Ulvophytes are attractive model systems for understanding the evolution of growth, development, and environmental stress responses. They are untapped resources for food, fuel, and high-value compounds. The rapid and abundant growth of Ulva species makes them key contributors to coastal biogeochemical cycles, which can cause significant environmental problems in the form of green tides and biofouling. Until now, the Ulva mutabilis genome is the only Ulva genome to have been sequenced. To obtain further insights into the evolutionary forces driving divergence in Ulva species, we analyzed 3905 single copy ortholog family from U. mutabilis, Chlamydomonas reinhardtii and Volvox carteri to identify genes under positive selection (GUPS) in U. mutabilis. We detected 63 orthologs in U. mutabilis that were considered to be under positive selection. Functional analyses revealed that several adaptive modifications in photosynthesis, amino acid and protein synthesis, signal transduction and stress-related processes might explain why this alga has evolved the ability to grow very rapidly and cope with the variable coastal ecosystem environments.