Sex determination in fish is a very flexible process with respect to evolutionary patterns observed among genera and families, and within individuals is subject to modification by external factors.

and并列的是二个系表结构。句子的结构式Sex determination in fish is a very flexible process and is subject to modification by external factors. 第一个系表结构被with respect to介词短语修饰，第二个系表结构被within介词短语修饰。只不过第二个系表结构的状语提前到了系表结构之前。

理解这个句子的关键是第二个系表结构的is, 其主语应该是个单数或不可数名词，你只需要在句子中少一个单数或不可数名词，并且这个名词与系表结构表达一个符合逻辑的意义，你的理解就是正确的。