Rampant drift in the endangered tidewater goby (Eucyclogobius newberryi): comparing genetic variation of naturally and artificially fragmented populations

The objective of this study was to determine whether a species that is adapted for living in naturally fragmented habitats can maintain genetic variation in an artificially fragmented context. Here, I evaluate the population structure and genetic diversity of the endangered tidewater goby (Eucyclogobius newberryi), a small fish that is restricted to discrete coastal lagoons and estuaries in California, USA. I use five naturally fragmented coastal populations from a 300 km spatial scale as a standard to assess migration and drift in eight artificially fragmented bay populations from a 30 km spatial scale. Using nine microsatellite loci in 621 individuals, and a 522 base fragment of mitochondrial DNA control region from 103 individuals, I found striking differences in the relative influences of migration and drift on the genetic variation at these two scales. Overall, the artificially fragmented bay populations exhibited a consistent pattern of higher genetic differentiation combined with significantly lower genetic diversity relative to the naturally fragmented coast populations. Consistent with a recent history of drift in extreme isolation, I found no mitochondrial DNA sequence variation, no relationship between genetic differentiation and geographic distance, widespread fixation of polymorphic microsatellite loci, and a strong correlation between habitat area and allelic richness in the artificially fragmented bay populations. In contrast, the naturally fragmented coast populations exhibited considerable mitochondrial DNA variation, a strong pattern of genetic isolation-by-distance, substantial microsatellite diversity, and no relationship between habitat area and allelic richness, consistent with demographic stability and periodic migration between neighboring populations. I conclude that artificial fragmentation has restricted gene flow and caused rampant genetic drift in the bay populations, and that this species' suitability to living in naturally fragmented environments is not sufficient to prevent genetic degradation in an artificially fragmented situation.