How can variation lead to 2 different species that can no longer interbreed?

1) Over a period of time, a species will gradually evolve 2) Both genes and the environment can cause variation, but only genetic variation can be passed on to the next generation 3) If two populations of one species become increasingly different in phenotype that they can no longer interbreed to form fertile offspring, this can result in the formation of two species EXAMPLE: Peacocks - females are more likely to mate males with more colourful tails, overtime peacocks have become more colourful, Bacteria - becoming resistant to antibiotics