Extinction and recolonization in metapopulations may either increase or decrease genetic differentiation among populations, but recent genetic models predict increased differentiation under most circumstances of recolonization. I examine this prediction empirically using tidepool populations of the marine tidepool copepodTigriopus californicus. The probability of extinction of tidepool populations was sufficiently high to invoke the model's predictions, but varied among populations. Nearly 75% of colonizing groups consisted of 10 or fewer individuals. The genetic effective size of colonizing groups might be as high as 18, depending on assumptions, but colonists probably originated from a subset of local populations. In contrast to my predictions, genetic differentiation was smaller among younger tidepool populations than among older populations on each of three rock outcrops, suggesting that genetic differentiation was reduced by metapopulation dynamics. The discrepancy between the prediction and the results may be explained by the unmet assumptions of classical metapopulation structure underlying the genetic models.