Research suggests that students tend to explain the origin of biological traits in terms of needs or purposes and/or as the direct product of genes, rather than as the outcome of evolutionary and developmental processes. We suggest that in order for students to be able to construct scientific explanations, it is important to clearly and explicitly distinguish between causes that make a difference to the production of effects and the processes through which these effects are produced in developing evolutionary and developmental explanations for the origin of biological traits. However, these distinctions are not usually made in textbooks and curricula, and it is unclear how biology teachers themselves explain the origins of biological traits. In this study, we present the conclusions from the analysis of the explanations that 19 preservice biology teachers gave for the origins of biological traits. The results indicate that they often referred to causes and processes in their evolutionary explanations, but that they did not always do this in their developmental explanations. We suggest that future biology teachers should understand the structure of explanations for the origins of traits to efficiently teach their students to construct such kinds of explanations.