Biol. J . Linn. Soc.. 5 : 289-295. With 4 figures
September 191 3
Divergence and evolution in Darwin’s finches
HUGH A. FORD*, DAVID T. PARKINt
and ALASTAIR W. EWINGS
Depurtrnerit of’ Zoology, West Muiris Roud, Ediribitrgh
Accepted March I 9 7 3
l h e medium and large ground finches of the Galapagos archipelago Geospira fortis and
C.niagnirosrris are distinguished by their different body size and bill dimensions on most of the
islands where they both occur. On the island of Indefatigable this distinction is not complete
and a group of birds with intermediate bill dimensions is present. The origin of this group could
be explained by sympatric divergence of G. forfis or by hybridization, between this species and
C. magnirostris. Although the conditions for sympatric divergence are severe it seems likely that
strong disruptive selection for different optimal bill sizes may be operating on G. fortis, due to
the presence of several ecological niches, separate categories of size and hardness of seeds the
birds eat. Ir is suggested that islands in the Galapagos archipelago, and perhaps other oceanic
islands, may provide conditions extremely conducive to sympatric divergence, or even
sympatric speciation.
CONTENFS
Introduction
. . . . . . . . . . . . . . . . . . . . . .
Methods
. . . . . . . . . . . . . . . . . . . . . . .
Results
. . . . . . . . . . . . . . . . . . . . . . .
Discussion
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . .
Acknowledgements
References
. . . . . . . . . . . . . . . . . . . . . .
289
290
291
291
295
295
INTRODUCTION
Mather (195 5 ) suggested that disruptive selection could cause sympatric
divergence (divergence without any geographical isolation) and that this would
result in polymorphism. Smith (1966) has outlined the rather severe conditions
necessary for sympatric divergence:
( 1 ) there must be separation of the environment into two or more niches;
(2) the size of the populations occupying each niche must be separately
regulated ;
Present addresses:
Department of Zoology, University of Adelaide, Adelaide, South Australia 5001.
t Department of Genetics, Nottingham University, University Park, Nottingham, NG7 2RD.
$ Department of Biological Sciences, Portsmouth Polytechnic, Portsmouth, Hants.
289
290
H. A. FORD ET A L .
( 3 ) the selective advantages of each type in its “own” niche must be large;
(4) once divergence has taken place it is likely that it will be followed by
reproductive isolation between the two populations. This will be aided if
the characters leading t o assortative mating are associated with those
adapting the organism to the different niches.
Sympatric divergence is only likely to occur when strong disruptive selection
is operating on a population; that is, where there is selection for differing
optima. Laboratory experiments on the fruit fly, Drosophilu melutioguster have
repeatedly shown that disruptive selection can lead t o divergence and, in one
case, t o some degree of reproductive isolation (see Thoday & Gibson, 1970 for
review). However, relatively few natural situations have been explored with this
mechanism in mind. One case was investigated by Bush (1969), who suggested
that the “biological races” of the apple fly, Rhugoletis. could have evolved
through syrnpatric speciation due to the existence of different niches, in the
form of different food plants in the same area.
An interesting situation is presented by the ground finch, Geospizu fi)rtis, on
the island of Indefatigable in the Galapagos archipelago, which we believe may
be explained by disruptive selection leading to sympatric divergence. We also
believe that the conditions necessary for sympatric speciation are present and
that there is a possibility that this type of speciation may have taken place in at
least some of the species of Darwin’s finches, Geospizitzue.
Four species of the genus, Geospizu, are present on the island of
Indefatigable; a fifth was present but now is believed t o be extinct. One,
G. scandens. is quite distinctive. I t has a long beak and feeds mainly on the
fruit and flowers of the cacti, Oputitiu and Cereus. The other three species all
eat seeds found mostly on the ground and differ chiefly in body size and in the
size and shape of their beaks. The smallest, G. Jirfigiirosu, is clearly separate (on
beak measurements) from the other two, C. fortis and G. mugtiirosfris.
Although the majority of the rest are easily assignable t o one of the latter two
species there are several individuals which are intermediate with respect to their
beak measurements.
Lack (1945, 1947), also observed this group of birds but suggested that they
were G. fortis and that this species shows a very large range of beak size as it
has partially moved into the niche of the rare, G. mugriirostris. Bowman (1961)
also believed that the intermediate individuals were large G. fbrtis and
suggested that they resulted from occasional introgression between G. f0rfi.s
and C. r?iugtzirostri.s. However, he later suggested that little or no interspecific
hybridization occurs. Snow (1966) measured a large number of G. .fi)rti.s and
G. mugnirostris and suggested that the complete gradation of both beak size
and song indicates that interspecific introgression has taken place and
proceeded to such a degree that the two species have merged into one highly
variable form.
