-10advantage some thousands
of years from now.
UCRL-3644
Some brief speculations re-
garding the extreme limits of variation may be offered:
(a) Humans and other long-lived animals have, as a corollary of their
longevity, a less frequent natural mutation rate per gene per unit of time
than short-lived species. As an approximate rule, each species appears to
have about the same mutation rate per generation time. Thus, it appears
that species are in some balance between generation time (or life span) and
stability of genetic structure.
(b) Testing biological capacity for survival under circumstances that increase genetic variation is possible only with species having relatively short
generation times. They have a common feature of a potentially great ratio
of progeny per parent. These species can therefore survive even if a relatively
high proportion of conceptions are incapable of survival and reproduction.
Humans in natural selection are at some disadvantage in comparison with
species producing a large number of offspring per generation, such as the
mouse or the fly. Thus, human genetic tolerance should not be judged from
effects of radiation exposures on these more fertile populations.
(c)
For survival of a species, the ratio
Reproducing offspring
Individual
must exceed l.
In humans, lowering of infectious disease toll has brought this ratio to approximately 2. As.a consequence, the human population is now doubling in numbers
approximately every 40 years. Thus, humans have already achieved some
protective reserve against genetic changes toward lower fertility.
(d) Ona scale of catastrophic genetic misfortune, humans also have the
protection of vast numbers of individuals. There are 2. 6 billion inhabitants
of the world. While this number is small compared with numbers of insects
and small mammals, it is still a very large number compared with that of
any previous age in the history of man. Radiation exposure, as a cause of
genetic change and increase of genetic variance, would be expected to produce
that change in a random way. Thus, even with a large increase in genetic
variance induced by radiation exposure, if population numbers are sufficiently
great, some individuals will remain relatively unchanged. If these individuals
were favored in selection, they might replace the less fit, less fertile fraction
of the population. Thus, if survival of mankind were the only consideration,
population numbers might, through reduction and segregation, achieve selective
retention of adequately functional humans.
Several approaches to evaluation of extreme tolerance of human
populations to radiation exposure with respect to health and genetic constitution
are presented in Appendix B. These methods of estimation are difficult and
speculative; but they indicate that an additional 2 r/yr
(or 50 r per gen-
eration time) of chronic radiation exposure to the average individual in the
human population would eventually cancel health and life-span gains we have
achieved recently. Such estimations of impairment of health and estimations
of the cost of increasing the genetic variance suggest that human population