Utilisation of different populations 3. Selection in sub-populations
W .G. Hill Livestock Production Science 63 2000 99 –109
105
Whereas mean fitness is likely to drop due to overall effective size is SN , where the effective size
ej
directional selection on other traits, whether stabilis- of subpopulation j is N . If there is limited migration
ej
ing or pleiotropic effects apply, variation in fitness is among them and contributions to the pool do not
likely to rise because genes that are deleterious with differ greatly, the effective size increases; indeed if
respect to fitness are maintained segregating by the there is no migration at all, variation is maintained
artificial selection. Furthermore, the observed selec- indefinitely in the set of populations as a whole, even
tion response in the trait under selection is likely to if individual populations become fixed. These calcu-
be less than expected from the standing genetic lations in terms of effective size are probably of
variation, because of the negative correlated response limited relevance to the case of animal breeding, for
from natural selection Hill and Mbaga, 1998. they refer to asymptotic changes in unselected
populations over what may be very long time scales.
5. Utilisation of different populations 5.3. Selection in sub-populations
5.1. Multiple objectives Selection within sub-populations, whether a breed-
er’s nucleus herd or a whole breed, has the effect of The discussion has been restricted to single closed
reducing the effective size of each. The amount of populations, but applies to any populations that are
variability retained in this set of populations will currently of commercial importance in satisfying
depend on the extent to which the same selection demand for a particular product or in utilising a
criteria apply in each. It is obvious that variation is particular environment. There is clearly then justifi-
maintained if, for example, both selected and un- cation in retaining and improving those populations
selected populations are kept. The question is then where the market share justifies doing so. Maintain-
whether such variation can be utilised. This depends ing populations in anticipation of long-term commer-
on critical assumptions, among which is the extent cial need requires good luck or insight, however. A
that selection objectives may change — essentially separate consideration is how to make best use of
an unknowable. If selection objectives are assumed resources in maintaining long term response for any
to remain constant, issues that affect the value of single population and objective, options for which
maintaining subpopulations with different sizes could involve population subdivision with selection
selection intensities include the following. or conservation, which impacts on effective popula-
tion size and useful variability. 1. Useful genes in any subpopulation are more likely
to be retained and fixed favourably if selection is 5.2. Effective population size
practised, although the optimum may be at rather weak selection because of the trade-off between
Firstly consider the case of no selection. Then, selection intensity and effective population size.
while migration generally increases the variation 2. The rate of response in the population as a whole
present in any single population, providing there are is generally maximised if it is kept as a single
not big differences in the contributions of individual population. This is because subdivision and re-
populations to the pool, genetic variation in the crossing in the absence of between line selection
population as a whole is retained longest if there is leads to lower mean responses at any time as N
e
no migration. The population dispersion can be and thus genetic variance are lower in each
described in terms of variance in gene frequencies, subpopulation Robertson, 1960. There are ex-
summarised by Wright’s F-statistics, specifically ceptions to this general result, however. If there is
F . The asymptotic change in variance in the total recessive gene action or particular forms of
ST
population can be described by its effective popula- epistasis in which variation rises when the popu-
tion size, N see Wang and Caballero 1999 for a lation undergoes a bottleneck in size, then in-
e
recent review of the effective size of a subdivided creased rates can be achieved. Also, if between
population. At its simplest, providing there is sub- line subpopulation selection is practised, there
stantial migration among a set of subpopulations, the are transient situations whereby the performance
106 W
.G. Hill Livestock Production Science 63 2000 99 –109
of the best of a group of small lines is expected to knowledge and technology. There are other possi-
be greater than that of one large population. This bilities.
is, of course, the main function of inbreeding- selection schemes.
1. Marker assisted introgression is essentially a way 3. It is important to distinguish between the total
of backcrossing in a particular region QTL or amount of variation present within and among a
gene, whereby performance of the recipient popu- set of populations and that utilisable by selection
lation is increased whilst retaining most of its in finite time. Thus if unselected populations are
previous variability. The problem is to locate the maintained alongside selected populations to con-
QTL, and then to do the backcrossing with the serve variation, the total variation in the set of
recipient population of sufficient effective size populations is increased, but it may not be
that substantial background variation is not lost, possible to utilise the conserved variation in an
except in the region very close to the introgressed effective and timely way as the unselected popu-
QTL. A first and major problem is to identify the lations lag further behind. If an unselected or
QTL to introgress. These are likely only to be backward population is to be used, the loss of
found by intensive study of well-recorded popula- performance from bringing in inferior genes has
tions or of crosses between populations. Detailed to be made up quite quickly, either by backcros-
QTL mapping in large numbers of populations sing out all but specified loci, which requires
would be a major undertaking. information on relevant marker genes, or because
2. Induced mutagenesis of unspecified quantitative genetic variation is much higher. Assuming the
variation is an old idea that seems still to be used infinitesimal model, the increment in additive
in a limited way in plant breeding, but has not variance is proportional to the genetic distance
been applied successfully in animals. The reasons between the populations which is proportional to
are not entirely clear, but traditionally X-rays the sum of their inbreeding coefficients sub-
have been used; they cause chromosomal damage, sequent to derivation from the same source. With
so mutations are ‘unfit’. Chemical mutagenesis is accurate selection, such that response is nearly
being widely used to screen for mutants in mice proportional to standard deviation, the propor-
to serve as models for particular diseases in man. tional increment in œV , about half that in V , is
For quantitative traits, because mutants are at low
A A
relevant. These issues need further consideration frequency, those of small effect take a long time
in design of conservation programmes. to become useful; and laboratory experiments
4. A more interesting and difficult case is where have typically been too short. Experiments using
selection objectives change in the medium or long mutagenesis have in fact indicated an increase in
term. If these go through a process of gradual genetic variance, but these have not been regarded
evolution rather than revolution, variation present in most cases as large enough to be worthwhile.
in current commercial populations can be utilised. Spectacular changes have, however, been found
Populations maintained just to increase the total in some cases, for example from mobile P-ele-
variation present among and within populations ment mutagenesis in Drosophila by Mackay
remain unlikely to be able to contribute usefully 1985, and smaller responses using retroviral
unless relevant QTL have been identified. insertions in mice Keightley et al., 1993. Per-
haps the subject needs revisiting, if only to provide a better understanding of how to in-