[Campbell Biology P.554] Refer to the provided figure showing the allele frequencies of lap9... | Practice Question

Refer to the provided figure showing the allele frequencies of lap94 and other lap alleles in Mytilus edulis mussels. How does the frequency of the lap94 allele generally change as salinity increases from Long Island Sound towards the open Atlantic Ocean, and what evolutionary mechanism best explains this pattern, considering the information provided?

  • A: The `lap94` allele frequency decreases; explained by genetic drift, as less selective pressure exists in higher salinity.
  • B: The `lap94` allele frequency increases; explained by natural selection, as this allele is advantageous in high-salinity environments.
  • C: The `lap94` allele frequency increases; explained by gene flow from the open ocean into Long Island Sound.
  • D: The `lap94` allele frequency decreases; explained by directional selection favoring other `lap` alleles in higher salinity.

Explanation

The map indicates that salinity increases towards the open ocean from Long Island Sound to the Atlantic Ocean . Observing the pie charts from site 1 through 11, the purple section representing lap94 alleles generally becomes larger as you move from left to right increasing salinity . The text explicitly states that 'The lap94 allele helps mussels maintain osmotic balance in water with a high salt concentration.' This suggests a selective advantage for lap94 in high-salinity environments, making natural selection the most fitting evolutionary mechanism to explain its increasing frequency with increasing salinity.