genome of each species to the genome of this ancestor is calculated and shown in the table below. Which species diverged from the common ancestor first?

Question

A scientist sequences the genomes of several species and analyzes their evolutionary relationships to a single common ancestor.  The percentage of similarity between the genome of each species to the genome of this ancestor is calculated and shown in the table below.  Which species diverged from the common ancestor first?

Quizplus · Accepted Answer

11ecba2d_1013_cb2d_a3bf_6b58529f3b26_MD0008_00 Based on the neutral theory of molecular evolution, the majority of genetic mutations are neutral, which means that they do not affect the fitness of an organism. These neutral mutations are randomly fixed or lost due to genetic drift and occur at a fairly constant rate over time and across different species. After two species diverge (separate), the accumulation rate of neutral mutations in a region of homologous DNA is mostly similar in both species. Accordingly, this "molecular clock" of evolution is said to be linear because the number of neutral mutations generally increases with time. Therefore, the molecular clock model allows researchers to measure evolutionary time by analyzing random changes in the genome over time. For example, species that diverged more recently from a common ancestor have accumulated fewer mutations across a shorter time and are more genetically similar to one another and their common ancestor. In contrast, species that diverged less recently (eg, a long time ago) from a common ancestor have accumulated more mutations across a longer period and are less genetically similar to one another and their common ancestor. In this scenario, the genome of Species D is least similar to that of the common ancestor compared with the other species. This means that Species D diverged from the common ancestor first and accumulated more mutations across a longer time, which led to a small percentage similarity of 22% with the common ancestor. (Choice A) Because Species A is most similar to the common ancestor, it has had less time to accumulate mutations and diverged from the common ancestor last. (Choices B and C) Species B and Species C have 88% and 35% genome similarity with the common ancestor, respectively. This means that both species likely diverged from the common ancestor after Species D, which diverged first. Educational objective: The molecular clock model uses the theory that most genetic mutations are neutral and occur at a fairly constant rate across organisms to estimate the amount of time elapsed since species diverged from their common ancestor.

A Scientist Sequences the Genomes of Several Species and Analyzes