Environments are constantly changing, so the organism must adapt. Sexual reproduction ensures genetic diversity in the progeny. If one or a few of the progeny have new traits that can help it survive in its changing environment, it is more likely to reach adulthood and reproduce.
If a species becomes isolated, or reproduces asexually it will not adapt to its environment, its traits will be exact copies of its parents and the species will eventually go extinct, that particular scenario only works in areas where the environment is stable and not subject to change.
