Abstract
Ecotypes, within-species lineages adapted to particular environments, are common in vertebrate and invertebrate animals and well-studied in plants. They are so familiar and ubiquitous in the spectrum of taxonomic variation that it is easy to overlook a surprising 'trick' that they achieve: their identities persist sometimes even in the face of apparently overwhelming gene flow. Here I examine the origin and persistence of ecotypes in three-spined stickleback. In British Columbia, genomic evidence suggests benthic stickleback may have a single origin. In the Atlantic, two contrasting migratory and resident ecotypes, with clearly different morphologies and behaviours, are found sympatrically in several, different, saltwater locations. Genomic data suggest that these arose from secondary contact between divergent lineages, following long-distance dispersal from allopatric glacial refugia. The ecotypes appear to have different adaptive proclivities. Their existence and persistence may have important consequences for evolution and speciation that are sometimes forgotten.