http://www.manhattangmat.com/forums/paleontologist-stephen-jay-gould-has-argued-that-many-t1413.html
Your first thought here probably was - and should be - parallelism. However, you're right that 'are not ... but are' and 'are not ... but' would both be legitimately parallel constructions in this context. So, the reasoning goes, we should take the one with one fewer word. Right?
Here's the problem: In the context of this longer, more complicated sentence, the 'but' (without 'are' after it) is, unfortunately, ambiguous. Consider this sentence:
Jimmy was not the typical class president, loved by most of the students, but also hated by a fair number as well; rather, he was loved by all the students.
The most probable reading here is that all of the words 'loved by most of the students, but also hated by a fair number as well' are a description of 'the typical class president.'
If you look at choice C, it can be read this way; one could (probably should, even) take ALL the words '...favored because they enhance reproduction or survival, but simply random byproducts...' as an elaboration on the phrase 'products of natural selection'.
It's harder to see than in the example of Jimmy, above, for two reasons:
(1) Because of the way the original sentence is written, you're biased toward reading the sentence the way it's 'supposed' to be written. (In the example with Jimmy, there's no prompt sentence, so you're free to read it as you like.)
(2) It's longer.
-- by 会员 willways (2010/8/10 0:08:12)
这句话的意思是：某学家SJG认为很多生物特征不是物竞天择产生的，因为这些特征提高了繁衍或是生存，而是进化过程中随机的简单的副产品（意外结果）（PS：我翻译得很菜哈哈哈·~(*^__^*) 嘻嘻……）
这个地方LZ在看到选项的时候应该很快可以缩小选择范围，即是CD（PS：我发现一般这种两个选项大体一致，个别不一样的时候，基本答案就是这两者之一）
比较下来发现D比C多一个are
又因为but的用法: but做介词,表示除了… 还可以用在词组not…but……，所以为了不产生歧义就选D了
但是LZ说的从这个地方得到的判定我认为不可以作为绝对错误来做题
GMAT的语法是选优，所以LZ可以在用绝杀完后的比较当中再来择优
-- by 会员 魏小妞要出国 (2010/8/10 0:05:58)