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Evolution of Character Displacement in Darwin's
Finches
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(print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the ScienceAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright2006 by the American Association for the Advancement of Science; all rights reserved. The title have anything but a mild competitive effect onG. fortis. Their numbers gradually increased asa result of local production of recruits, aug- mented by additional immigrants (22, 25), andreached a maximum of 354 T 47 (SE) in 2003(Fig. 3). Little rain fell in 2003 (16 mm) and 2004 (25 mm), there was no breeding in eitheryear, numbers of both species declined drasti- Competitor species can have evolutionary effects on each other that result in ecological cally, and from 2004 to 2005 G. fortis ex- character displacement; that is, divergence in resource-exploiting traits such as jaws and beaks.
perienced strong directional selection against Nevertheless, the process of character displacement occurring in nature, from the initial encounter of competitors to the evolutionary change in one or more of them, has not previously been Selection differentials in G. fortis were uni- investigated. Here we report that a Darwin’s finch species (Geospiza fortis) on an undisturbed formly negative for both males and females Gala´pagos island diverged in beak size from a competitor species (G. magnirostris) 22 years after treated separately (Table 2). Average selection the competitor’s arrival, when they jointly and severely depleted the food supply. The observed differentials in standard deviation units for the evolutionary response to natural selection was the strongest recorded in 33 years of study, and six measured traits that quantify bill size and close to the value predicted from the high heritability of beak size. These findings support the role shape and body size were 0.774 for males and of competition in models of community assembly, speciation, and adaptive radiations.
0.649 for females. Compared with values re-ported in other studies elsewhere (27), they are Characterdisplacement(1,2)isanevolu- termined by the tradeoff in energetic rewards unusually large. The six traits are positively tionary divergence in resource-exploiting from feeding on small and large seeds, and the correlated to varying degrees. Selection gradi- tradeoff is affected by variation in beak mor- ent analysis helps to identify which particular caused by interspecific competition (3–5). It has traits were subject to selection independent of the potential to explain nonrandom patterns of replenishment (7, 18, 19). Competitors can mod- correlations among traits (28). However, bill co-occurrence and morphological differences depth and width are so strongly correlated in between coexisting species (6–10). Supporting these samples (r 0 0.861 for males, 0.946 for evidence has come from phylogenetic analyses with the arrival of a new competitor species, females) that their independent effects on sur- (11) and from experimental studies of stickle- setting up the potential for character displace- vival cannot be distinguished. Selection gradient backs, in which the role of directional selection ment to occur. Between 1973 and 1982, a few analysis without these two variables shows bill in character divergence has been demonstrated individuals of the large ground finch (G.
length to be the only significant entry into the (12). The process of character displacement magnirostris; È30 g) visited the island for gradient, for both males Epartial regression co- occurring in nature, from the initial encounter short periods in the dry season but never bred efficient (b) 0 –0.931 T 0.334 SE, P 0 0.0079; of competitors to the evolutionary change in (15). In late 1982, a breeding population was R2 0 0.190^ and females (b 0 –0.814 T 0.295, P 0 one or more of them as a result of direction- established by two females and three males at al natural selection, has not previously been the beginning of an exceptionally strong El NiDo event that brought abundant rain to the Table 1. Proportions of seeds in the diets of three The situation on the small Gal"pagos island island (1359 mm) (20–22). G. magnirostris is finch species. Small seeds are a composite group of Daphne Major (0.34 km2) has been referred a potential competitor as a result of diet of 22 species, medium seeds are O. echios, and to as the classical case of character release overlap with G. fortis (Table 1), especially in large seeds are T. cistoides. N is the number of (1, 2, 13), which is the converse of character the dry season when food supply is limiting observations. There is strong heterogeneity in the displacement. Here, in the virtual absence of (9, 23). The principal food of G. magnirostris the small ground finch (Geospiza fuliginosa; is the seeds of Tribulus cistoides, contained 0.0001). The reduction in G. fortis feeding on weighing È12 g) and released from compe- within a hard mericarp and exposed when a Tribulus in 2004 makes a significant contribution tition, the medium ground finch (G. fortis; finch cracks or tears away the woody outer (X 2 0 3.912, P G 0.05). Data were obtained by È18 g) is unusually small in beak and body size.
covering (Fig. 1). Large-beaked members of observations in the first 3 months of each year.
