Ecological consequences of dispersal and gene flow in an intertidal alga

Gene flow across a changing environment can counter natural selection and prevent adaptation; this may have profound consequences for the way that species are distributed in space. Here I present research examining the effects of gene flow on adaptation to, and distribution across, a strong environmental gradient, using the intertidal alga Silvetia compressa as a model system. This work includes (1) measuring dispersal directly by mimicking colonization events; (2) assessing local adaptation to tide height with reciprocal transplant experiments; (3) documenting variation in a key trait, embryo emersion tolerance, within and across populations, and investigating the source of this variation; and (4) testing the consequences of gene flow across the gradient by conducting controlled crosses and measuring progeny performance.; Zygotic dispersal was found to be consistently limited (approximately one meter) across experimental benches, regardless of differences in region, slope, and exposure. Across sites, S. compressa populations occupy different portions of the intertidal gradient (i.e. variation in upper and lower limits as well as the total range of tidal heights), and zygotes collected from nearby populations differed significantly in ability to withstand emersion stress in lab trials.; Within sites, transplant experiments with both adults and zygotes revealed evidence of local adaptation to tidal height at some sites but not others. This difference was not explained by simple predictions about slope, but may be due to variation in small scale heterogeneity in the gradient. At sites where transplant experiments indicated a home-height advantage, I found that progeny of upper limit individuals survive emersion stress significantly better than lower limit progeny; a common garden experiment revealed that this difference was robust to maternal environment during gametogenesis in two of three populations.; I investigated fine-scale spatial patterns in emersion tolerance and its underlying sources at one of the sites (Greyhound Rock) by conducting controlled crosses and comparing the emersion tolerance of half-sib families. These experiments provided strong evidence that the trait is partially under genetic control. Surprisingly, I also found that emersion tolerance was greatest at the center of the intertidal distribution, which may reflect the combined effects of genetic and maternal environmental components.