Heritable genetic variation of dispersal
In our house sparrow metapopulation data set there were 2118 resident
adults and 484 adults that had dispersed between islands prior to
recruitment. Among the dispersers, 399 individuals dispersed to an
island of the same habitat type as their natal island, whereas 85
individuals either dispersed from a farm habitat island to a non-farm
habitat island (N = 42) or in the opposite direction (N = 43).
Proportions of dispersing recruits produced by adult sparrows was
considerably higher on non-farm habitat islands than farm habitat
islands (Figure 2; Supplementary Table S1), and within each habitat type
there was a tendency for disperser parents of both sexes to produce a
somewhat higher proportion of dispersing recruits than parents that were
residents (Figure 2). The interchange of individuals between islands of
different habitat types enabled us to use genetic groups animal models
to separate heritable genetic causes from environmental causes of
spatial variation in individual dispersal propensity, because house
sparrows with genomes partially originating from the farm genetic group
were present in the non-farm habitat and vice versa (Supplementary
Figure S1).
The basic GGAM analysis showed that the dispersal probability in our
house sparrow metapopulation had a heritable genetic basis, with
additive genetic variance explaining approximately 10% of the observed
variation in dispersal probability (Table 1). Furthermore, the basic
GGAM indicated that a considerable portion of the observed variation in
dispersal probability among individuals was explained by environmental
differences between natal islands (ca. 25%) and hatch years (ca. 1 %),
and provided strong evidence that females had a higher probability to
disperse than males (Table 1). Finally, there was strong evidence from
our basic GGAM that the estimated mean genetic value (i.e. mean breeding
value) for dispersal was lower for the non-farm habitat than the farm
genetic group (Table 1).
Our extended GGAM analysis showed that the farm genetic group had a
higher additive genetic variance for dispersal probability than the
non-farm genetic group (Figure 3, Table 1). The posterior difference in
additive genetic variance between the farm and non-farm genetic groups
had a mode of 0.556, with a 95% credible interval (CI) ranging from
0.045 to 1.134 (Supplementary Materials, Figure S2), providing clear
evidence that the additive genetic variances were different.
Correspondingly, the heritability of dispersal was higher in the farm
habitat than the non-farm habitat (h2 = 0.124
and 0.017, respectively; Table 1). In contrast, the proportions of
variation in dispersal probability explained by differences between
hatch years and natal islands were similar in the two habitat types and
similar to estimates from the basic GGAM (Table 1). In agreement with
the basic GGAM, there was also strong evidence from the extended GGAM
for a sex difference in dispersal probability, and that the estimated
mean genetic value (i.e. mean breeding value) for dispersal was lower
for the non-farm than the farm genetic group (Figure 3, Table 1).