M ETt IOD S
To obtain fuller data on this situation we captured and measured about 600
Geospizu in August and September 1968. The birds were captured in mist nets
and traps, mainly near the Charles Darwin Research Station at Academy Bay,
DIVERGENCE A N D EVOLUTION IN DARWIN'S FINCHES
29 1
Indefatigable island. The traps were baited with rice which is readily eaten by
G. jltliginosa and G. fbrris. It is possible that the rather shyer G. rriugriirostris
was under-represented by this method of capture. The t w o parameters,
beak-length and beak-depth were measured; the former was taken from
beak-tip to front of nares and the latter from forehead t o throat. The first one
hundred of these birds were ringed; as only two were recaptured very few birds
in the whole sample of 600 can have been measured more than once.
K E sU L1's
The entire range of measurements for beak-length and beak-depth, with the
exception of G. .sca/ideri.s, is presented on Figs 1 and 2. Individuals with at least
some black were separated from the rest to limit any bimodality that might be
due to sex differences. (The presence of black plumage indicates an adult male.
However some adult males show no black, so that separation into adult males,
and females and young is not completely accurate.)
The measurements show that G. firligirzosa, the small ground finch is clearly
separate. Like those of Snow (1966) and Lack (1947) the measurements also
show that there is a large class of medium sized finches, corresponding t o
typical G.fi)rti.s, a few very large individuals corresponding to G. rrzug/iiro.stri,s
and a group of individuals of intermediate size which cannot be classified
definitely as either G. jbrtis or G'. riiugriiro.stri.s. The results are particularly
valuable because the sample is large and was collected in a restricted area over a
limited period of time and therefore probably represents a single population.
Tests for bimodality can thus be made.
The accumulative measurements for beak-depth of all birds except G.
firligi/io.sa were plotted on probability paper. A heterogeneity in these
measurements would be revealed by a change in the slope in this graph.
Figures 3 and 4 show these graphs for beak-depth in black males and the rest.
An inflection is apparent around 11.5 mm in the former and around 11.0 mm
in the latter, which strongly indicates bimodality. A further change in slope
may be present above 14.5 mm which could correspond to G'. //zug/iirostrisbut
the number of individuals of this size is small. A similar pattern is apparent for
beak-length though the change in slope is less pronounced.
Measurements of finches from the island of James (Lack, 1947) show that
C; ,fi)rti,s and G'. t~iug/iiro,stri.vare clearly distinct there and that n o birds of
intermediate size are present. The mean beak measurements of G fi)rti.s from
James Island are the same as the modes shown on Indefatigable Island; i.e. the
mean measurements for beak-length and -depth of the Indefatigable finches are
higher than those from James due to the presence of several very large
individuals rather than due to a shift in the whole distribution.
DISCUSSION
I t seems very likely that a separate population is present on Indefatigable
Island, intermediate in size between G f i ) r f i sand G mug/iiro.sfris. Its ecological
and evolutionary relation t o these two species must be considered. 'Ihe likely
alternative explanations for the origin of this group are sympatric divergence or
hybridization.
H. A. FORD ET A L .
292
401
30
I
- Black
males
---Others
Beak length (mm)
Figure 1 . Distribution of beak-length of the genus, Geospiza, with the exception of G. scandens.
Black males and others are shown separately.
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29 3
DIVERGENCE AND EVOLUTION IN DARWIN'S FINCHES
1790
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Figure 3. Accumulative measurements of the beak-depth of black males of C.fortis and
G. magnirostris.
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Figure 4. Accumulative measurements of the' beak-depth of females and non-black males of
G. fortis and G. mogniroatris.
294
11. A. FORD ET A L .
The conditions of Smith (1966) must be satisfied for sympatric divergence
to be a likely explanation. l h e r e must be clear separation of the environment
into separate ecological niches. Although this could only be proved beyond
doubt by an exhaustive study of all possible food sources, the work of Bowman
(1961) has shown that the predominant food of this genus, seeds, can be
classified into fairly distinct categories. From stomach contents he showed that
G. fiiligiriosu tends to feed on the smallest seeds, G. f o r f i s on the small and
medium seeds and G tuugrzirostris on the largest and hardest seeds. Especially
interesting was the presence of the large hard seeds of Burscru and Cordiu in the
stomachs of large individuals of G. fi)rtis yet not in birds of the typical size.
Thus this intermediate group appears t o be utilizing a food source which
typical G‘. f o r t i s cannot, and is therefore occupying a slightly different niche, at
least for some seasons of the year.
