Coelacanths and Lungfish I

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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.

The passage provides information in support of which of the following assertions?

Review: Coelacanths and Lungfish I


Explanation 

In this question, as in every question that involves a statement "inferred from" or "supported by" a passage or prompt, we should first look for something that is required. If one of these statements is required by the passage, then it's certainly well supported. This slightly non-obvious relationship is not really a testmaker trick; rather, it is a byproduct of the testmaker's efforts to write questions in which one answer choice is objectively better than the others.

Digging in, we look for a statement that must be true or which is assumed by the passage. Choice (A) may be the most confusing, so we can come back to it. Choice (B) involves physical similarities, choice (C) involves fossils and choice (D) involves physical similarities. What is the key point in this passage about fossils and physical similarities? It's that fossils are hard to obtain, so scientists used physical similarities in their studies. That's how the c-fish and the lungfish become fishes of interest. So the passage must believe (B); it's grounds for the line of reasoning. (C) and (D) are somewhat opposite; if the author believed them, they would pose a threat to the passage, so the passage does not support them.

At this point, we could go fairly confidently with (B) or we could confirm objective flaws with choices (A) and (E). Choice (A) is not supported by the passage. The passage admits a degree of uncertainty about who is the closest relative, but it ends up decisive that the lungfish, not the c-fish, is the closest relative of the land animals; this implies that a path has been identified as correct, contrary to (A). So (A) is out. (E), meanwhile, is a jumble of fragments from the passage. The c-fish and the lungfish became species of interest because of their morphology, their skeletons, not specifically because their DNA hadn't changed.

The correct answer is (B).


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