Fitness and Selection

 Absolute fitness measures the number of genes an organism has passed down to its offspring or the total number of surviving offspring it produces. Relative fitness measures an organism's number of surviving offspring compared to those in the population. Let's take a population of dolphins for example. One dolphin has a really strong tale that makes him able to outswim predators making him more likely to reproduce. In his lifetime he has produced 30 surviving offspring with this stronger tail trait making his absolute fitness 30. The majority of the dolphin population does not have this stronger tail making them less likely to escape predators, so the dolphin's population average of surviving offspring is 20 making the individual dolphin's relative fitness 1.5. 

If the dolphin population went through positive selection the trait that would go through this selection would be the stronger tail. As this trait continues to be passed down through the generations the dolphins become better at escaping their predators, so the population is better at reproducing, so the population grows over multiple generations. This change would take many generations of reproducing to spread this trait. This change is driven by the dolphin's ability to escape their predators.