Competition for food happens when there is a limited amount of food resources available. The members of a species that eat that food who are best adapted to access the food are more likely to reproduce and pass on their genes to the next generation.
For example, we have a bird feeder with a narrow opening for food. The small-headed birds can reach the food, the bigger-headed birds cannot. If this was the only food source for this area, then over time, there would be more smaller-headed birds than large-headed birds, due to the competition for food and success/lack of success.
It is thought that life arose on earth around four billion years ago. The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes.
