So I have been looking at the past psychology papers and found one question that is similar in marks and question layout as the evolutionary explanation of aggression one we recieved today. The question was 'Discuss genetic factors involved in aggressive behaviour (4 + 16 marks)' and the mark scheme for this question said
"Note that although the question refers to ‘factors’, there are no partial performance criteria onthis question part. Answers providing detailed outlines of one factor may earn AO1 marks inthe top band, while examiners should also be sensitive to depth-breadth trade-offs inanswers that cover more than one factor"
Therefore, I believe that writing about only one (or writing about both) will be awarded marks as AQA are aware of time constraints and want answers to be focused and coherent. I am not an examiner, obviously, but we should not panic and fingers crossed, we will all be fine. Too late to change it now!