|Experimental studies on adaptive evolution in Gasterosteus aculeatus L.|Heuts, M.J. (1947). Experimental studies on adaptive evolution in Gasterosteus aculeatus L. Evolution 1: 89-102
In: Evolution. Society for the Study of Evolution: Lancaster, PA.. ISSN 0014-3820, more
Gasterosteus aculeatus Linnaeus, 1758 [WoRMS]; Marine; Brackish water; Fresh water
Populations of the stickleback, Gasterosteus aculeatus L., vary greatly in morphological and physiological characters. The populations of Belgium and neighboring countries are differentiated into two sharply defined adaptive types. These types differ in morphological characters, especially in the mean plate numbers and in mean body size, and also in physiological traits, the most important of which concern the chlorine regulation of the blood. In Belgium one type occupies fresh water habitats and the other type salt water habitats. The gene exchange between these types is very limited because of the physiological properties of the hybrid eggs. Representatives of the same type coming from different countries are often distinct in the mean plate number. These geographical differences form north-south gradients. Indirect evidence is given, based on the study of the type, which in Belgium lives in fresh water, that the geographical distinctions are adaptive. The establishment of the geographical gradients was in all probability an adaptive process. An hypothesis is advanced, according to which the two types have arisen through a series of primary and secondary linkage rearrangements starting from an originally single polygenic complex. In this way, two physiologically distinct groups of populations might have been produced. Although these types intergrade morphologically, they are adapted to different combinations of temperature and salinity. The morphological intergrades are not physiologically intermediate between the two types; they belong physiologically, and therefore ecologically, to one or the other type. The physiological differentiation between and within the two types is large enough to explain the geographical and morphological divergence as well as the maintenance of the two types by natural selection. Natural selection is shown to act not only on the adult stage but even more sharply on the egg stage.