Behavioral Genetics and Animal Domestication - Genetics and the Behavior of Domestic Animals - page 50

Drift
Drift is the name given to random fluctuations in gene frequency due to
stochastic processes ( Falconer and Mackay, 1996 ), and is generally most
important in small populations (with domestic populations frequently having
low effective population sizes). It is also most pronounced when substruc-
ture, for example inbreeding, has occurred to further decrease the effective
population size. In this instance it can readily lead to a loss of allelic diver-
sity, and is therefore similar in effect to inbreeding. Founder effects (when
limited numbers of individuals are used to generate the initial population)
and bottlenecks (whereby only a limited number of individuals are used to
generate the next generation at certain points) will also decrease this initial
diversity (in the case of founder effects) or reduce it during their occurrence
(in the case of bottlenecks) ( Hedrick et al., 2001; Mickett et al., 2003;
Sunden and Davis, 1991 ).
Allelic diversity is important to long-term adaptation ( Allendorf, 1986 )
and is lost more readily than heterozygosity. However, the same measures to
avoid the loss of heterozygosity should also help with drift, for instance,
with the use of large founder populations with efforts made to maintain
diversity ( Ferguson et al., 1991 ) or back-crossing back to wild individuals
( Vil` et al., 2005 ). Effects of this loss of diversity are potentially somewhat
equivocal, though generally thought to be negative. A study with bottle-necked
Drosophila populations found a decreased tolerance to NaCl compared to
control populations ( Frankham et al., 1999 ), whilst effects on within-line vari-
ance ( Avery and Hill, 1977 ) and variance in response ( Wray and Hill, 1989 )
have been shown. However, Carson (1990) found no effects on variance after
bottleneck events in Drosophila and house-fly populations.

GENETIC ARCHITECTURE OF DOMESTICATION
The term “genetic architecture” refers to the location, number and effect size
of the number of loci (Quantitative Trait Loci, or QTL for short) that under-
pin the variation in domestication phenotypes ( Falconer and Mackay, 1996 ).
A QTL is the name given to a genetic locus that causes variation in a pheno-
typic trait. For example, one allele at this particular locus may cause an
increase in the expression of a trait, whereas another may cause a decrease.
The effect of the locus may be caused by a single base change within the
coding region of a gene (referred to as a SNP, single nucleotide polymor-
phism), a single base change in a non-coding (regulatory) region, or indeed
any number of different changes in the genetic code (duplications, deletions,
etc.). The strong directional selection associated with domestication can
potentially lead to a distinctive genetic architecture ( Andersson and Georges,
2004 ), whilst the form of this genetic architecture appears to be remarkably
consistent

in both plant and animal species ( Wright et al., 2010 ). This