Evolution is the change over time of a species. It is thought that all organisms originated from a common ancestor. Evolution occurs on the molecular level- from changes arising due to the change of genetic material (DNA). Sometimes these changes are due to random mutations that end up benefiting the organism. Most often, evolution is due to natural selection. Natural selection occurs when variations within a species are selected by the environment in which the species lives. Organisms that contain variations that are advantageous to the environment survive and more likely to find a mate. Thus, their genes are passed on to future generations. Individuals with less advantageous traits die due to the external pressures of the environment (nature). Thus, their genes are eventually lost from the species' gene pool.
Defense mechanisms are traits or behaviors of a species that helps that species defend itself against its predators. Examples can be viewed via this hyperlink. Thus, if a species has evolved a better defense mechanism, then it will be able to defend itself better against its attackers. The species (prey) is more likely to survive. Thus, this defense mechanism will be passed on to future generations of the species. If the defense mechanism is strong enough, the predator will be less likely to prey on the species. If this occurs, then the population size of the prey may grow. If the prey was the main source of the predator, then some of the predators may starve. This would result in a decreased population size of the predators.