Species evolve over time as a result of natural selection. As per Darwin's theory, a species tends to overpopulate. This results in a competition. No two individuals will have the exact genetic makeup therefore, any individuals with an adaptation that is beneficial in a particular environment, may win the struggle for existence. If it survives, it may pass down its traits to offspring. If this occurs, it is considered to be "fit"--thus the term, survival of the fittest. Over time, as enough of these variations are passed down which provide an advantage in a particular environment, ultimately, speciation may occur and a new species may evolve from the ancestral population. Of course, environments are subject to change therefore, an adaptation in one environment that may be beneficial, may not be if that environment should change. Therefore, evolution is an ongoing process.