Long-term threats to persistence

The threats to long-term persistence are those associated with loss of the genetic variability necessary for adaptation to evnironmental change. One way of assessing this threat is to ask how large the population must be to balance the loss of additive genetic variance through drift with the input of additive genetic variance through mutation. Several reviews suggest that the mutation rate for quantitative characters is on the order of $10^{-3}$ [10,11]. The rate of loss, recall, is $1 \over 2N_e$. This would suggest that an $N_e$ of 500 is necessary to ensure long-term evolutionary potential [3].¹⁰ Again, it would appear that populations large enough to buffer environmental stochasticity will also be large enough to maintain the additive genetic variance necessary to respond adaptively to environmental change.

When we recall that the main effect of population bottlenecks is to reduce the number of alleles present, there is even more reason to suspect that this is a reasonable conclusion.

• Most of the genetic variance responsible for an evolutionary response to natural selection (the additive genetic variation) is found in high-frequency alleles, precisely those least likely to be lost in small populations.

• Many low-frequency alleles are probably unconditionally deleterious and are maintained in the population only by recurrent mutation. There loss may actuall benefit the population.

• Low-frequency alleles have a short lifetime in populations. They are quickly lost through drift. Current low-frequency alleles are unlikely to provide the source of evolutionary novelties. They are more likely to arise as a result of mutation in the future.

• Alleles that are selectively advantageous are unlikely to be lost from populations in a short time as a result of drift unless the populations are very small.

But it is important to remember that when we manage species, they may respond not only through changes that are primarily ecological, they may adapt to the new management regime to which they are subject. This is most obvious when the managed environment is very different, as when captive populations are bred in zoos, aquaria, or botanical gardens, but it may also happen in the wild if populations are restricted to a part of their former range.As Stockwell and his colleagues point out, refuge populations may diverge from those in other parts of their range making it more difficult to use them as source populations for re-introduction or restoration efforts.