Beschreibung:
<jats:p>In an effort to analyze the nature of a taxonomic species in the microalgae, the major biological characters of various isolates of <jats:italic>Pandorina morum</jats:italic> have been studied. Seventy heterothallic clones from 31 collecting sites on three continents were found to represent 20 reproductively isolated groups, based on their ability to form zygotes when paired in all possible combinations. Successful germination of the zygotes and demonstrated fertility of their products suggest that each reproductively isolated group represents a syngen—a group of organisms sharing a common gene pool. A further indication of the close relationship of members of the same syngen is that its representatives all share a common chromosome number, time of division, and zygote distribution pattern, characters which vary between syngens. The known range of 18 of the syngens is less than 500 km. The other two syngens may have a world‐wide distribution; they both have a haploid chromosome number of 5. Crosses between geographically isolated clones in these two syngens and one other give evidence of both pre‐ and postzygotic partial isolation within a syngen. Analysis of collections from two revisited sites demonstrated that representatives of a syngen can be recovered after 19 years have elapsed, and that a single pond may contain as many as four syngens. The results obtained for <jats:italic>P. morum</jats:italic> are compared with the information available on other microalgae with a similar breeding system, and the possible causes and consequences of the species structure are discussed.</jats:p>