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 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 extremophiles' habitats have remained relatively unchanged, then their adaptation to extreme conditions should not have required significant genetic changes. This would result in relatively shorter branches on a phylogenetic tree (i.e., less genetic change) for the extremophiles compared to non-extremophilic archaeans. Therefore, B is a valid statement, while A, C and D are not supported by the information given.

A Large Proportion of Archaeans Are Extremophiles, So Called Because