One of Many Scenarios (IMAGE) Brown University Caption The fitness of a genetic trait (an allele) may vary over time, rather than remain constant. In this simple model, populations with two different alleles (black or yellow) see-saw between advantage and disadvantage as their relative fitness changes over time (blue line below). Credit Weinreich et. al. Usage Restrictions with credit License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.