The purpose of this study is to examine effect of guide materials developed in ‘adaptation and natural selection’ subject on grade 8 students’ alternative conceptions. This study was conducted with 31 grade 8 students in spring semester of 2009-2010. In this context, to define the students’ conceptual problems cited in related literature, an extensive literature review was initially carried out. Adaptation and Natural Selection Concept Test was employed as a pre-test one week before the teaching intervention while the same test was immediately re-administered as a post-test after the teaching intervention. Moreover, to investigate its influence on their longterm memory, the same test was administered as a retention test two months after the posttest. Pre, post and retention test results were exposed to descriptive statistics and post-hoc tests using SPSS 15.00TM. The results showed that the guide materials of the “adaptation and natural selection” subject were not only efficient in enabling the grade 8 students to achieve conceptual change but also substantially eliminating their alternative conceptions. Some recommendations were made in light of the results of the study

Keywords: Alternative conceptions, adaptation, natural selection, guide material