Ground Finch Research Articles

The flippingt‐ratio test: Phylogenetically informed assessment of the Pareto theory for phenotypic evolution

AbstractAn organism cannot be fully optimized for all tasks that are needed for its survival because of the existence of intrinsic trade‐offs among those tasks. It was recently proposed that an economics‐originated theory, the Pareto theory, is a general theory explaining the rules of phenotypic evolution under such trade‐offs. To date, many phenotype datasets have been argued to fit the Pareto theory based on a statistical method named thet‐ratio test.Here, we show that this test suffers a serious defect in that general phenotype data can be wrongly concluded to fit the Pareto theory with a very high false‐positive rate and that the claim that the Pareto theory is a general theory should definitely be considered with caution. This is because thet‐ratio test assumes that all phenotypic traits are independent of one another, but this assumption does not hold true—different traits of organisms have usually been affected by the same phylogenetic history and are thus typically not independent.We developed the flippingt‐ratio test to accurately test the Pareto theory by considering phylogenetic background as well as artefacts that can be induced during dimensionality reduction. Using this improved method, we confirm that the phenotype data analysed in previous studies, including the well‐known Darwin's ground finch dataset, no longer support the Pareto theory.We hope that the flippingt‐ratio test will contribute to examining which phenotype datasets truly fit the Pareto theory and understanding how diverse phenotypes evolve in natural ecosystems.

Structural variation and phylogenetic relationship of Geospiza magnirostris based on mitochondrial control region

The control region of mitochondrial genome is the main non-coding region. There is no details about Geospiza magnirostris’s control region, who is the only species of Geospiza in Darwin finches whose mitochondrial genome has been sequenced. In this study, the control region mitochondrial genome of G. magnirostris was analyzed. Compare research for 15 Passerine species, belonging to 4 families (Darwin ground finch, Thraupidae, Icteridae, Emberizidae), was also been done. The result showed the length was 1202 bp and it could been divided into three parts, domainI, domainII and domainIII. Genetic distances showed average divergence of different family ranged from 0.36% (Darwin ground finch) to 9.20% (Emberizidae), indicated that the Darwin ground finch may be a very conservative single-source species. Some specific sequence fragments such as conservative sequence box, boxes C, D, F and E, HSP/LSP, CSB-1, TAS-1 were also found in the control region of G. magnirostris, while B-box, CSB-2 and CSB-3 were not found in it. The F-box was detected in all selected 15 species and was 29 bp in length with no variant sites. In the E-box, 20 of the 21 nucleotide positions were identical as conserved among the selected species. There are two variant sites in the D-box, and five variant sites in the C-box. Basing the phylogenetic tree, it can be clearly seen that the relationship of G. magnirostris and family Thraupidae is the closest in all selected family. Through this study, the structure characteristics of G. magnirostris control region were explored and understood. This study provided more basic information for in-depth research of G. magnirostris. It’s also reminded us that we should pay more attention to explore the characteristics of the control region of Darwin’s finches.

Temporally varying disruptive selection in the medium ground finch (Geospiza fortis).

Disruptive natural selection within populations exploiting different resources is considered to be a major driver of adaptive radiation and the production of biodiversity. Fitness functions, which describe the relationships between trait variation and fitness, can help to illuminate how this disruptive selection leads to population differentiation. However, a single fitness function represents only a particular selection regime over a single specified time period (often a single season or a year), and therefore might not capture longer-term dynamics. Here, we build a series of annual fitness functions that quantify the relationships between phenotype and apparent survival. These functions are based on a 9-year mark-recapture dataset of over 600 medium ground finches (Geospiza fortis) within a population bimodal for beak size. We then relate changes in the shape of these functions to climate variables. We find that disruptive selection between small and large beak morphotypes, as reported previously for 2 years, is present throughout the study period, but that the intensity of this selection varies in association with the harshness of environment. In particular, we find that disruptive selection was strongest when precipitation was high during the dry season of the previous year. Our results shed light on climatic factors associated with disruptive selection in Darwin's finches, and highlight the role of temporally varying fitness functions in modulating the extent of population differentiation.

