It is unclear to which other species it is closely related.

直接的特殊疑问句，可以把介词放在特殊疑问词之前。例如：

1. Which other species is it closely related to? 

2. To which other species is it closely related? 

它与其他哪些物种有密切关系？

把上面两句变成名词性从句：

1. I don't know which other species it is closely related to.

2. I don't know to which other species it is closely related.

网友的句子， to which other species it is closely related 是主语从句，it 是形式主语。

我再举几个介词提到特殊疑问词之前的例子：

To what degree are you interested in botany? 

你对植物学的兴趣达到什么程度？

For what do we live? 我们活着是为了什么？

For whom, for what, was that bird singing? 

鸟儿啊,你这样唱,是为了谁人, 是为了什么?

来自英汉文学