Hi Vineet,
The last sentence of the passage is a subsidiary or intermediate conclusion. An intermediate conclusion is a conclusion because it itself is supported by other premises. But it is intermediary because it is relied upon to support the main conclusion. The rhetorical function of all intermediate conclusions is, therefore, that of evidence.
An easy way to realize that the second sentence is the main conclusion is pattern recognition. If the author opens with something like "many economists claim" or "the traditional theory is", and then follows this up with a contrast keyword, nine times out of ten the author's motivation in arguing is to discredit what economists claim or to discredit the traditional theory. Therefore, the author's conclusion is just "the economist's claim is wrong".
Here, the first two sentences are:
"It is well known that many species adapt to their environment, but it is usually assumed that only the most highly evolved species alter their environment in ways that aid their own survival. However, this characteristic is actually quite common."
As soon as we get to "however", we can anticipate that the author's conclusion is that that which is "well-known" is in fact incorrect.
What is "well-known"?...that only the most highly evolved species alter their environment in ways that aid their own survival.
"However" tells us that the author is arguing the negation of this. What's the negation of the"well-known" position?...that even species that are NOT highly evolved (ie, even simple) species can alter the environment in ways which aid their own survival. That's the author's conclusion, which again, we coud have anticipated as soon as we got to "however". And, sure enough, the rest of the argument is designed to support that conclusion.
If you are not sure which of two statements is the conclusion, another tactic you can apply is asking yourself "which idea supports the other?". The supporting idea is evidence while the supported idea is conclusion. Here: does the last sentence work to support the idea that even simple species can alter the environment in ways which aid their own survival?...Yes, it does....therefore, the last sentence must be evidence.
Kaplan Teacher in Toronto