On Biology.SE, all other StackExchange sites, and until recently Stack Overflow, the answer that appears first is the accepted answer (if there is one), the one that the OP decides is to be marked "correct". It is followed by all the others in order of votes by default (users can also change to sort by Active or New).
As a result of user feedback, especially about "accepted answers" that have become outdated, and a test that suggests users on Stack Overflow benefit from the top answer being the one with most votes rather than the one that is accepted, this behavior has now changed on Stack Overflow.
Also quoted from that Meta announcement:
We can change the way the engine sorts answers in site settings. We would like to hear from you all if it is something you want to see on your site. (Please let me acknowledge in advance that we will not be able to run a test on each site.)
Currently we are planning to move forward with one of two scenarios, based on your feedback:
Unpin the accepted answer on all SE sites by default and pin it back on a few sites that ask us to do so.
Keep the accepted answer pinned on all SE sites by default and unpin it on a few sites that ask us to do so.
Please let us know what you think will work best for your site! If you can discuss this question with your community it would be awesome. We are going to collect feedback before the end of September 19th.
So, what do y'all think? Whichever way the default ends up going, how would you like things to behave on this site?