Human influences on antipredator behaviour in Darwin's finches.

Humans exert dramatic influences upon the environment, creating novel selective pressures to which organisms must adapt. On the Galapagos, humans have established a permanent presence and have altered selective pressures through influences such as invasive predators and urbanization, affecting iconic species such as Darwin's finches. Here, I ask two key questions: (a) Does antipredator behaviour (e.g., flight initiation distance - FID) change depending on whether invasive predators are historically absent, present, or eradicated? and (b) To what degree does urbanization affect antipredator behaviour? This study is one of the first to quantify antipredator behaviour in endemic species after the eradication of invasive predators. This will help to understand the consequences of invasive predator eradication and inform conservation measures. I quantified FID, an antipredator behaviour, in the small ground finch, across multiple islands in the Galapagos that varied in the presence, absence, or successful eradication of invasive predators. On islands with human populations, I quantified FID in urban and non-urban populations of finches. FID was higher on islands with invasive predators compared to islands with no predators. On islands from which invasive predators were eradicated ~11years previously, FID was also higher than on islands with no invasive predators. Within islands that had both urban and non-urban populations of finches, FID was lower in urban finch populations, but only above a threshold human population size. FID in larger urban areas on islands with invasive predators was similar to or lower than FID on islands with no history of invasive predators. Overall, these results suggest that invasive predators can have a lasting effect on antipredator behaviour, even after eradication. Furthermore, the effect of urbanization can strongly oppose the effect of invasive predators, reducing antipredator behaviour to levels lower than found on pristine islands with no human influences. These results improve our understanding of human influences on antipredator behaviour which can help inform future conservation and management efforts on islands.

The ecology and evolution of seed predation by Darwin's finches on Tribulus cistoides on the Galápagos Islands

AbstractPredator–prey interactions play a key role in the evolution of species traits through antagonistic coevolutionary arms races. The evolution of beak morphology in the Darwin's finches in response to competition for seed resources is a classic example of evolution by natural selection. The seeds of Tribulus cistoides are an important food source for the largest ground finch species (Geospiza fortis, G. magnirostris, and G. conirostris) in dry months, and the hard spiny morphology of the fruits is a potent agent of selection that drives contemporary evolutionary change in finch beak morphology. Although the effects of these interactions on finches are well known, how seed predation affects the ecology and evolution of the plants is poorly understood. Here we examine whether seed predation by Darwin's finches affects the ecology and evolution of T. cistoides. We ask whether the intensity of seed predation and the strength of natural selection by finches on fruit defense traits vary among populations, islands, years, or with varying finch community composition (i.e., the presence/absence of the largest beaked species, which feed on T. cistoides most easily). We then further test whether T. cistoides fruit defenses have diverged among islands in response to spatial variation in finch communities. We addressed these questions by examining seed predation by finches in 30 populations of T. cistoides over 3 yr. Our study reveals three key results. First, Darwin's finches strongly influence T. cistoides seed survival, whereby seed predation varies with differences in finch community composition among islands and in response to interannual fluctuations in precipitation. Second, finches impose phenotypic selection on T. cistoides fruit morphology, whereby smaller and harder fruits with longer or more spines exhibited higher seed survival. Variation in finch community composition and precipitation also explains variation in phenotypic selection on fruit defense traits. Third, variation in the number of spines on fruits among islands is consistent with divergent phenotypic selection imposed by variation in finch community composition among islands. These results suggest that Darwin's finches and T. cistoides are experiencing an ongoing coevolutionary arms race, and that the strength of this coevolution varies in space and time.

