Climate
Let's take climate as an example. In a cold climate, animals need certain characteristics to survive, such as perhaps a warm furry coat, the ability to make burrows to live in and the ability to collect and store food for the winter. The selective pressure of cold weather means that animals that don't have these characteristics are less likely to survive and reproduce. In a hot, dry climate, plants will have an advantage if they have phenotypes such as the ability to store water, large root systems to absorb what little water is in the soil, and perhaps ways to prevent water loss even at high temperatures.