Biodiversity forms patterns not just across space, but also across time. New species emerge, old ones become extinct; rates of diversification speed up and slow down. These long-term patterns in evolution get their start in the generation-to- generation processes of natural selection, genetic drift, and reproductive isolation. When a lineage of organisms evolves over a few million years, these processes can potentially produce a wide range of patterns (see ). Natural selec- tion may produce a significant change in a trait such as body size, for example. On the other hand, the average size of a species may not change significantly at all (a pattern known.
