The desert is hot and dry, while the tundra is cold and frozen. In a desert climate, anatomical structures and behaviors that protect an organism from heat are useful adaptations. Most organisms have adaptations that help them conserve water, because water can be scarce in desert climates. For example, many organisms in the desert are nocturnal, which means they’re most active at night when temperatures are cooler.
A genetic bottleneck also known as a population bottleneck is one that describes a sharp reduction in the size of a population caused by environmental events like in the above or caused by human activities.