One of the key objectives in evolutionary biology is to understand the underlying sources of phenotypic variation in natural populations. Natural genetic variation is essential for organisms to adapt to their local environment and to changing environments. Studying phenotypic and genetic variation will enable us to understand how natural selection shapes genetic architecture and phenotypic variation among natural populations. Little is known about the patterns of genetic variation underlying adaptive traits. Using restriction site-associated DNA sequencing (RAD-seq), and re-sequencing methods, I investigated genome-wide patterns of genetic variation in between and within populations of the freshwater zooplankton Daphnia pulex. Daphnia makes an ideal organism to study the relationship between phenotypic and genetic variation because it has been used as a model system for ecology and quantitative genetics for decades. The availability of a complete reference genome, our genomic data combined with phenotypic data will increase our understanding of the complex interaction between genomic structure and phenotypic variance. In this study I used a combination of phenotypic data from a common-garden life-history assay and SNPs derived from RAD-seq and re-sequencing data to examine populations of Daphnia pulex for genome-wide analysis.