In 1977 (only), when G. fortis experienced direc- Lack (14) proposed that its small size reflects the G. fortis population are capable of this tional selection against small bill size, the pro- an evolutionary shift enabling G. fortis to take maneuver—indeed, survival in the 1977 drought portion of large seeds in the diet rose to 0.304 to a large extent depended on it (13, 16)—but available by the absence of its competitor. Sub- on average they take three times longer than G.
sequent field studies demonstrated an associa- magnirostris to gain a seed reward (13, 24).
tion, previously only inferred, between beak The smallest G. fortis never attempt to crack sizes and seed diets (13, 15). In 1977, a drought them (18, 24). G. magnirostris compete with G.
on Daphne revealed that small seeds are pre- ferred when they are abundant, but when they Tribulus feeding sites and by reducing the are scarce, finches turn increasingly to large density of Tribulus fruits to the point at which and hard seeds that only the large-beaked mem- it is not profitable for G. fortis to feed on them, bers of the population can crack (13, 15). Most owing to handling inefficiencies in relation to finches died that year, and mortality was heavi- search and metabolic costs (7, 13, 18, 24). By est among those with small beaks (13, 16, 17).
depleting the supply of Tribulus fruits, G.
Thus, a population_s mean beak size is de- magnirostris was predicted to cause a selective shift in G. fortis in the direction of small beak Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544–1003, USA.
The predicted shift occurred in 2004 (Fig. 2).
Initially, the population size of G. magnirostris *To whom correspondence should be addressed: E-mail: was too small in relation to the food supply to 0.0130; R2 0 0.455). Inclusion of either bill Thus, character displacement in G. fortis consumed by a G. magnirostris individual each depth or bill width made no difference to occurred in 2004–2005. Four lines of evidence day are sufficient for two G. fortis individuals if these results. Overall bill size rather than bill support the causal role of G. magnirostris. First, they feed on nothing else (13). Moreover, a length is identified as the most important fac- the potential impact of G. magnirostris was much higher fraction of G. magnirostris than G.
tor distinguishing survivors from nonsurvi- greatest at the beginning of 2004 because their fortis feed on Tribulus, as inferred from feeding numbers (150 T 19) were closer to those of G.
observations (Table 1). As a result of their joint size) was a selected trait in both sexes, where- fortis (235 T 46) than at any other time (Fig. 3), reduction of seed biomass, G. fortis fed on and their population biomass was about the Tribulus in 2004 only half as frequently as in either. There was little effect on body size, same, because a G. magnirostris individual was other years (Table 1). We did not quantify food unlike in the 1977 episode. In contrast to G.
approximately twice the mass of a G. fortis supply; nevertheless, food scarcity was evident fortis, the heavy mortality experienced by G.
from the exceptionally low feeding rates of G.
magnirostris was apparently not selective: Four Second, G. magnirostris are largely depen- magnirostris. In 2004, a minimum of 90 in- surviving males did not differ from 32 non- dent on an important food resource, Tribulus dividuals were observed foraging for Tribulus survivors in any of the six measured traits (all seeds, which are not renewed during droughts.
mericarps for 200 to 300 s, and none obtained P 9 0.1), and only 1 of 38 measured females G. magnirostris deplete the Tribulus seed sup- seeds from more than two mericarps; whereas ply faster than do G. fortis. The seeds that are under the more typical conditions prevailing inthe 1970s, a total of eight birds observed for thesame length of time fed on 9 to 22 mericarps,with an average interval between successivemericarps of only 5.5 T 0.5 s (SE) (24).
