7 Adaptive Recovery after Fitness Reduction:

the Role of Population Size

R.F. Hoekstra

Laboratory of Genetics, Wageningen University, Arboretumlaan 4, 6703 BD

Wageningen, The Netherlands

Introduction: Deleterious Mutations

Organisms may experience sudden reduc-
tions of fitness. Perhaps the most common
reason is a change in environmental condi-
tions. Populations tend to consist of geno-
types that are well adapted to the
prevailing conditions. When these condi-
tions change, previously well-adapted
genotypes may no longer be advantageous
or may even become deleterious. For exam-
ple, a mutation conferring resistance to an
antibiotic may have risen to high frequency
in the presence of the antibiotic, but is
likely to become disadvantageous in an
environment without this antibiotic for rea-
sons explained in the next paragraph.
Another possible cause of a sudden reduc-
tion in fitness is the fixation of a deleterious
mutation due to genetic drift in a (tem-
porarily) very small population.

Genomes are subject to the inevitable

occurrence of mutations. In the great majority

of organisms having DNA genomes, muta-

tions occur roughly at a rate of 10^5 per locus,

in some viruses with RNA genomes, e.g. those

causing influenza or AIDS, the mutation rate

may be several orders of magnitude higher.

Deleterious mutations are expected to disap-

pear again from the population due to the

action of natural selection. Occasionally, how-

ever, a deleterious mutation may reach a high

frequency in the population as a consequence

of genetic drift. The likelihood of such an

event is very small in large populations, but

may be relatively high in small populations –

even when a normally large population expe-

riences a ‘bottleneck’: a strong reduction in

size for only one or two generations.

This chapter considers the micro-evolu-

tionary adaptive recovery following a sud-

den reduction in fitness.

© CAB International 2003. Quality Control and Production of Biological Control Agents:
Theory and Testing Procedures (ed. J.C. van Lenteren) 89

Abstract

The transfer of natural enemies from the field to a mass-production facility may result in a sudden reduc-
tion in fitness. A sharp decrease in population size during the season in the mass-production facility can
lead to further reduction in fitness and will, in addition, enhance the possibility of fixation of deleterious
mutations in the population by genetic drift. Such reductions in fitness can be prevented by keeping very
large populations in the mass-production unit and by regularly replacing laboratory populations with
new, field-collected individuals.