An important complication that the emergence of sexual reproduction brought was the Alli effect, i.e. the phenomenon of the dependence of the effectiveness of reproduction on the density of the population, accompanied by a threshold population density necessary for effective reproduction. If an individual of a sexually reproducing species comes to a new territory, sooner or later it will die without establishing a new population. Even when this individual consists in the legendary “fertilized female” or even a small population of individuals of both sexes, it is highly probable that they will die out without establishing a basis for a long-term population. Amongst sexually reproducing species, there is always a threshold density below which the population cannot permanently exist. If the size of the population or, more precisely, the density of the population, i.e. the number of individuals occurring in a certain space, decreases below this value, the population is fated to die out. Individuals of the opposite sex or their gametes are not capable of finding one another sufficiently effectively. Assuming that organisms have not created special ethological mechanisms, the frequency of meeting of individuals is directly proportional to the square of the population density. In this case, the transition between sufficient and insufficient population density can be very sharp and the difference between the actual size of the population, determined, for example, by the resources in the environment, and the threshold density can be very small. Thus, sexually reproducing organisms are permanently exposed to the risk that, on a random fluctuation in the size of the population, they will get below their threshold population density and die out.