A large proportion of archaeans are extremophiles, so called because they inhabit extreme environments with high acidity, salinity, and/or 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 the following 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 the amount of genetic change, the branches of the extremophiles should be shorter than 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 habitats have remained relatively unchanged, then modern extremophiles should still possess many of the ancestral traits that enabled their ancestors to adapt to extreme conditions. Therefore, choice A is incorrect. The fact that extremophiles are adapted to extreme environments suggests that they have undergone genetic changes that allowed them to survive in these environments. Thus, the branches of the extremophiles should be longer, not shorter, than those of non-extremophilic archaeans. Choice B is also incorrect. There is no reason to believe that extremophiles contain genes that originated in eukaryotes, so choice C is incorrect. Finally, because they are adapted to specific environments, there is no reason to believe that extremophiles are undergoing a high level of horizontal gene transfer with non-extremophilic archaeans, so choice D is also incorrect.

A Large Proportion of Archaeans Are Extremophiles, So Called Because