Darwin's Finches Research Articles

Heritability of morphological traits in Darwin's finches: misidentified paternity and maternal effects.

We studied the influence of extra-pair paternity on heritability estimates of morphological traits in a population of the Medium Ground Finch (Geospiza fortis) on Isla Daphne Major, Galápagos. Data from eight microsatellite loci were used to determine parentage. Six morphological traits measured on each finch were represented by two separate principal components analyses, one for the three bill measurements and one for the body size measurements. Heritabilities were calculated using weighted regressions of offspring on their parents and also offspring on their grandparents. We found that 20% of all offspring were extra-pair young but all offspring matched their mothers. Heritabilities derived from midparent-offspring regressions were all high and significantly different from zero. Removing all extra-pair young from the data set increased father-offspring regressions by an average of 21%, but mother-offspring resemblance still exceeded father-offspring resemblance by up to 42%. These results and grandparent-offspring regressions provide evidence for maternal effects, comparable in magnitude to those reported in other studies of wild birds.

Ecology and Evolution of Darwin’s Finches

Being old enough to remember the era of vinyl record albums, in which record companies often repackaged the same songs in reissues which usually differed only in the album cover, I was transported back to those days by this book. Not because Peter Grant has suddenly become a country-and-western singer, but because Princeton University Press has copied the recording industry's tactics of the 1970s by reissuing his 1986 classic book on Darwin's finches, complete with new cover and the equivalent of 'liner' notes in a foreward by Jonathan Weiner.

Ecology and Evolution of Darwin's Finches Peter R. Grant

Ecology and Evolution of Darwin's Finches Peter R. Grant

On the origin of Darwin's finches.

Darwin's finches comprise a group of 15 species endemic to the Galápagos (14 species) and Cocos (1 species) Islands in the Pacific Ocean. The group is monophyletic and originated from an ancestral species that reached the Galápagos Archipelago from Central or South America. Descendants of this ancestor on the Archipelago then colonized Cocos Island. In the present study, we used sequences of two mitochondrial (mt) DNA segments (922 bp of the cytochrome b gene and 1,082 bp of the control region), as well as two nuclear markers (830 bp of numt2, consisting of 140 bp of mtDNA control region and 690 bp of flanking nuclear DNA; and 740 bp of numt3, consisting of 420 bp of mt cytochrome b sequence flanked by 320 bp of nuclear DNA) to identify the species group most closely related to the Darwin's finches. To this end, we analyzed the sequences of 28 species representing the main groups (tribes) of the family Fringillidae, as well as 2 outgroup species and 13 species of Darwin's finches. In addition, we used mtDNA cytochrome b sequences of some 180 additional Fringillidae species from the database for phylogeny reconstruction by maximum-parsimony, maximum-likelihood, minimum-evolution, and neighbor-joining methods. The study identifies the grassquit genus Tiaris, and specifically the species Tiaris obscura, as the nearest living relative of Darwin's finches among the species surveyed. Darwin's finches diverged from the Tiaris group shortly after the various extant species of Tiaris diverged from one another. The initial adaptive radiation of the Tiaris group apparently occurred on the Caribbean islands and then spread to Central and South America, from where the ancestors of Darwin's finches departed for the Galápagos Islands approximately 2.3 MYA, at the time of the dramatic climatic changes associated with the closure of the Panamanian isthmus and the onset of Pleistocene glaciation.

Correlated evolution of morphology and vocal signal structure in Darwin's finches.

Speciation in many animal taxa is catalysed by the evolutionary diversification of mating signals. According to classical theories of speciation, mating signals diversify, in part, as an incidental byproduct of adaptation by natural selection to divergent ecologies, although empirical evidence in support of this hypothesis has been limited. Here I show, in Darwin's finches of the Galápagos Islands, that diversification of beak morphology and body size has shaped patterns of vocal signal evolution, such that birds with large beaks and body sizes have evolved songs with comparatively low rates of syllable repetition and narrow frequency bandwidths. The converse is true for small birds. Patterns of correlated evolution among morphology and song are consistent with the hypothesis that beak morphology constrains vocal evolution, with different beak morphologies differentially limiting a bird's ability to modulate vocal tract configurations during song production. These data illustrate how morphological adaptation may drive signal evolution and reproductive isolation, and furthermore identify a possible cause for rapid speciation in Darwin's finches.

