Coelacanths and Lungfish VII

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     How aquatic vertebrates evolved into land vertebrates has been difficult for evolutionary biologists to study, in part because the shift from water to land appears to have occurred rapidly and has yielded a scarce fossil record. Prior to the advent of DNA sequencing, the primary guideposts in tracing the emergence of tetrapods have been morphological considerations, which have highlighted the coelacanth and the lungfish as species of interest.
     Coelacanths and lungfish are distinct from other fish in that they are lobe-finned species. Lobe-finned species, like ray-finned fishes such as tuna and trout, possess not cartilage but a bony skeleton, a key prerequisite for survival on land. Lobe-finned fish species are distinguished from ray-finned species by fins that are joined to a single bone and which thus have the potential to evolve into limbs. Coelacanths and lungfish are two of the only lobe-finned species that are not extinct, and since they have evolved minimally since the time of the appearance of tetrapods, they are sometimes referred to as "living fossils." In fact, the first live coelacanth was discovered more than 100 years after the species had been discovered in fossilized form.
     Whether the coelacanth in particular is rightly called a living fossil and whether it is the closest living relative of the original tetrapods are two questions that have been illuminated more recently by genetic analysis. The coelacanth's genome has recently been sequenced, and this analysis has led to the conclusion that the lungfish is the closer relative of tetrapods. Moreover, the coelacanth DNA has shown evolution over time--although at a rate much slower than that of most animals. Possibly, the fish's morphology and its environment deep in the Indian Ocean have created favorable conditions allowing a more slowly evolving species to have survived for the last 400 million years.

In the passage, the author is primarily concerned with doing which of the following?

Review: Coelacanths and Lungfish VII


Explanation 

If we believe we are equipped to make a prediction to a question prior to looking at the answer choices, it's almost always worth taking a moment to articulate that prediction. There are two reasons. First, arming ourselves with a prediction will make it easier to run the gauntlet of temptation of the answer choices without getting drawn away by something that sounds plausible. Second, having a prediction makes the evaluation of answer choices swifter.

So, in this case: we want the author's primary purpose. We answer to ourselves: the author describes how the bodies of c-fish and lungfish shed light the shift from water to land that occurred in animals. Let's look for that in the answer choices. (A) is quite close. (B) is out, because we don't hear all that much about the evolutionary history, especially about the lungfish. (C) happens in the passage, but it's not the main point; the term "living fossil" comes up only in the context of explaining the main point, which is described by (A). (D) is out because it describes a degree of opinion that never comes in the passage. (E) describes a fact which is false, because lungfish can't survive on land; they are still fish.

The correct answer is (A). Passage 8
















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