We know a great deal about the origins of mammals

The fossil record for the origin of mammals is superior to that for the origin of any other major group. It is therefore an important test case for general theories about how major evolutionary transitions take place.

Two important conclusions from the historical narrative:

• changes from reptilian to mammalian characters evolved in gradual stages;

• the large-scale differences between mammals and reptiles concern adaptations.

The mammals have a high-energy, high metabolic rate kind of physiology, with locomotory adaptations for rapid movements (upright rather than sprawling gait) and adaptations for powerful and efficient feeding (the mammalian teeth and jaw articulation). This jaguar opposite provides a good example of all these traits. These are surely adaptive changes, which would have been brought about by natural selection. Thus macroevolution is taking place by the same process - natural selection and adaptive improvement - as has been observed within species and at speciation; but the process is operating over a much longer period. This can be thought of as the “neo-Darwinian” theory of macroevolution.

In population genetics, neo-Darwinism explains microevolution by changes in the frequencies of pre-existing variants: most characters show variation and and the character evolves as its frequency distribution is altered by selection. Likewise, macroevolution can be explained without recourse to extraordinary kinds of variation: natural selection, on ordinary variation and mutation, is enough. That is how the mammals evolved from the reptiles.