Survival Rate

in biology, the average probability of survival and reproduction of the organisms of each generation of a species (population). The survival rate is measured by the ratio of the number of adults that reproduce to the number born in each generation (or the number of eggs de-posited, spawn laid, seeds ripened, and so forth).

Under unfavorable conditions the survival rate (like fertility) decreases, and a population declines. Under favorable conditions the survival rate increases, stabilizing or augmenting the population. The survival rate increases by many orders in the course of progressive evolution. Thus, the average survival rate increases from 10~7-10~6 percent in bacteria, unicellular organisms, and plants to 10-30 percent in higher animals. This is associated with the development of several systems that promote the safety of the organism and reduce loss in all phases of ontogeny (development of multicellularity, differentiation of organs, perfection of self-regulation and homeostasis, increase in the amount of yolk in an egg, shift to internal fertilization, viviparity, active con-cern for offspring, and so forth). An increase in the survival rate during evolution is accompanied by a regular decrease in fertility. For example, in animals with small eggs deficient in yolk that are laid directly in water, the females deposit many million eggs at a time; however, in species that have large well-protected eggs and are capable of guarding their young, the females deposit only ten to 100 eggs. Maintenance of the optimum survival rate is important for evolution because higher rates can dangerously lower the effectiveness of natural selection and the evolutionary flexibility of the species.

The term “survival rate” is also used in research on the effect on the organism of various unfavorable factors, such as ionizing radiation. It means the percentage of organisms surviving exposure to such factors.

K. M. ZAVADSKII