It occurs with natural selection ...For example, there is a whole colony of fish in a pond. Some of the fish are faster than the others. They reproduce and pass on their genes - these genes make their offspring fast as well. The other fish that aren't as fast, die out because they are not fast enough. The fast fish soon become more common, making the species as a whole faster. This occurs in several stages, where the slowest fish are removed each time, and the faster ones reproduce more often. Gradually, the fish get faster and faster as a group.
Accumulation of variations for a long period of time, leads to evolution.
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