Ecology and Evolution of Darwin's Finches

Ecology and Evolution of Darwin's Finches

Effects of El Nino Events on Darwin's Finch Productivity

Effects of El Nino Events on Darwin's Finch Productivity

EFFECTS OF EL NIÑO EVENTS ON DARWIN'S FINCH PRODUCTIVITY

We studied the effects of heavy and prolonged rainfall associated with four El Nino events on the reproduction of Darwin's finches on the Galapagos island of Daphne Major. Rainfall varied in the El Nino years from 195 mm to 1359 mm, exceeding the maximum in the other years by 40% to 1000%. Two species were studied: Geospiza fortis, Medium Ground Finch, and G. scandens, Cactus Finch. Almost all eggs, nestlings, and fledglings produced by banded females were recorded in the El Nino years of 1983, 1987, 1991, and 1998. Finch production in these years was compared with production in 10 other years of breeding in the period 1976-1990; there was no breeding in three drought years, 1985, 1988, and 1989. Breeding differed in the two sets of years in several ways. More broods were produced in El Nino years than in non-El Nino years (maximum 10 clutches per female vs. five clutches), the period of breeding was longer (maximum eight months vs. four), average clutch sizes (range 2-6 eggs) were distinctly larger (four vs. three), and average egg and fledgling production per female was larger by a factor of four. The two species differed in some features of breeding, but differences were minor in comparison with the marked seasonal and annual variation. Finch production varied among El Nino years, being greatest in the year of most rain over the longest period (1983), and least in the next wettest year (1998). The surprisingly low production in 1998 is attributed in part to interactions with other finches, and in part to exceptionally high temperatures. Temperature, although postively correlated with rainfall, had an independent negative effect upon hatching and overall breeding success of G. fortis. Breeding by both species in the year of birth (hatch) occurred in two El Nino years with the most extended wet seasons: 1983 and 1987. Young breeders had lower clutch sizes and breeding success than did contemporaneously breeding older birds. Observations in different El Nino years show that finch population responses to major climatic perturbations such as elevated rainfall vary for two major reasons: the perturbations themselves vary in strength and duration, and responses to them are determined, in part, by preceding conditions. Those preceding conditions, in turn, are determined by whether drought or normal conditions precede the perturbation, and on the interval since the previous El Nino event. Thus, perturbations of natural systems can be fully understood only in a broad temporal context.

Non-random fitness variation in two populations of Darwin's finches.

Darwinian fitness of an individual is measured by the number of recruits it contributes to the next generation. We studied variation in fitness among members of three cohorts of two species of Darwin's finches living on the Galipagos island of Daphne Major: the medium ground finch (Geospiza fortis) and cactus finch (Geospiza scandens). Individuals of both species live for up to 16 years. Variation in fitness was neither random nor heritable. Non-randomness arises as a result of a few individuals living for an exceptionally long time and breeding many times. For each cohort, the number of recruits per breeder is strongly predicted by the number of fledglings per breeder. In turn, the number of fledglings is strongly predicted by longevity of the breeder. These results suggest that the most important determinant of fitness is the ability of an individual to survive to breed in many years. Morphological traits affect this ability. Although morphological traits are heritable they do not change unidirectionally because they are selected in opposite directions, and in different combinations, under fluctuating environmental conditions. Non-random fitness variation in fluctuating populations implies much smaller genetically effective sizes than breeding population sizes.

THE MITOCHONDRIAL AND NUCLEAR GENETIC HOMOGENEITY OF THE PHENOTYPICALLY DIVERSE DARWIN'S GROUND FINCHES.

