In the United States, lay-adults with a range of educational backgrounds often conceptualize species change within a non-Darwinian adaptationist framework, or reject such ideas altogether, opting instead for creationist accounts in which species are viewed as immutable. In this study, such findings were investigated further by examining the relationship between religious belief, scientific expertise, and ecological reasoning in 132 college-educated adults from 6 religious backgrounds in a Midwestern city. Fundamentalist and non-fundamentalist religious beliefs were differentially related to concepts of evolution, adaptation, and extinction. Biological expertise (r = .28) and creationism (r = –.46) were significantly and differentially related to the endorsement of the Darwinian concept of common descent. Yet, creationists were more likely to reject macroevolutionary than microevolutionary concepts. Overall, the greater the taxonomic distance between species, the less likely were participants to agree that species-pairs had common ancestors. It is argued that lay adults from contemporary industrialized societies adopt a view of evolution in which species adapt to novel environments, but remain the same "kind" despite changes. Therefore, extinction is considered unlikely and the relations between micro- and macroevolution misconstrued. Lay-adults' species concepts appear to be an amalgam of a common-sense understanding of species and of evolutionary ideas, modified but not transformed by religious and scientific beliefs. Finally, it is argued that the development of scientific expertise does not involve the radical transformation of ingrained worldviews. Rather, scientists select specializations that are compatible with their existing philosophies, then consciously apply the constructs of their disciplines in order to transcend their common-sense folk beliefs.