Darwin's finches: a model of landscape effects on metacommunity dynamics in the Galápagos Archipelago

Darwin's finches represent a dynamic radiation of birds within the Galápagos Archipelago. Unlike classic island radiations dominated by island endemics and intuitive ‘conveyer belt’ colonization with little subsequent dispersal, species of Darwin's finches have populations distributed across many islands and each island contains complex metacommunities of closely related birds. Understanding the role of metacommunity and structured population dynamics in speciation within this heterogeneous island system would provide insights into the roles of fragmentation and dispersal in evolution. In this study, a large multi‐species dataset and a comparative ground finch dataset (two co‐distributed lineages) were used to show how landscape features influence patterns of gene flow across the archipelago. Factors expected to regulate migration including distance and movement from large, central islands to small, peripheral islands were rejected in the multi‐species dataset. Instead, the harsh northeast islands contributed individuals to the larger central islands. Successful immigration relies on three factors: arriving, surviving and reproducing, thus the dispersal towards the central islands may be either be due to more migrants orienting towards these land masses due to their large size and high elevation, or may reflect a higher likelihood of survival and successful reproduction due to the larger diversity of habitats and more environmentally stable ecosystems that these islands possess. Further, the overall directionality of migration was south‐southwest against the dominant winds and currents. In comparing dispersal between the common cactus finch and medium ground finch, both species had similar migration rates but the cactus finch had approximately half the numbers of migrants due to lower effective populations sizes. Significant population structure in the cactus finch indicates potential for further speciation, while the medium ground finch maintains cohesive gene flow across islands. These patterns shed light on the macroevolutionary patterns that drive diversification and speciation within a radiation of highly‐volant taxa.

Dispersal of fern spores by Galápagos finches

Fern sporangia may provide an important source of energy for bird species, which in turn can act as potential dispersers of viable spores. This study reports the first case of fern spore dispersal by land birds. We document the consumption of fern sporangia and evaluate the potential spore dispersal by Galapagos finches on Santa Cruz Island. Overall, 18% of the 34 sampled individuals of three finch species, the Vegetarian Finch (Platyspiza crassirostris), the Small Ground Finch (Geospiza fuliginosa) and the Medium Ground Finch (Geospiza fortis), were found to disperse viable spores of two native ferns, Asplenium auritum and Asplenium feei.

Minimum longevity and age-related male plumage in Darwin’s finches on Floreana Island

Several Darwin’s finch populations on the Galapagos Islands are declining and/or locally extinct. Acoustic surveys provide useful information about population size, but do not provide information on the age or morphology of birds. This study uses mist netting data collected during 2004–2016 on Floreana Island with the aim of evaluating minimum longevity in Darwin’s finches. The study species are the Small Tree Finch (Camarhynchus parvulus), hybrid tree finches (Camarhynchus spp.), Medium Tree Finch (Camarhynchus pauper), and Small Ground Finch (Geospiza fuliginosa). In total, 1032 Darwin’s finches were mist netted and 86 of 707 males and 14 of 325 females were recaptured across years. We used the proportion of black plumage to age males, and the age at first capture plus the number of years between recapture to estimate minimum longevity. Minimum longevity ranged from 12 to 15 years and was lowest in the critically endangered C. pauper (12 years). The average number of years between first capture and last recapture was significantly lower in females than in males. Because long-term mist netting provides information on age structure, recruitment and longevity in males and females, it should be a key component of effective conservation planning.

Species Limits and Phylogenomic Relationships of Darwin's Finches Remain Unresolved: Potential Consequences of a Volatile Ecological Setting.

