Adaptation has produced more than a million different species of animals on Earth. To make sense of it all, scientists divide them into groups according to how they are related.
The animal kingdom consists of several big groups called classes. Reptiles, birds and mammals are all different classes.
Animals in each class share certain features that make them different from animals in other classes. For example, reptiles, such as snakes, are cold-blooded. Birds diverged from reptiles during the Jurassic period 201.3-145 million years ago. Reptiles have scaly skin and most of them lay eggs. Birds lay eggs too, but they are warm-blooded and their skin is covered in feathers instead of fur.
Mammals, such as cats, are also warm-blooded, but instead of laying eggs they give birth to live young, which they suckle on milk.
Orders and families
Each class is divided into smaller groups called orders, and each order is divided into families.
For instance, ducks, geese and swans belong to one order of birds that all have waterproof feathers and webbed feet for swimming.
Swans are one family within this order. They all have long necks for reaching underwater to feed on the bottom.
A family contains a number of species – or types. Each species is different from all others. It can raise healthy young only by breeding with others of its own kind. So, in the case of the swan family, the mute swan is one species. You can tell it from other swans by its unique features, like the black knob on its red beak.
There are nearly 10,000 different species of bird. They range from the huge ostrich, which stands taller than a person, to the tiny bee hummingbird, which weighs less than an acorn. Many, such as the chiffchaff and willow warbler, look very similar to each other. But each one is adapted to cope with the environment in its own unique way.