Apple maggot flies are thought to have evolved from hawthorn flies by sympatric speciation. Hawthorn flies are native to North America. They lay their eggs in the berries of the hawthorn bush. The eggs hatch into maggots and the maggots develop into adult flies. At different distances from the front edge of the glacier, there will be variations in soil conditions. These soil conditions include: soil depth, the carbon : nitrogen ratio and the organisms present in the soil. These differences depend on the length of time the ground has been exposed by the retreating glacier. Which term describes the changes in soil conditions and organisms with time? (1)

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