If the frequency changed to 60% and 40%, the population would be evolving.
Evolution is determined as changes in the population of a species. This implies over time one or more changes will occur in a population, these changes are usually a change in the frequency of a trait, in this case, the frequency of snails with/without bands.
The population is evolving if the Hardy-Weinberg equilibrium is lost, this means the alleles are changing from one generation to the other.
Based on this, evolution is occurring in this population of snails.
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