so为何替代主谓了

As my own studies have advanced, I have been increasingly impressed with the functional similarities between insect and vertebrate societies and less so with the structural differences that seem, at first glance, to constitute such an immense gulf between them.

以前一直了解so可以替代宾语，但这句话中明显是替代主谓了，这是什么替代规则？

less so =I have been increasingly less impressed的省略形式

so可以是分句替代词，替代前面说过的一个句子。比如，Has he left? I don't think so. = I don't think he has left. so替代he has left这个分句。你应该见过so的这种使用方法的。