The most extensively studied group of Darwin's finches is the genus Geospiza, the ground finches, and yet little is known about the evolutionary history and genetic relationships of these birds. Studies using either allozyme or morphological data have been unable to resolve relationships between the six species and numerous populations of ground finches. In this paper we report the results of a study using mitochondrial control region and nuclear internal transcribed spacer (ITS) 1 sequence data. The differentiation of the ground finch species based on morphological data is not reflected in either mitochondrial or nuclear DNA sequence phylogenies. Furthermore, there is little concordance between the mitochondrial haplotypes and ITS alleles found within individuals. We suggest that the absence of species-specific lineages can be attributed to ongoing hybridization involving all six species of Geospiza. There are no long term selective pressures against hybridization within this genus, and therefore a genetically homogenous genus may be maintained indefinitely. Hybridization has apparently played a role in the adaptive radiation of Darwin's finches.

Phylogenetics of Darwin's Finches: Paraphyly in the Tree-Finches, and Two Divergent Lineages in the Warbler Finch

The Galapagos Darwin's finches (Geospizinae) have been classified as three major groups based on morphology and behavior: ground-finches, tree-finches, and the Warbler Finch (Certhidea olivacea). Little is known about the evolutionary relationships within and among these groups, which is partly due to the lack of a phylogeny based on molecular sequence data. We used mitochondrial sequence data to reconstruct a phylogeny of Darwin's finches. These data show that within the tree-finches, only one genus is conclusively monophyletic, and another is conclusively paraphyletic. It may be appropriate to uphold the classification of the tree-finches into two genera. The Warbler Finch complex is paraphyletic, as revealed by two divergent genetic lineages contained within this species. Stochastic lineage sorting within relatively recently diverged species and interspecific and intergeneric hybridization are the two most likely explanations for the sharing of haplotypes among taxa.

Phylogeny of Darwin's finches as revealed by mtDNA sequences.

Darwin's finches comprise a group of passerine birds first collected by Charles Darwin during his visit to the Galápagos Archipelago. The group, a textbook example of adaptive radiation (the diversification of a founding population into an array of species differentially adapted to diverse environmental niches), encompasses 14 currently recognized species, of which 13 live on the Galápagos Islands and one on the Cocos Island in the Pacific Ocean. Although Darwin's finches have been studied extensively by morphologists, ecologists, and ethologists, their phylogenetic relationships remain uncertain. Here, sequences of two mtDNA segments, the cytochrome b and the control region, have been used to infer the evolutionary history of the group. The data reveal the Darwin's finches to be a monophyletic group with the warbler finch being the species closest to the founding stock, followed by the vegetarian finch, and then by two sister groups, the ground and the tree finches. The Cocos finch is related to the tree finches of the Galápagos Islands. The traditional classification of ground finches into six species and tree finches into five species is not reflected in the molecular data. In these two groups, ancestral polymorphisms have not, as yet, been sorted out among the cross-hybridizing species.

A phylogeny of Darwin's finches based on microsatellite DNA length variation

Allele length variation at 16 microsatellite loci was used to estimate the phylogeny of 13 out of the 14 species of Darwin's finches. The resulting topology was similar to previous phylogenies based on morphological and allozyme variation. An unexpected result was that genetic divergence among Galapagos Island populations of the warbler finch (Certhidea olivacea) predates the radiation of all other Darwin's finches. This deep split is surprising in view of the relatively weak morphological differentiation among Certhidea populations and supports the hypothesis that the ancestor of all Darwin's finches was phenotypically similar to Certhidea. The results also resolve a biogeographical problem: the Cocos Island finch evolved after the Galapagos finch radiation was under way, supporting the hypothesis that this distant island was colonized from the Galapagos Islands. Monophyletic relationships are supported for both major groups, the ground finches (Geospiza) and the tree finches (Camarhynchus and Cactospiza), although the vegetarian finch (Platyspiza crassirostris) appears to have diverged prior to the separation of ground and tree finches. These results demonstrate the use of microsatellites for reconstructing phylogenies of closely related species and interpreting their evolutionary and biogeographic histories.

A new pressure sensory mechanism for prey detection in birds: the use of principles of seabed dynamics?

