Darwin states that natural selection is the main reason for the evolution of life. The fight for food, water and other necessities benefits those creatures who are well adapted for the struggle. Those that cannot survive, die with no offspring to continue their genetic line. Natural selection is also called survival of the fittest. Another related idea to evolution is gradualism.
Genetic drift is another way that scientists define evolution.
When two of a species mate, their offspring gets 23 chromosomes from both parents. When a gene does not split and combine correctly, a mutation occurs. This mutation will get passed down from the creature to its offspring. Scientists who have accepted the general theory of evolution as fact disagree among themselves about the ratio of importance between natural selection and genetic drift. They also disagree about what caused the apparent gaps in fossil layers.