Island biotas have become paradigms for illustrating many evolutionary processes. The fauna of the Galapagos Islands includes several taxa that have been focal points for evolutionary studies. Perhaps their most famous inhabitants, Darwin's finches, represent a go-to icon when thinking about how species originate and adapt to the environment. However, unlike other adaptive radiations, past morphological and molecular studies of Darwin's finches have yielded inconsistent hypotheses of species limits and phylogenetic relationships. Expecting that idiosyncrasies of prior data and analytic methods explained different proposed classifications, we were surprised to observe that three new phylogenetic hypotheses derived mostly from the same genomics data were topologically inconsistent. We found that the differences between some of these genomics trees were as great as one would expect between two random trees with the same number of taxa. Thus, the phylogeny of Darwin's finches remains unresolved, as it has for more than a century. A component of phylogenetic uncertainty comes from unclear species limits, under any species concept, in the ground finches (Geospiza) and tree finches (Camarhynchus). We suggest that past authors should have tested the species limits of Lack, rather than uncritically accepting them. In fact, the impressive amount of genomics data do not provide unambiguous hypotheses of the number of species of Geospiza or Camarhynchus, although they imply greater species diversity than Lack's taxonomy. We suggest that insufficient sampling of species populations across islands (35.6% for morphometrics and 20.4% for genomics) prevents accurate diagnoses of species limits. However, it is unknown whether samples from a greater number of islands might result in bridging differences between species, or reveal many new ones. We conclude that attempts to interpret patterns of variation among the finches under standard evolutionary paradigms have obscured some major messages, most specifically the ongoing reciprocal interactions between geographic isolation and lineage divergence, and dispersal and gene flow caused by the volatile ecological conditions in the islands. Although the finches provide textbook examples of natural selection, better understanding of species limits and a robust phylogenetic hypothesis are required to corroborate past hypotheses of speciation and adaptive radiation in the finches of the Galapagos.

Beak of the pinch: anti-parasite traits are similar among Darwin’s finch species

Darwin’s finches are an iconic example of adaptive radiation. The size and shape of the beaks of different finch species are diversified for feeding on different size seeds and other food resources. However, beaks also serve other functions, such as preening for the control of ectoparasites. In diverse groups of birds, the effectiveness of preening is governed by the length of the overhanging tip of the upper mandible of the beak. This overhang functions as a template against which the tip of the lower mandible generates a pinching force sufficient to damage or kill ectoparasites. Here we compare feeding versus preening components of the beak morphology of small, medium, and large ground finches that share a single parasite community. Despite adaptive divergence in beak morphology related to feeding, the three species have nearly identical relative mandibular overhang lengths. Moreover, birds with intermediate length overhangs have the lowest feather mite loads. These results suggest that Darwin’s finches maintain an optimal beak morphology to effectively control their ectoparasites.

Baseline and stress-induced blood properties of male and female Darwin’s small ground finch (Geospiza fuliginosa) of the Galapagos Islands

Birds are renowned for exhibiting marked sex-specific differences in activity levels and reproductive investment during the breeding season, potentially impacting circulating blood parameters associated with stress and energetics. Males of many passerines often do not incubate, but they experience direct exposure to intruder threat and exhibit aggressive behaviour during the nesting phase in order to defend territories against competing males and predators. Nesting females often have long bouts of inactivity during incubation, but they must remain vigilant of the risks posed by predators and conspecific intruders approaching the nest. Here, we use 33 free-living male (n = 16) and female (n = 17) Darwin's small ground finches (Geospiza fuliginosa) on Floreana Island (Galapagos Archipelago) to better understand how sex-specific roles during the reproductive period impact baseline and stress-induced levels of plasma corticosterone (CORT), blood glucose and haematocrit. Specifically, we hypothesise that males are characterised by higher baseline values given their direct and relatively frequent exposure to intruder threat, but that a standardised stress event (capture and holding) overrides any sex-specific differences. In contrast with expectations, baseline levels of all blood parameters were similar between sexes (13.4 ± 1.9 ng ml−1 for CORT, 13.7 ± 0.4 mmol l−1 for glucose, 58.3 ± 0.8% for haematocrit). Interestingly, females with higher body condition had lower baseline haematocrit. All blood parameters changed with time since capture (range 1.2–41.3 min) in both sexes, whereby CORT increased linearly, haematocrit decreased linearly, and glucose increased to a peak at ∼20 min post-capture and declined to baseline levels thereafter. Our results do not support the hypothesis that sex-specific roles during the reproductive period translate to differences in blood parameters associated with stress and energetics, but we found some evidence that blood oxygen transport capacity may decline as finches increase in body condition.

