An example of biological evolution is the evolution of the tiger, during which time the tiger's ancestors grew in size due to an abundance of larger prey animals. This led to the large size and musculature, as well as the stripe adaptations to more easily blend into their specific hunting ground.
Charles Darwin is credited with the first well-documented argument for the scientific ideas behind evolution, citing the processes of natural selection. Natural selection is the process at which specific traits are favored through environmental conditions, where more cunning or adapted animals are more likely to survive and pass on their evolutionary changes to their offspring. The common modern definition of evolution is also known as descent with modification, especially in the scientific circles where this distinction is more important.
Attributes like the intelligence in humans, or the coexistence of cells in any complex organism can be reasoned to be caused by evolution. When a species goes extinct by natural disadvantages, either from flaws in its own adaptations or radical changes in its environment, this is considered a natural outcome of natural selection.
Predators evolve stronger sight, smell and hearing to catch prey, and prey develop the same to avoid being eaten. Plants evolve new chemical processes to discourage, or encourage, animals to eat them. Wherever an adaptation happens, the other living things, whether plant or animal, change to match it, making life a continual fluctuating chain that constantly demands new approaches to survive.