Third, numbers of G. fortis declined to a lower level (83) in 2005 than at any time sincethe study began in 1973, and numbers of G.
magnirostris declined so strongly from the2003 maximum that by 2005, only fourfemales and nine males were left. The popu-lation was almost extinct, apparently as a result of exhaustion of the standing crop oflarge seeds and subsequent starvation. Of the137 G. magnirostris that disappeared in 2004–2005, 13.0% were found dead, and so were21.7% of 152 G. fortis. Consistent with thestarvation hypothesis, the stomachs of all deadbirds (23 G. magnirostris and 45 G. fortis,banded and not banded individuals combined)were empty.
The principal alternative food for both spe- cies is the seeds of Opuntia cactus, butproduction in 2004 was low, the fourth lowestsince records were first kept systematically in1982 (23). Not only were cactus seeds insuffi-cient for the two granivore species to escape the Fig. 1. Large-beaked G. fortis (A) and G. magnirostris (B) can crack or tear the woody tissues of dilemma of a diminishing supply of their pre- T. cistoides mericarps (D), whereas small-beaked G. fortis (C) cannot. Five mericarps constitute a ferred foods, they were insufficient for the single fruit. In (D), the left-hand mericarp is intact. The right-hand mericarp, viewed from the other cactus specialist G. scandens (È20 g), whose (mesial) side, has been exploited by a finch, exposing five locules from which seeds have been numbers, like those of G. fortis, fell lower (to extracted. Mericarps are È8 mm long and are shown at twice the magnification of the finches.
50) than in any of the preceding 32 years. The only escape was available to the smallest, mostG. fuliginosa–like, members of the G. fortispopulation, which are known to feed like G.
fuliginosa on small seeds with little individual energy reward (13, 18). We have no feeding observations to indicate that they survived as a result of feeding on the typical components of the G. fuliginosa diet: the very small seeds of (13, 15, 23). Nevertheless, it may be signifi- cant that two G. fuliginosa individuals were present on the island in 2004 and both survived The fourth line of evidence is the contrast sequent changes in themean. Sample sizes vary between the directions of strong selection on the G. fortis population in the presence (2004) and near absence (1977) of G. magnirostris. In values are reversed so that mean size increases from the origin.
1977, a year of only 24 mm of rain and no breeding, body size and beak size of both male be expected from strong directional selection to null or neutral models (6, 9). Replicated and female G. fortis considered separately were against large size (32). This was observed. The subject to selection (Table 2). Average selec- mean beak size (PC1 ) of the 2005 generation needed to demonstrate definitively the causal tion differentials were 0.642 for males and measured in 2006 was significantly smaller role of competition, not only as an ingredient 0.668 for females, and they were uniformly than that in the 2004 sample of the parental of natural selection of resource-exploiting positive. In the intervening years, 1978–2003, traits (12) but as a factor in their evolution there was a weaker selection episode favoring 0.0001). The difference between generations is (33). Our findings should prove useful in 0.70 SD, which is exceptionally large (27, 29).
designing realistic experiments, by identifying It may be compared with the range of values ecological context (high densities at the start of an small seeds and scarcity of large ones after the predicted from the breeders equation, namely environmental stress) and by estimating the El NiDo event of 1982–1983 (20, 21, 23). At the product of the average selection differential that time, G. magnirostris were rare (22, 25); numbers varied from 2 to 24. The selection intervals of the heritability estimate. The ob- events of 1977 and 2004 stand out against a 1. W. L. Brown Jr., E. O. Wilson, Syst. Zool. 5, 49 background of relative morphological stability predicted range of 0.66 to 1.00 SD. Although (29) (Fig. 2). Immediately before 2004 there a small component of the response is probably 2. P. R. Grant, Biol. J. Linn. Soc. 4, 39 (1972).
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26. See methods in supporting material on Science Online.
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34. We thank K. T. Grant, L. F. Keller, K. Petren, and U. Reyer for help with recent fieldwork, and the Charles Darwin Research Station and Gala´pagos National Park Service forpermission and support. The research was supported by www.sciencemag.org/cgi/content/full/313/5784/224/DC1 5 April 2006; accepted 25 May 200610.1126/science.1128374

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