There is a special button that allows you to do just that.

Make sure you don't have any experiments already running in the Laboratory. Then tap on the habitat where you have a single animal settled.

Tap the "paw" button on the right side. You will see a button with a flask icon.

Tap it to open the Laboratory. You will see two animals set up for an experiment that can give you a mate for the one in the habitat.

Be careful: If you've never gotten this species in the Laboratory or this species cannot be made via experiments, you will see the animals from your last experiment selected in the Laboratory instead.