An understanding of microevolution, or adaptation, is necessary for predicting the effects of disturbances on ecosystems, including climate change and other human impacts. This paper focuses on students’ reasoning about adaption in the complex ecosystem environment. That is, when the traits involved are providing an advantage in a community situation more complicated than simple predator-prey interactions with strong selection pressures (e.g., situations involving competitors or mutualists) or when the trait under selection is continuously variable or phenotypically plastic. Our research goal is to develop a learning progression framework to describe how grade 6-12 students reason about evolution in complex contexts. We administered semi-structured interviews about evolution in complex situations (n=46 grade 6-12 students, 3 undergraduates, and 4 graduate students). We used grounded theory coding to analyze interview transcripts and uncover emerging patterns. We can interpret our data using two progress variables, the nature and origin of phenotypic traits and the nature of fitness or adaptation. Nature and origin of phenotypic traits: The less sophisticated students focused on the development of traits during individual life cycles and inheritance of these acquired traits. Changes were often described as due to need, learning or conscious effort. After the seventh grade, some students included genes and mutations in their accounts but they still invoked “need” of individual organisms as the cause of mutation instead of random events. More sophisticated secondary school students were able to trace the appearance of novel traits to a genetic origin that was not due the wants or needs of the organism. They were also able to distinguish learned behaviors from inherited traits. However, only the most sophisticated students, the undergraduate and graduate students, described genetically controlled phenotypic plasticity. They understood that the ability to change in response to the environment is itself a genetic trait that is inherited. Nature of fitness or adaptation: Less sophisticated students understood fitness as strength, speed, endurance, intelligence, etc.—traits that organisms need and use. More sophisticated students described fitness as reproductive success, including survival throughout the life cycle and, less often, changes in reproductive strategies or other life history traits. Our findings support the notion that to understand micro-evolutionary processes in populations, students must be able to reason at the scale of the individual and molecules (DNA). This work demonstrates that the idea of the central dogma of molecular biology is an important “rule” that must be followed in order to understand the nature of phenotypic traits (i.e., genetic traits are not affected by the environment). We assert that the reason many students don't invoke natural selection more often is because they don't believe in the absoluteness of the central dogma; that is, they do not consistently constrain their accounts by this genetic “rule”.