A key question in evolutionary biology is to understand which factors shape biological diversity. This is especially true at the intra-specific level, where evolutionary and ecologically processes interact to shape population genetic and phenotypic structure.

Here, we examine phenotypic and genetic diversity in relation to ecological characters of small benthic Arctic charr (Salvelinus alpinus) populations in twenty caves around Lake Mývatn. Fish were individually tagged and monitored twice a year for growth, morphology and movement. Our results strongly indicate that within each of these cave genetically distinct populations of Arctic charr can be found, despite the close proximity of some of these caves. These populations are however often very small. Patterns of fish movement among and within years confirm the genetic and phenotypic structures. The result of the study will give us important information on repeated evolution of small populations in relation to ecological characteristics.