Biome spread

The evil biomes and the Hallow spread themselves, unlike all other biomes. This means that they slowly convert certain susceptible tiles which are near their own, biome-exclusive tiles, to their respective biome counterparts, effectively increasing the area of the biome. They can possibly convert the entire world this way. Biome spread starts immediately upon world generation, is greatly enhanced and sped up after defeating the Wall of Flesh, and is slowed down again after defeating Plantera.

Spreading tiles
Only the following tiles of a biome are capable of converting susceptible tiles in their vicinity:

Pre-Hardmode spread
In pre-Hardmode, the three biomes can only spread to a very limited number of tiles:

Hardmode spread
In Hardmode, the spread is greatly sped up and many more tiles become susceptible to conversion:

Defeating Plantera for the first time reduces the speed of the biomes' spread to 1/2 (50%) of the default Hardmode speed.

Trivia

 * Even though Crimson and Hallowed plants and spread their biomes, neither do Corruption plants nor Vile Mushrooms.