population on Daphne is reproductively isolated
from one of the parental populations, G. fortis,
but whether it is reproductively isolated from the
other, G. conirostris on Española, is unknown be-
cause experiments have not been done there.
Nevertheless, it is likely that the founder popu-
lation has already become reproductively iso-
lated from G. conirostris, as bill size has changed
in relation to body size (Fig. 3A). Together, these
traits are used as cues in the choice of mates,
arising from cultural, nongenetic imprinting
(12, 13). Of particular relevance, experiments on
Daphne Major with G. scandens showed that
altering bill size in relation to body size of finch
models significantly reduced responses from males
(28). Additionally, males of the founder popula-
tion sing a different song from G. conirostris on
Española and Gardner, probably as a result of im-
perfect copying of a Daphne Major finch by the
founder after it had first learned its father’s song
on Española (or Gardner) (13). Song and mor-
phology are cues that are used in mate choice
and typically result in the avoidance of inter-
“…to understand the mechanism of speciation,
the focus should be on cases of incipient speci-
ation rather than on completed ones” (29). We
have taken advantage of witnessing a rare colo-
nization event to directly document the fate of a
population founded by a single immigrant and
his G. fortis mate. The newly founded population
of Darwin’s finches is an incipient hybrid species,
reproductively isolated and ecologically segregated
from coexisting finch species (Fig. 3D). The key
features of success of the new lineage are repro-
ductive isolation based on learned song and
morphology, transgressive segregation produc-
ing new phenotypes, and the availability of
underexploited food resources. Homoploid hy-
brid speciation is believed to be a generally slow
process extending over hundreds of generations
(29), but, as the present example shows, it can be
established in only three generations. Thus, in
small islands or island-like settings, it may be easier
to achieve than is currently believed (1, 30–32).
Homoploid hybrid speciation of the Big Bird
lineage exemplifies the potential evolutionary
importance of rare and chance events. Expansion
of the population from two individuals to three
dozen was conditioned on the founder being a
male with a distinctive song (14) and facilitated by
the chance occurrence of strong selection against
large bill size in a competitor species, G. fortis, in
2004 to 2005 (12, 26). The selection event, in turn,
was mediated by G. magnirostris, a species that
established a breeding population in 1983. Joint
occurrence of rare and extreme events such as
these may be especially potent in ecology and
evolution (33, 34).
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PC (Body size)
0 23 6
P = 8 x 10-4, ß = 0.24
P = 0.39, ß = 0.06
10 12.5 15 17.5
Bill length (mm)
0 23 654 1
0 2 -2 3 -3 1 -1
Fig. 3. Morphology. (A) Bill-size variation in relation to body size among the Big Bird lineage (blue)
and the two parental species G. fortis (green) and G. conirostris (red). All 42 Big Birds lie above a
line connecting the two parents, indicated by black squares, and above a line connecting the
means of the two populations. The ordinary least squares regression slopes of the three relationships
(table S3) are homogeneous (ANCOVA, F2, 161 = 1.4, P = 0.26), whereas the intercepts differ
(ANCOVA with species–bill size interaction removed, F2, 163 = 140.9, P = 0.0001). The 99%
confidence limits on the Big Bird intercept estimate do not overlap those of the other two intercepts,
whereas the 95% confidence limits of the G. conirostris and G. fortis slopes do overlap. This pattern
is repeated in two of the components of bill size: depth and width [fig. S3; for bill width, see (14)].
PC, principal component. (B) Mean bill depth increased over generations (F1,42 = 12.9, P = 0.0008,
adj r2 = 0.22, slope = 0.25 ± 0.07). The relationship holds for generations 1 to 6 alone, i.e., without the
founder and its mate (F1,40 = 9.1, P = 0.004, adj r2 = 0.16, slope = 0.24 ± 0.08). Note the bill depth
of the single member of generation 1 is 10.9 mm, which is close to the midpoint of the parental
measurements (11.0 mm). Transgressive segregation for bill depth in the Big Bird lineage is possibly
indicated by the fraction of individuals that exceeded parental phenotypic values (highlighted in
blue) (10), which was estimated at 0.5, 0.3, 0.4, and 0.3 in generations 3 to 6, respectively. (C) Mean
body size remained unchanged across generations (F1,42 = 0.76, P = 0.39, adj r2 = 0.00, slope = 0.06 ±
0.07). (D) Members of the Big Bird lineage (blue) are in unoccupied morphological space among the
coexisting ecological competitor species. Ellipses contain 95% of individuals. G. magnirostris,
yellow; G. fortis, green; G. scandens, aqua.