Which choice best describes the function of the underlined portion in the text as a whole?
A
It explains why parasites are less susceptible to horizontal gene transfer than their hosts are.
B
It clarifies why some genes are more likely to be transferred horizontally than others are.
C
It contrasts how horizontal gene transfer occurs among vertebrates with how it occurs among invertebrates.
D
It describes a means by which horizontal gene transfer might occur among vertebrates.
Explanation
Correct answer: D.
Choice D is the best answer. The text defines horizontal gene transfer and then gives one possibility for how it happens in vertebrates (via infection by parasites). The underlined part describes how that mechanism could work.
Why the other choices are wrong
Choice A
Choice A is incorrect. The underlined portion doesn’t do this. Parasites are only described as the mechanism that does the transferring, not the species that gives or receives the genes.
Choice B
Choice B is incorrect. The underlined portion doesn’t do this. The text never discusses which genes are more likely to be transferred.
Choice C
Choice C is incorrect. The underlined portion doesn’t do this. The text never discusses how horizontal gene transfer occurs among invertebrates.