Stickleback fish found in lakes rapidly evolve a low armor plate morphology when compared to marine populations. Analysis of the
gene responsible shows that populations with low plating form one clade and marine populations form another. However, gene trees formed with other genes show that the low plating phenotype probably evolved independently multiple times. How have scientists resolved these contradictory gene trees. A. The mutation first appeared in a single lake, but gene flow between lakes connected by rivers has spread the mutation to new populations where it increased in frequency.
B. All of the populations in lakes are very closely related. The other genes suggesting different topologies of relatedness are biased by geographic covariation.
C. High armor evolved independently from a low armor state in the oceanic population, where it increased in frequency to near fixation.
D. A single origin of the low plating gene in the oceanic population that remained at a low frequency until lakes were colonized. Then the gene independently became fixed in each.
E. Mating between individuals of different populations has allowed introgression of the gene into many lake populations.
The correct answer is option D. "A single origin of the low plating gene in the oceanic population that remained at a low frequency until lakes were colonized. Then the gene independently became fixed in each".
Explanation:
In this example, the adaptation of low platting in Stickleback fish made difficult to establish a clade based on the gene responsible of this trait, because the analysis with other genes suggest that the genes evolved independently. The scientists resolved the contradictory gene trees by suggesting the existence of a single origin of the low plating gene in the oceanic population that remained at a low frequency until lakes were colonized. Then the gene independently became fixed in each. This explains why the fish share the gene, and gives a coherent explanation of how an external factor (colonization) affected the genetic linage of this trait.
