What is descent with modification?

Populations experience random and systematic changes in genetics over multiple generations (decent) that can lead to adaptations to their environment (modifications).

Random changes in genetic sequence occur primarily through . Systematic changes in genetic code exist in many forms: natural variation by (think of two brothers who are not twins that look different) and gene flow (as populations migrate and mate with each other) are two examples.

Adaptations to the environment is nature's way of ensuring the "best" genetics are most likely passed on to future generations. This was Darwin's major contribution to the theory of evolution.