In Hawaii, there are two types of fruit flies that look very similar in appearance. Both species live in the same places, feeding on the sap of the same woody plant. In this case, one species eats juice flowing down the trunks and branches in the upper tiers of the tree, while the other – puddles of juice on the forest floor. Crossing between these species never occurs due to their spatial disunity. An interesting example of behavioral isolation is demonstrated by various types of fireflies. Each of the species living together is characterized by a specific light path and the types of light signals emitted. The trajectories can be zigzag, straight or in the form of a loop, and light pulsations can be short or long in the form of stable reflections. When mating, individuals pick each other, strictly focusing on the type of light signal. This example shows that isolation between populations can be fixed by the formation of certain types of behavior – the development of reflex reactions only to signals of one type or another. The pollen of some plant species, such as orchids, is carried only by certain species of animals, whose instinctive behavior is the guarantee that gene exchange will occur only among individuals of their species. In animals with external fertilization, insulating mechanisms work at the molecular level. In starfish and some species of mollusks, the role of isolating factors is played by differences in the structure of special protein molecules that bind sperm and eggs. Being on the surface of the ovum, these molecules react only to spermatozoa of “their own” species, which excludes the possibility of fusion of sexual products of different species. In animals with internal fertilization, this role is played by differences in the structure of the genital organs. Finally, in many animals, the breeding season begins with strictly defined combinations of external factors (for example, temperature and light). These factors act on them as signals to the start of mating. Different species react to the same factors in different ways, due to this, the breeding times for them do not coincide. Isolating mechanisms impede the development of an organism from a zygote formed as a result of the fusion of gametes of a male and a female of different species. Hybrids that arise in this way usually die quickly or remain barren. Thus, hybrids of the hare and the hare, the marten and sable, the horse and the donkey (mule) are barren.