untitled

additive genetic variance is by far the most important and for present purposes can

be discussed as if it constituted the total genetic variance.

The intricacies of genetic variance are beyond the scope of this book but all the

necessary points can be made by using heterozygosity Has an index of additive genetic

variance. Those factors that influence additive genetic variance also influence hetero-

zygosity and they do so by similar amounts and in similar ways. The mathematics

describing the dynamics of the two are much the same.

At this stage we make an important point on a subject that is widely misunder-

stood: the nature of “genetic diversity.” It is usually conceptualized by conservationists

as the number of distinct alleles in the population. The loss of one of those alleles

is seen as a reduction in genetic diversity and therefore a bad thing. That notion of

genetic diversity is trivial and entirely inappropriate to conservation management.

Rather, the important measure is genetic variance, which can be conceptualized as

a parameter closely akin to heterozygosity Hwhich is the proportion of loci that are

heterozygous in an average individual. Consequently the amount of heterozygosity

carried within a sample comprising a couple of dozen individuals will closely

approximate the amount of heterozygosity within the population from which that

sample was drawn. By extension, the amount of genetic variance carried by a

relatively small sample of the population will closely approximate the magnitude of

the genetic variance of that population as a whole. It is the genetic variance of the

population that we seek to conserve, not the “genetic variation” or “genetic diver-

sity” represented by the total number of distinct alleles within the population.

That misunderstanding carries over to populations established by translocation.

It is often argued that since these were usually started by a nucleus of only a few

individuals, and since those individuals must carry only a small fraction of the genetic

variability of the population from which they were drawn, the subsequent robust health

of the population that developed by the liberation indicates that a population needs

little genetic variability. Those populations tend either to increase rapidly or to go

extinct (often apparently by simple demographic stochasticity) within the first few

years following translocation. If the former, they are shortly free of any further loss

of genetic diversity. The condition that gets a population into the greatest trouble is

a history of small population size that persists for many generations. That seldom

occurs for a wildlife population founded by translocation.

In the absence of immigration and mutation, the number of different alleles at a

locus in the population as a whole can either remain constant or decrease. It can-

not increase. In practice it will always decrease because alleles will be lost under the

influence of non-random mating and unequal reproductive success between individuals.

Heterozygosity thus decreases also. Its rate of decline is a function of population

sizeN, the proportion of heterozygous loci in the population as a whole being

reduced by the fraction 1/(2N) per generation. Over one generation Hchanges

according to:

H 1 =H 0 [1 −1/(2N)]

and over tgenerations:

Ht=H 0 [1 −1/(2N)]t

294 Chapter 17

17.3.3Drift and
mutation