Thermoregulatory windows in Darwin's finches

Abstract Darwin's finches have been the focus of intense study demonstrating how climatic fluctuations coupled with resource competition drive the evolution of a variety of bill sizes and shapes. The bill, as other peripheral surfaces, also plays an important role in thermoregulation in numerous bird species. The avian bill is vascularized, while limbs have specialized vasculature that facilitate heat loss or heat conservation (i.e. they are thermoregulatory windows). The Galápagos Islands, home to Darwin's finches, have a hot and relatively dry climate for approximately half of the year, during which thermoregulatory windows (i.e. surfaces) could be important for thermoregulation and the linked challenge of water balance. We hypothesized that Darwin's finch bills have evolved in part for their role in thermoregulation, possibly co‐opted, following adaptation for other functions, such as foraging. We predicted that bills of Darwin's finches are effective thermoregulatory windows, and that species differences in bill morphology, along with physiology and behaviour, lead to differences in thermoregulatory function. To test these hypotheses, we conducted a field study to assess heat exchange and microclimate use in three ground finch species and sympatric cactus finch (Geospiza spp.). We collected thermal images of free‐living birds during a hot and dry season and recorded microclimate data for each observation. We used individual thermographic data to model the contribution of bills, legs and bodies to overall heat balance and compared surface temperatures to those from dead birds to test physiological control of heat loss from these surfaces. We derived and compared species‐specific threshold environmental temperatures, which are indicative of a species’ thermally neutral temperature. In all species, the bill surface was an effective heat dissipater during naturally occurring warm temperatures. As expected, we found that finches controlled surface temperatures through physiology and that young birds had higher surface temperatures than adults. Larger bills contributed proportionally more to overall heat loss than smaller bills. We demonstrate here that related, sympatric species with different bill sizes exhibit different patterns in the use of these thermoregulatory structures, supporting a role for thermoregulation in the evolution and ecology of Darwin's finch morphology. A plain language summary is available for this article.

Epigenetic variation between urban and rural populations of Darwin\u2019s finches

BackgroundThe molecular basis of evolutionary change is assumed to be genetic variation. However, growing evidence suggests that epigenetic mechanisms, such as DNA methylation, may also be involved in rapid adaptation to new environments. An important first step in evaluating this hypothesis is to test for the presence of epigenetic variation between natural populations living under different environmental conditions.ResultsIn the current study we explored variation between populations of Darwin’s finches, which comprise one of the best-studied examples of adaptive radiation. We tested for morphological, genetic, and epigenetic differences between adjacent “urban” and “rural” populations of each of two species of ground finches, Geospiza fortis and G. fuliginosa, on Santa Cruz Island in the Galápagos. Using data collected from more than 1000 birds, we found significant morphological differences between populations of G. fortis, but not G. fuliginosa. We did not find large size copy number variation (CNV) genetic differences between populations of either species. However, other genetic variants were not investigated. In contrast, we did find dramatic epigenetic differences between the urban and rural populations of both species, based on DNA methylation analysis. We explored genomic features and gene associations of the differentially DNA methylated regions (DMR), as well as their possible functional significance.ConclusionsIn summary, our study documents local population epigenetic variation within each of two species of Darwin’s finches.

Host-specific associations affect the microbiome of Philornis downsi, an introduced parasite to the Galápagos Islands.

The composition and diversity of bacteria forming the microbiome of parasitic organisms have implications for differential host pathogenicity and host-parasite co-evolutionary interactions. The microbiome of pathogens can therefore have consequences that are relevant for managing disease prevalence and impact on affected hosts. Here, we investigate the microbiome of an invasive parasitic fly Philornis downsi, recently introduced to the Galápagos Islands, where it poses extinction threat to Darwin's finches and other land birds. Larvae infest nests of Darwin's finches and consume blood and tissue of developing nestlings, and have severe mortality impacts. Using 16s rRNA sequencing data, we characterize the bacterial microbiota associated with P.downsi adults and larvae sourced from four finch host species, inhabiting two islands and representing two ecologically distinct groups. We show that larval and adult microbiomes are dominated by the phyla Proteobacteria and Firmicutes, which significantly differ between life stages in their distributions. Additionally, bacterial community structure significantly differed between larvae retrieved from strictly insectivorous warbler finches (Certhidea olivacea) and those parasitizing hosts with broader dietary preferences (ground and tree finches, Geospiza and Camarhynchus spp., respectively). Finally, we found no spatial effects on the larval microbiome, as larvae feeding on the same host (ground finches) harboured similar microbiomes across islands. Our results suggest that the microbiome of P.downsi changes during its development, according to dietary composition or nutritional needs, and is significantly affected by host-related factors during the larval stage. Unravelling the ecological significance of bacteria for this parasite will contribute to the development of novel, effective control strategies.

