In each generation , the fitness of every individual in the population is evaluated , and multiple individuals are stochastically selected from the current population ( based on their fitness ) , and modified ( recombined and possibly randomly mutated ) to form a new population . In each generation , the fitness of every individual in the population is evaluated , multiple individuals are stochastically selected from the current population ( based on their fitness ) , and modified ( recombined and possibly randomly mutated ) to form a new population .