Conclusions on the LSAT do one of three things:
1) Interpret evidence
2) Make a recommendation
This one does #3 - The hint is "It is a mistake to think, as ecologists once did..." which means, "I disagree with ecologists who said..."
"After all..." indicates premise--information they're asking you to take for granted. This premise is followed by a semicolon, which basically means, "I'm about to expand on, support, or say the same thing another way."--in other words, here comes another premise.
You can double check this by taking what we think is the conclusion, ask, "Why should I believe this?" and seeing if the premise answers your question.
Why is it a mistake to think natural selection will result in perfectly adapted organisms? Well, because perfect adaptation is impossible.
After finding this, we'll see (B) sums up the conclusion nicely.