Adandelion seed floats across a lake and lands on an island. the seed sprouts and dandelion plants gradually spread across the island. these plants have larger flowers than most of the other dandelion plants on the shoreline of the lake. why are the dandelion plants on the island genetically different from the larger population

This genetic difference is due to the founder effect.
The founder effect is an evolutionary term referring to any change in the genetic pool due to a migration of a small group. When a small number of individuals leaves the bigger population and travels to another environment to establish a new population, then the genetic variation is decreased and as a result, the new population has different genotypic and phenotypic characteristics.
In this context, a dandelion seed migrates and establishes a new population on an island. The plants of the new population differ genetically and phenotypically (size of their flowers) from the bigger population. 

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Explanation:

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