# File lib/libgenetics.rb, line 159
  def mutation(mutation_rate)
    parents = @individuals.dup # genomes that haven't been parents yet
    next_population = Population.new(0)
    while parents.length > 0
      parent = parents.pop
      child = parent.mutate mutation_rate
      next_population.individuals << child
    end
    next_population
  end