The adaptive genomic landscape of beak morphology in Darwin's finches.

Beak shape in Darwin's ground finches (Geospiza) is emblematic of natural selection and adaptive radiation, yet our understanding of the genetic basis of beak shape variation, and thus the genetic target of natural selection, is still evolving. Here we reveal the genomic architecture of beak shape variation using genomewide comparisons of four closely related and hybridizing species across 13 islands subject to parallel natural selection. Pairwise contrasts among species were used to identify a large number of genomic loci that are consistently related to species differences across a complex landscape. These loci are associated with hundreds of genes that have enriched GO categories significantly associated with development. One genomic region of particular interest is a section of Chromosome 1A with many candidate genes and increased linkage. The distinct, pointed beak shape of the cactus finch is linked to an excess of intermediate frequency alleles and increased heterozygosity in significant SNPs, but not across the rest of the genome. Alleles associated with pointier beaks among species were associated with pointier-beaked populations within each species, thus establishing a common basis for natural selection, species divergence and adaptive radiation. The adaptive genomic landscape for Darwin's finches mirrors theoretical expectations based on morphological variation. The implication that a large number of genes are actively maintained to facilitate beak variation across parallel populations with documented interspecies admixture challenges our understanding of evolutionary processes in the wild.

Low diversity, activity, and density of transposable elements in five avian genomes

In this study, we conducted the activity, diversity, and density analysis of transposable elements (TEs) across five avian genomes (budgerigar, chicken, turkey, medium ground finch, and zebra finch) to explore the potential reason of small genome sizes of birds. We found that these avian genomes exhibited low density of TEs by about 10% of genome coverages and low diversity of TEs with the TE landscapes dominated by CR1 and ERV elements, and contrasting proliferation dynamics both between TE types and between species were observed across the five avian genomes. Phylogenetic analysis revealed that CR1 clade was more diverse in the family structure compared with R2 clade in birds; avian ERVs were classified into four clades (alpha, beta, gamma, and ERV-L) and belonged to three classes of ERV with an uneven distributed in these lineages. The activities of DNA and SINE TEs were very low in the evolution history of avian genomes; most LINEs and LTRs were ancient copies with a substantial decrease of activity in recent, with only LTRs and LINEs in chicken and zebra finch exhibiting weak activity in very recent, and very few TEs were intact; however, the recent activity may be underestimated due to the sequencing/assembly technologies in some species. Overall, this study demonstrates low diversity, activity, and density of TEs in the five avian species; highlights the differences of TEs in these lineages; and suggests that the current and recent activity of TEs in avian genomes is very limited, which may be one of the reasons of small genome sizes in birds.

Naris deformation in Darwin’s finches: Experimental and historical evidence for a post-1960s arrival of the parasite Philornis downsi

