In this post, I will argue that this requirement is commonly met by many types of simple, natural systems involving positional inheritance.
The thesis here will be that macroscopic variations in resources are common - and result fairly directly in heritable fitness. If resources are very evenly distributed, then the condition that fitness is heritable would not be met. Such extremely even distribution of resources can happen if the environment is near to equilibrium, for example.
With diffusion-limited aggregation systems, the concentration of aggregating particles can be greater in some places than others. In electrical discharge systems, the potential gradients can be greater in some places than others. With propagating cracks, the medium can be more brittle in some places than others. These situations are all commonplace ones.
The existence of heritable fitness is consistent with observed adaptations in these types of systems. Drainage basins are well adapted to rapidly dissipate the potential energy in the incoming rainwater - and form similar structures to drainage systems designed by engineers. Lightning strikes take the shortest path from the cloud to the ground. Cracks seek out lines of weakness - resulting in an adaptive fit between the actual cracks and the weak points of the material.
In practice, the requirement for heritable fitness is a pretty trivial condition which is almost always met. To evolve adaptations some additional, more stringent conditions are also required. Essentially, the selection pressure needs to out-weigh the mutation pressure. If it doesn't do so, you get an error catastrophe - and no adaptations. In other words, devolution - rather than adaptive evolution.