A large proportion of archaeans are "extremophiles," so called because they inhabit extreme environments with high acidity and/or high temperature. Such environments are thought to have been much more common on the primitive Earth. Thus, modern extremophiles survive only in places that their ancestors became adapted to long ago. Which of these is, consequently, a valid statement about modern extremophiles, assuming that their habitats have remained relatively unchanged?
A)Among themselves, they should share relatively few ancestral traits, especially those that enabled ancestral forms to adapt to extreme conditions.
B)On a phylogenetic tree whose branch lengths are proportional to amount of genetic change, the branches of the extremophiles should be shorter, relative to branches of the non-extremophilic archaeans.
C)They should contain genes that originated in eukaryotes that are the hosts for numerous species of bacteria.
D)They should currently be undergoing a high level of horizontal gene transfer with non-extremophilic archaeans.

Question

Quizplus · Accepted Answer

If the extremophiles' habitats remained relatively unchanged, then their ancestors became adapted to these extreme conditions long ago. As a result, the genetic changes required for adaptation have already occurred in the ancestors, which means that the branches of modern extremophiles on the phylogenetic tree should be shorter than those of non-extremophilic archaeans. Thus, choice B is the best answer. Choice A is incorrect because modern extremophiles would still share ancestral traits that enabled their ancestors to adapt to extreme conditions. Choices C and D are unsupported by the information provided in the passage.

A Large Proportion of Archaeans Are "Extremophiles," So Called Because