Useful Concepts

Natural Selection and Adaptation
Individuals inherit traits, or features, from their parents. Those members of a species with traits that help them survive in a particular environment—like the sharp, piercing canines and slicing molars of carnivores versus the flat grinding teeth of mammals that eat tough grasses—pass on these characteristics. Generation after generation, individuals with the advantageous trait, or adaptation, will survive longer and produce more offspring, until most or even all members of the species possess it. Called natural selection, this is an important mechanism of evolution.

Convergent Evolution
This refers to situations in which different groups evolve similar adaptations because they live in similar environments. These species may even live on different continents and be far apart on their family trees. For example, high-crowned grinding teeth for chewing tough grasses have evolved independently in a diverse suite of mammals, including bison, horses, elephants, some rodents, and many extinct mammal groups.

Mammal Reproduction

The ancestor of all mammals almost certainly laid eggs, as do most vertebrates and a tiny minority of living mammals, the monotremes—like the platypus and echidnas of Australia and New Guinea. But the vast majority of modern mammals are placentals. They've evolved to give live birth to babies that are nourished for a long time inside the mother's body, using an organ called a placenta. A few hundred other living species (and many more fossil forms), like koalas and kangaroos, are marsupials; their young are born very immature and much of their development occurs while drinking milk, typically inside a pouch on their mother's belly. The group Mammalia is named for mammary glands, which produce milk—as all mammal species do.