What two key insights allowed darwin to come up with the therory of natural selection?

The key insights were that any species develops varieties of which some are more adapted to the environment than others. Along with this was the changes in the environment or the physical world by natural geological processes such as the creation of savannas from forested areas or also from the abilities and type of predators that would favor say man's upright gait in the former instance and a fleet footed coyote on the other hand to escape a predator.

Basically, the concept of natural selection refers to competition between species requiring the same resources within the same environment.

The theory suggests that species that are better adopted to the environment have a better chance to survive while the weak are phased out over time.

Charles Darwin was influenced by:

1) Weakening of genes for species that inter breed

2) Elimination of butterflies that didn't adopt to the changing environment in the UK occasioned by onset of industrialization.