We demonstrate a novel mechanism for prey detection in birds. Red knots (Calidris canutus), sandpipers that occur worldwide in coastal intertidal areas, are able to detect their favourite hard-shelled prey even when buried in sand beyond the reach of their bills. In operant conditioning experiments designed to find out whether the birds could tell buckets containing only wet sand from buckets containing hard objects in wet sand, we show that they detect the presence not only of deeply buried live bivalves but also of stones. The latter finding virtually excludes, under experimental conditions, prey-detection mechanisms based on vision, acoustics, smell, taste, vibrational signals emitted by prey, temperature gradients and electromagnetic fields. A failure to discriminate between food and non-food trays with dry sand indicates that pore water is involved. Based on the presence of large arrays of Herbst corpuscles (sensory organs that can measure the acceleration due to changes in pressure), the specifics of foraging technique and the characteristics of sediments on which red knots feed, we deduce that the sensory mechanism involves the perception of pressure gradients that are formed when bills probe in soft sediments in which inanimate objects block pore water flow. To our knowledge, this mechanism has not been described before. It is argued that repeated probing in soft, wet sediments allows red knots to induce a residual pressure build-up of sufficient strength to detect the pressure disturbance caused by a nearby object. The cyclic process of shaking loosely packed sand grains followed by gravitational settling into a closer packing, leads, owing to insufficient drainage of the sediment, to a locally increased pressure disturbance that is 'pumped up' at each shake.

LIKELIHOOD OF ANCESTOR STATES IN ADAPTIVE RADIATION.

Theories of ecological diversification make predictions about the timing and ordering of character state changes through history. These theories are testable by "reconstructing" ancestor states using phylogenetic trees and measurements of contemporary species. Here we use maximum likelihood to estimate and evaluate the accuracy of ancestor reconstructions. We present likelihoods of discrete ancestor states and derive probability distributions for continuous ancestral traits. The methods are applied to several examples: diets of ancestral Darwin's finches; origin of inquilinism in gall wasps; microhabitat partitioning and body size evolution in scrubwrens; digestive enzyme evolution in artiodactyl mammals; origin of a sexually selected male trait, the sword, in platies and swordtails; and evolution of specialization in Anolis lizards. When changes between discrete character states are rare, the maximum-likelihood results are similar to parsimony estimates. In this case the accuracy of estimates is often high, with the exception of some nodes deep in the tree. If change is frequent then reconstructions are highly uncertain, especially of distant ancestors. Ancestor states for continuous traits are typically highly uncertain. We conclude that measures of uncertainty are useful and should always be provided, despite simplistic assumptions about the probabilistic models that underlie them. If uncertainty is too high, reconstruction should be abandoned in favor of approaches that fit different models of trait evolution to species data and phylogenetic trees, taking into account the range of ancestor states permitted by the data.

Simulation of Ecological and Evolutionary Interactions of Darwin's Finches—A Multimedia Learning Tool

Dr. Quinn is one of a team of six biology professors from six different CSU campuses collaborating on this pilot project. EvolvelT is a web-based method for students to learn the fundamentals of natural selection and speciation by simulating natural processes. The simulation will be modeled on the evolution of Darwin's Finches in the Galapagos Islands. Learners will manipulate variables such as initial population size, variability and heritability of bill morphology, and quantity and quality of seeds, and then observe changes with time in population size and bill morphology. The interactive model will allow variables to be changed and simulations to be repeated, producing results that can be graphed and statistically analyzed. The Integrated Technology Strategy (ITS) of the California State University System (CSU) is using the Internet to create new and more flexible learning opportunities. Recently the ITS brought together biologists from several CSU campuses to explore ways to use technology to improve learning in introductory biology laboratories for non-science students. These laboratories were chosen because they affect large numbers of students at all campuses. Development criteria include applicability across the CSU, improvement in learning quality, accessibility to large numbers of students, and measurable success. We selected evolution as a topic for web-based learning because it is a central concept of biology, and it is relatively difficult to teach in conventional introductory biology laboratories. Our development team will work with multimedia design specialists to insure that the web presentation promotes scientifically sound and efficient learning. We are collaborating via e-mail and occasional video conferences and face-to-face meetings. We will work on the actual teaching materials via a web page. The initial prototype will be ready by early summer 1997 and will be tested, modified, and released for beta testing by summer's end.

