According to Darwin's Theory of Natural Selection, each generation produces more individuals than the ecosystem can sustain. However, not all individuals are identical and phenotypical variation exists.
Due to this variation, some antelopes are better at escaping lions, perhaps due to speed and/or stamina. Thus, the individuals that are slower and thus less fit will be killed and eliminated from the herd. The antelopes with heritable traits that makes them better suited to the environment, will escape and thus, survive and reproduce.
In Darwin's Theory predation is an evolutionary force.
Cats and dogs are both mammals because they have hair, are warm-blooded, and nourish their young with milk. Dogs and cats are distinct species, incapable of producing offspring together because of the many differences in their genome.