There is, at present, no evidence that these finches show maximum
efficiency at utilizing the seeds which they eat most frequently. The work of
Kear (1962), on British finches, shows how large-billed finches are far more
efficient at husking large seeds, while small-billed finches are better at husking
smaller seeds. These are, respectively, the seeds most commonly eaten by the
finches suggesting a relation between the food actually eaten and the food dealt
with most efficiently. The same is likely t o be true of Darwin’s finches on the
Galapagos 1slands .
The abundance of the different kinds of seeds will not vary synchronously,
indeed at some times there may be shortages of certain foods due t o the very
variable nature of the Galapagos climate both in the long and short terms
(Colinvaux, 1972). Hence the size of these niches and the sizes of the
populations occupying them will be separately regulated. However, the flora of
Indefatigable island is sufficiently diverse (about 300 species) t o favour many
specialists rather than a few generalists as occurs on the outer islands with more
restricted floras. (This idea is developed by Lack, 1969.)
Thus if separate niches do exist, and there is strong evidence that they do,
the conditions of Smith (1966) will be satisfied.
To consider the reproductive relationship between these two groups of
G. fi,rtis, Lack (1947) believes that beak size is the specific recognition
character in this genus, individuals selecting as mates birds which have beaks
similar in size and shape to their own. Bowman (pers. comm.) however believes
that song is more important. If beak shape be important in mate choice, then
the evolution of reproductive isolation between these two groups would be
greatly facilitated, since the character leading to assortative mating is the very
same as that leading to the ecological differences between the two groups. Song
could have diverged subsequently.
The alternative explanation for the origin of this group is hybridization
which differs from sympatric divergence in that the new niche can only be
occupied after a period of geographical isolation and subsequent introgression.
Although popular opinion, greatly influenced by Mayr ( 1 942, 1963 ) would
favour the hybridization hypothesis, we believe that as the necessary conditions
for sympatric divergence are probably present, and as it is more direct (not
requiring another island) it is equally likely.
If “large” G. fortis could have arisen from “typical” G. fortis without
geographical isolation (the case we have been arguing), then the step from
DIVERGENCE AND EVOLUTION IN DARWIN’S FINCHES
295
“large” G. fortis to G. magnirostris can also be considered. Sympatric divergence may be an important mechanism on oceanic islands where many
alternative niches are available for the first colonizers. It may be an alternative
to “double invasion” in explaining the presence of congeners on isolated
islands. More information is necessary on the disruptive nature of the
environment, on the comparative efficiency of seed utilization, and on the
reproductive relationships ‘between the different bird groups, before the
likelihood of sympatric speciation having taken place in this example can be
fully assessed. However we believe that this is a definite possibility, and is
worth considering because of the importance of these finches in the
development of our knowledge of evolution.
ACKNOWLEDGEMENTS
This work was supported by the Percy Sladen Memorial Fund, the Godman
Exploration Fund, the Carnegie Trust for Scottish Universities and the
Zoological Society of London. The authors are grateful for criticism and advice
from Prof. B. Clarke, Prof. A. J. Cain, Prof. J. Antonovics, Dr D. Lack and
Prof. P. R. Grant. The facilities offered by the Charles Darwin Foundation on
the Galapagos islands were much appreciated and this paper is contribution
No. 117 of that Foundation.
REFERENCES
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Calif Publs Zool., 5 8 : 1-302.
BUSH, G. L., 1969. Sympatric host race formation and speciation in frugivorous flies of the genus,
Rhagoletis, (Diptera, Tephritidae). Evolution. Lancaster Pa., 23: 237-51.
COLINVAUX, P. A.. 1972. Climate and the Galapagos Islands. Nature, Lond., 240: 17-20.
KEAR, J., 1962. Food selection in finches, with special reference to interspecific differences. Proc. 2001.
SOC.Lond., 138: 163-205.
LACK, D., 1945. The Galapagos finches (Geospizinae): A study in variation. Occ. Pap. Calif. Acad. Sci..
21: 1-159.
LACK, D.. 1947. Darwin ‘sfinches. Cambridge University Press.
LACK, D., 1969. Subspecies and sympatry in Darwin’s finches. Evolution, Lancaster Pa., 23.’ 252-63.
MATHER, K., 1955. Polymorphism as the outcome of disruptive selection. Evolution, Lancasfer Pa., 9:
52-61.
MAYR, E., 1942. Systematics and the origin of species. Columbia University Press.
MAYR, E.. 1963. Animal species and evolution. Harvard University Press.
SMITH, J. M., 1966. Sympatric speciation. A m . Nat.. 100: 637-49.
SNOW, D. W., 1966. Moult and the breeding cycle in Darwin’s finches. J. O m . , Lpz., 107: 283-91.
THODAY. J. M. & GIBSON, J. B., 1970. The probability of isolation by disruptive selection. Am. Nut.,
104: 219-30.