Hybridization between the Galápagos land and marine iguana (Canolophus subcristatusandAmblyrhynchus cristatus) on Plaza Sur

Hybridization plays an important role in the evolution of some of the vertebrate taxa on the Galápagos Islands, such as the Darwin finches. Conversely, only a single possible hybrid between the Galápagos marine iguana (Amblyrhynchus) and the land iguana (Conolophus) has been reported from the island Plaza Sur. In this paper, the hybrid status of a morphologically unusual iguana from this island is confirmed, using restriction fragment length polymorphism (RFLP) analyses of the nuclear ribosomal DNA (rDNA). Sequencing of the hybrid's mitochondrial cytochrome b gene revealed that it was the offspring of a female land iguana and a male marine iguana. Preliminary molecular analyses of morphologically typical marine and land iguanas from Plaza Sur did not detect introgression of nuclear or mitochondrial markers between species. The potential significance of hybridization for the evolution of the Plaza Sur iguana populations is discussed.

Immune defence, extra–pair paternity, and sexual selection in birds

Secondary sexual characters have been suggested to reliably reflect the ability of individuals to resist debilitating parasites, and females may gain direct or indirect fitness benefits from preferring the most extravagantly ornamented males. Extra–pair paternity provides an estimate of an important component of sexual selection in birds. Species with a high frequency of extra–pair paternity have a variance in realized reproductive success that is greater than the variance in apparent reproductive success, and extra–pair copulations and hence extra–pair paternity by females are often directly associated with the expression of male secondary sexual characters. If sexually dichromatic species have experienced a long period of antagonistic coevolution with their parasites, such species should have evolved larger immune defence organs than sexually monochromatic species. Bird species with sexual dichromatism had larger spleens for their body size than monochromatic species in a comparative analysis. Furthermore, species with a high frequency of extra–pair paternity were sexually dichromatic and had large spleens for their body size. These results are consistent with the hypothesis that females of dichromatic bird species seek extra–pair copulations to obtain indirect fitness benefits in terms of superior resistance of their offspring to virulent parasites.

Mating patterns of Darwin's Finch hybrids determined by song and morphology

Three species of Darwin's Finches hybridize on the Galapagos island of Daphne Major. We examined mating patterns to determine if hybrids exhibit mate preferences.Geospiza fortis×G. scandensF1hybrids backcrossed to both of the parental species, whereas all backcrosses bred with the parental species to which they were most related, or with hybrids. Paternal song was shown to be the crucial factor determining the mating pattern ofG. fortis×G. scandensF1hybrids and their offspring. Song is culturally inherited, transmitted faithfully from father to son (with few exceptions) as a result of an imprinting-like process. Size also contributes to the choice of mates.G. fortis×G. scandensF1hybrid females paired with largeG. scandens-likeG. fortismales.G. fortis×G. fuliginosaF1hybrids paired negatively assortatively with respect to the size of theirG. fortismates. Non-random mating of hybrids based on song, a non-genetic trait, has interesting evolutionary consequences. Song characteristics and nuclear and mitochondrial genes flow fromG. fuliginosainto theG. fortispopulation, whereas the direction of transfer of genetic and song information betweenG. fortisandG. scandensdepends on which song was sung by the father of the hybrids.

Mating patterns of Darwin's Finch hybrids determined by song and morphology

Three species of Darwin's Finches hybridize on the Galápagos island of Daphne Major. We examined mating patterns to determine if hybrids exhibit mate preferences.Geospiza fortis×G. scandensF1hybrids backcrossed to both of the parental species, whereas all backcrosses bred with the parental species to which they were most related, or with hybrids. Paternal song was shown to be the crucial factor determining the mating pattern ofG. fortis×G. scandensF1hybrids and their offspring. Song is culturally inherited, transmitted faithfully from father to son (with few exceptions) as a result of an imprinting-like process. Size also contributes to the choice of mates.G. fortis×G. scandensF1hybrid females paired with largeG. scandens-likeG. fortismales.G. fortis×G. fuliginosaF1hybrids paired negatively assortatively with respect to the size of theirG. fortismates. Non-random mating of hybrids based on song, a non-genetic trait, has interesting evolutionary consequences. Song characteristics and nuclear and mitochondrial genes flow fromG. fuliginosainto theG. fortispopulation, whereas the direction of transfer of genetic and song information betweenG. fortisandG. scandensdepends on which song was sung by the father of the hybrids.