Due to the overproduction of offspring with only finite resources, species begin to engage in competition for survival. In order to increase their chance of survival, certain species will begin to develop genetic variations that allow them survival advantages.
The pollen grain is actually the male gametophyte generation of seed plants. Inside the anther, pollen mother cells divide by meiosis to form pollen grains whose nuclei contain half the number of chromosomes characteristic of the parent plant.
(hope that helped answer the question)
