Aim To analyse how important body size is for geographic variation in microscopic organisms in the marine environment and thereby determine the validity of the ‘everything is everywhere’ hypothesis.
Location Marine environments, globally.
Methods Studies on geographic variation in all marine eukaryotes smaller than 1 mm and all marine copepods were compiled from the literature and incorporated in multiple binomial regressions to analyse the effect of body size, lifestyle, environmental isolation and taxonomic affinity on the probability of different regions being reciprocally monophyletic. Sample size was also analysed, because a negative relationship between sample size and probability of being reciprocally monophyletic would indicate biases due to undersampling in the original studies. Separate analyses were performed for three potential types of barriers to gene flow for marine organisms (the water barrier of the middle of the oceans, the temperature barrier of the equator, and the land barrier of the continents).
Results Environmental isolation was the only variable in the best-fitting model for the probability of populations from each side of an ocean being reciprocally monophyletic. The estimated importance of body size was quite large and the lack of this variable in the best-fitting model may be a power issue due to the small sample size (n= 40). Both environmental isolation and body size were important for the probability of monophyletic populations in different hemispheres. The analysis of isolation between different oceans did not produce clear results. The relationship between sample intensity and probability of being reciprocally monophyletic was positive for the isolation between different oceans and non-existent for the two other analyses.
Main conclusions The results showed that body size was an important factor governing the potential for geographic variation but not the only important factor by far. These results clearly weaken the ‘everything is everywhere’ hypothesis as they showed that microscopic organisms may have geographic variation, albeit to a lesser degree than larger organisms.