<p>Observation of diverse animal species A(show that) the B(most successful) in the struggle for survival are C(those which) are most D(adaptable to) changes in their world.</p>
<p>I think there are two possible errors here.</p>
<p>First one is A, which should've been "shows that"
and the second one is C, which should've been "those that"</p>
<p>I always thought that a comma is needed right after which, so I'm really confused.</p>
<p>They don’t distinguish between them in the sense that they don’t expect you to use one with a comma and one without a comma, if that’s what you mean.</p>