This study examines how third-graders' conceptions about the origin of species are affected by formal instruction and whether children can learn not only about evolution but also about natural selection. We interviewed the same group of third-grade children (8-9 years old) twice, before and after following a curriculum about these topics. Creationist answers were very rare at both pre- and post-tests. The number of evolutionist answers increased significantly after instruction. However, children learned about evolution in a piecemeal way; they gave evolutionist answers for only some of the animals they were asked about and made several errors when putting classes of vertebrates on a time line. In addition, only a minority of children learned something about evolutionary mechanisms. They said that new-born animals can be different from the parents because of mutations but did not mention differential survival. These results challenge the views, according to which, cognitive biases (i.e. essentialism, teleological and intentional thinking) make children unreceptive to evolution.