Answer:
The reason most species are successful is because of Genetic Variation.
Step-by-step explanation:
Let's say two species are place into a new environment, they have offspring.
Now lets say the mother, has a certain trait that's beneficial to this new environment. Later comes natural selection, then the offspring with that trait most likely live on to reproduce. Then so on happens throughout generations. Genetic Variation is one of many key factors that help with adaptation.