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![The Fickleness of Fitness The fitness of an allele may vary over time, rather than remain constant. In this simple model, populations with two different alleles (black or yellow) seesaw between advantage and disadvantage as their relative fitness changes over time (blue line). Courtesy Weinreich et al.](https://medicine.at.brown.edu/wp-content/uploads/2017/09/allele-fitness-351x185.png)
The Fickleness of Fitness
A new study explains why it’s difficult to predict the evolutionary fate of a new genetic trait.