Unless big changes to the environment happen at least once every generation, then there is no way that diversity of offspring will be selected for. Say you have

Unless big changes to the environment happen at least once every generation, then there is no way that diversity of offspring will be selected for. Say you have a population of 100,000 organisms and 1 randomly mutates to produce more diverse offspring. That gives the gene a 0.001% occurrence in the population. Hardy-Weinberg principle says that gene will remain at about that percent of the population unless it is selected for. So if a generation passes without selection for that trait, there is a very high probability that gene's occurrence drops back to 0%. In other words, without a constant source of selection, there is no way for a trait to become fixed in a population.