Biology Reference
In-Depth Information
decreased wing length) was performed for 13 generations, and the flight behav-
ior of individuals monitored. Individuals also were selected for flight time, and
those whose flight times totaled 30 minutes were considered “fliers,” whereas
those with a shorter flight time were labeled “nonfliers.” Response to selection
on wing length was rapid, and flight tests of the long- and short-winged insects
indicated there was a positive correlation between wing length and flight dura-
tion. Selection after two generations for flight or nonflight likewise resulted in
divergent responses, indicating a large genetic component to flight behavior.
11.4.2.2 Analysis of Selection Experiments
To estimate the degree of genetic influence on a specific behavior, two mea-
sures are used, the selection differential and the estimate of heritability. The
response to selection
(
R
) is the difference in mean phenotypic value between
the offspring of the selected parents and the mean phenotypic value of the
entire parental generation before selection (
Falconer 1989
).
2
RhS
=
R is the improvement or response to selection, h
2
is the heritability of the
characteristic under selection in the population, and S is the selection differen-
tial. The
selection differential
(
S
) is the average superiority of the selected par-
ents expressed as a deviation from the population mean (
Falconer 1989
). The
selection differential measures the difference between the average value of a
quantitative character in the whole population and the average value of those
selected to be parents of the next generation. It is measured in standard devia-
tion units.
Heritability
in the broad sense
is the degree to which a trait is genetically
determined. Because both genes and environment influence behavioral traits,
heritability is expressed as the ratio of the total genetic variance to the pheno-
typic variance (V
G
/V
P
). Heritability
in the narrow sense
is the degree to which a
trait is transmitted from parents to offspring, and is expressed as the ratio of the
additive genetic variance to the total phenotypic variance (V
A
/V
P
) (
Falconer 1989
).
Heritability could be estimated to be “zero” if the specific population being
selected had no variability for the behavioral attribute under study because it
was inbred. Heritability could be estimated to be “one” if the trait was com-
pletely determined by genes, and the environment had little effect on the phe-
notype, although this would be an unusual outcome.
Heritability estimates provide no information about the actual mode of inher-
itance of a quantitative trait because they represent the cumulative effect of all
