The specific aim is to test priority effects at the within-species level, and monitor the genetic composition of the resulting population through time using molecular markers (microsatellite loci). The experiment will be conducted in containers in the Doñana reserve and we will use Daphnia longispina individuals from the Doñana reserve. In order to define the time period necessary until priority effects between initial and invading clones are detectable, we will use one initial clone and a different clone as invader. To exclude stochastic effects, we intend to use four clonal combinations with reciprocal treatment and three replicates per treatment.