The rate of evolution depends on the strength of selection, which may be particularly strong for introduced parasites and their naive hosts. Because natural selection acts on phenotypes and because parasites can alter host phenotype, one fruitful starting point to measure the impact of novel pathogens is to quantify parasite-induced changes to host phenotype. Our study system is Darwin’s finches on Floreana Island, Galápagos Archipelago, and the virulent fly larvae of Philornis downsi that were first discovered in Darwin’s finch nests in 1997. We use an experimental approach and measure host phenotype in parasitized and parasite-free chicks in Darwin’s small ground finch (Geospiza fuliginosa). Beak size did not differ between the two treatment groups, but naris size was 106% larger in parasitized chicks (∼3.3 mm) versus parasite-free chicks (∼1.6 mm). To test if P. downsi was present prior to the 1960s, we compared naris size in historical (1899–1962) and contemporary birds (2004–2014) on Floreana Island in small ground finches (G. fuliginosa) and medium tree finches (Camarhynchus pauper). Contemporary Darwin’s finches had significantly larger naris size (including extreme deformation), whereas historical naris size was both smaller and less variable. These findings provide the first longitudinal analysis for the extent of P. downsi-induced change to host naris size and show that Darwin’s finches, prior to the 1960s, were not malformed. Thus natural selection on altered host phenotype as a consequence of P. downsi parasitism appears to be contemporary and novel.

Linked loci and Galapagos finch size

Evolutionary Genetics![Figure][1] Medium and large ground finches diverged in beak size after a severe drought. PHOTO: © FRANS LANTING STUDIO/ALAMY STOCK PHOTO Observations of parallel evolution in the finches of the Galapagos, including body and beak size, contributed to Darwin's theories. Lamichhaney et al. carried out whole-genome sequencing of 60 Darwin's finches. These included small, medium, and large ground finches as well as small, medium, and large tree finches. A genomic region containing the HMGA2 gene correlated strongly with beak size across different species. This locus appears to have played a role in beak diversification throughout the radiation of Darwin's finches. Science , this issue p. [470][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aad8786

A beak size locus in Darwin's finches facilitated character displacement during a drought.

Ecological character displacement is a process of morphological divergence that reduces competition for limited resources. We used genomic analysis to investigate the genetic basis of a documented character displacement event in Darwin's finches on Daphne Major in the Galápagos Islands: The medium ground finch diverged from its competitor, the large ground finch, during a severe drought. We discovered a genomic region containing the HMGA2 gene that varies systematically among Darwin's finch species with different beak sizes. Two haplotypes that diverged early in the radiation were involved in the character displacement event: Genotypes associated with large beak size were at a strong selective disadvantage in medium ground finches (selection coefficient s = 0.59). Thus, a major locus has apparently facilitated a rapid ecological diversification in the adaptive radiation of Darwin's finches.

Galápagos mockingbirds tolerate introduced parasites that affect Darwin's finches.

Introduced parasites threaten native host species that lack effective defenses. Such parasites increase the risk of extinction, particularly in small host populations like those on islands. If some host species are tolerant to introduced parasites, this could amplify the risk of the parasite to vulnerable host species. Recently, the introduced parasitic nest fly Philornis downsi has been implicated in the decline of Darwin's finch populations in the Galápagos Islands. In some years, 100% of finch nests fail due to P.downsi; however, other common host species nesting near Darwin's finches, such as the endemic Galápagos mockingbird (Mimus parvulus), appear to be less affected by P.downsi. We compared effects of P.downsi on mockingbirds and medium ground finches (Geospiza fortis) on Santa Cruz Island in the Galápagos. We experimentally manipulated the abundance of P.downsi in nests of mockingbirds and finches to measure the direct effect of the parasite on the reproductive success of each species of host. We also compared immunological and behavioral responses by each species of host to the fly. Although nests of the two host species had similar parasite densities, flies decreased the fitness of finches but not mockingbirds. Neither host species had a significant antibody-mediated immune response to P.downsi. Moreover, finches showed no significant increase in begging, parental provisioning, or plasma glucose levels in response to the flies. In contrast, parasitized mockingbird nestlings begged more than nonparasitized mockingbird nestlings. Greater begging was correlated with increased parental provisioning behavior, which appeared to compensate for parasite damage. The results of our study suggest that finches are negatively affected by P.downsi because they do not have such behavioral mechanisms for energy compensation. In contrast, mockingbirds are capable of compensation, making them tolerant hosts, and a possible indirect threat to Darwin's finches.