Trait frequency can change within a population lineage.
In this diagram, a population that was ancestrally fixed for white flowers passes through a period of polymorphism before ultimately becoming fixed for the derived trait of yellow flowers.
This image is linked to the following World Library of Science pages:
What do phylogenetic trees illustrate? These diagrams don't just organize knowledge of biodiversity - they also show us that living species are the summation of their evolutionary history.
