Long Neck card in evolutuon
Warning: This answer is about Evolution (with or without the Flight and Climate Change expansions) as you specified. It is possible you are actually asking about the related-but-different Evolution: The Beginning. See this answer for the answer about Evolution: The Beginning.

Read on if your Long Neck card reads:


  Long Neck: When the Food Cards are revealed, take 1 Plant Food from the Food Bank.




The species gets one extra food before anyone else. Population doesn't enter into this (except that you can't place more food on your species than its population).



Before anyone starts feeding, species with a Long Neck will be fed one food (from the Food Bank, not the Watering Hole).

This doesn't matter much if there's enough food to feed every species of every player[1]. But it helps if there's a food shortage.

For example, let's say there are four players, and each player only has a single species with a population of two and no traits. There are only 5 food in the Watering Hole.


Player 1 takes a food from the Watering Hole.
Player 2 takes a food from the Watering Hole.
Player 3 takes a food from the Watering Hole.
Player 4 takes a food from the Watering Hole.
Player 1 takes a food from the Watering Hole.


Result:


Player 1's species is satiated.
Player 2's species's population reduces to 1.
Player 3's species's population reduces to 1.
Player 4's species's population reduces to 1.


But what if Player 3's species had a Long Neck?


Player 3 takes a food from the Food Bank.
Player 1 takes a food from the Watering Hole.
Player 2 takes a food from the Watering Hole.
Player 3 takes a food from the Watering Hole.
Player 4 takes a food from the Watering Hole.
Player 1 takes a food from the Watering Hole.


Result:


Player 1's species is satiated.
Player 2's species's population reduces to 1.
Player 3's species is satiated.
Player 4's species's population reduces to 1.





It could matter if the species is a burrowing species. It also gives the species a chance to eat before any carnivore can reduce its population.