Explanation
Correct answer: D.
Choice D is the best answer. “That is” logically signals that this sentence clarifies the terms of the scientific debate introduced in the previous sentence by explaining the difference between dispersalists and vicariancists.
Why the other choices are wrong
Choice A
Choice A is incorrect because “furthermore” illogically signals that the information in this sentence is merely additional to (and separate from) the information in the previous sentence about the scientific debate. Instead, the information about dispersalists and vicariancists clarifies the terms of that debate.
Choice B
Choice B is incorrect because “by contrast” illogically signals that the information in this sentence contrasts with the information in the previous sentence about the scientific debate. Instead, the information about dispersalists and vicariancists clarifies the terms of that debate.
Choice C
Choice C is incorrect because “similarly” illogically signals that the information in this sentence is merely similar to the information in the previous sentence about the scientific debate. Instead, the information about dispersalists and vicariancists clarifies the terms of that debate.