The measured and predicted properties of a series RC circuit containing a battery, a constant resistance R, and a time-dependent capacitance C are reported and compared with those of a conventional RC circuit with constant R and C and a time-dependent input voltage. The apparatus needed for these experiments is available in most undergraduate physics departments and the experiments are straightforward. Despite the simplicity of these circuits, some of the experimental results are unexpected. We describe several mathematical techniques that can be used to provide a detailed understanding of these results.

The process of speciation in birds can be inferred from the pattern of diversification on islands, especially in archipelagos. The basic model is one of initial differentiation of allopatric populations, with further differentiation taking place at the time that sympatry is established. Differences that evolve in allopatry are reinforced by a regime of divergent selection on the taxa in sympatry arising from ecological pressures and not from reproductive (genetic) incompatibility. A low level of interbreeding (hybridization) at the secondary contact phase and subsequently may occur with little or no fitness loss. Introgressive hybridization has the potential to play a creative role in evolution, facilitating further divergence by enhancing genetic variation and relaxing genetic constraints on particular directions of evolutionary change under natural selection. Hybridization potential may last for many millions of years after two taxa diverge, implying that post-zygotic isolation evolves slowly. The main alternative model of speciation in island birds is the peripatric model. It emphasizes major genetic changes taking place in the founding of a new population by a small number of individuals. There is no direct evidence that would make it preferable to the standard allopatric model for islands.

Hybridization, the interbreeding of species, provides favorable conditions for major and rapid evolution to occur. In birds it is widespread. Approximately one in ten species is known to hybridize, and the true global incidence is likely to be much higher. A longitudinal study of Darwin's finch populations on a Galápagos island shows that hybrids exhibit higher fitness than the parental species over several years. Hybrids may be at an occasional disadvantage for ecological rather than genetic reasons in this climatically fluctuating environment. Hybridization presents challenges to the reconstruction of phylogenies, formulation of biological species concepts and definitions, and the practice of biological conservation.

Lande and Arnold's (1983) technique for measuring selection on correlated quantitative traits was used to identify the targets of selection and to reveal the direction of selection on three bill dimensions, during different stages of the life cycle in a population of Darwin's finches, Geospiza conirostris, on Isla Genovesa, Galápagos. There was a tendency towards disruptive selection during dry conditions, arising from differential survival. In terms of longevity and breeding success of females, the direction of selection was to increase bill length. For males competing for territories, selection acted to increase bill depth and bill length. The effects of male-male interactions were separated from those of female choice. Male-male interactions selected for deep and long bills, whereas females chose their mates on the basis of a male's territory position and plumage coloration. The results reveal three factors constraining changes in bill dimensions: a tendency for the mean of a dimension to shift in one direction is counteracted by selection in the opposite direction on 1) another, positively correlated, bill dimension, 2) the same dimension in the other sex, and 3) the same dimension at another stage of the life cycle. If these factors are overcome by strong directional selection at one stage of the life cycle and relaxation at another, there can be an evolutionary response because the bill dimensions in this population are known to be heritable. The results complement those found in studies of G. fortis on another island and strengthen the view that these populations of Darwin's finches are frequently subjected to natural selection.

AbstractMales of Geospiza conirostris, the large cactus finch, on Isla Genovesa, Galápagos, sing a single, precisely copied song. There are two song types in the population, and these are sub-divided into 3 and 5 discrete song sub-types respectively. Adult males sing only one song sub-type throughout life. Sons sing the same song sub-type as their fathers' and do not copy the songs of either natal or breeding territory neighbours. Significantly more of the males holding territories with neighbours of unlike (heterotypic) song type obtain a female than males in territories with no heterotypic neighbour. Pairs in territories with a heterotypic neighbour fledge significantly more young than do pairs in territories with no heterotypic neighbour. Significantly more of those young born on territories with a heterotypic neighbour, that survive to become adults, obtain a mate, than do those surviving young born on territories with no heterotypic neighbour. The data are consistent with the hypothesis that the relative greater importance of song for species recognition in this population has made it advantageous for males to have a single, short, distinct, precisely copied and stable song. The necessity for recognizing both song types as conspecific has produced a unique mating pattern which favours pairs in territories with a heterotypic song neighbour. This allows young to associate visual and auditory cues of father and neighbouring male during the short period of imprinting. A small effective population size, combined with the observation that no female has been known to mate with a male of the same song sub-type as her father's, suggests a possible kin recognition system to avoid inbreeding.

Etude sur une population de Geospiza Conirostris sur l'ile Genovesa, Galapagos, de la variation de certaines de ses caracteristiques morphologiques, ethologiques, ecologiques

Niche Shifts and Competition in Darwin's Finches Get access B. R. Grant, B. R. Grant Search for other works by this author on: Oxford Academic Google Scholar P. R. Grant P. R. Grant Search for other works by this author on: Oxford Academic Google Scholar BioScience, Volume 33, Issue 2, February 1983, Pages 116–117, https://doi.org/10.2307/1309174 Published: 01 February 1983

Six bill dimensions, and wing, tarsus and hallux lengths were measured on almost all museum specimens of Geospiza species available, and up to 20 specimens from each population of the remaining species of Darwin’s finches. The data were subjcrted to univariate and multivariate analyses in order to provide a quantitative description of size and shape differences among populations and between species. Each spccirs of GeospiZa varies among islands in size, and most of the remaining species do so as well. There is more variation in shape among species than among populations of the same species, especially in bill proportions. Allometric relations differ among species. Approximate morphological counterparts to the ground finch species, Geospzza, can be identified among the tree finches. There is a small amount of overlap in multivariate spare between a ground finch species and a tree finch species (two rases), but no overlap between any two species within each group. Size variation among populations is not grnerally correlated with geographical variables such as latitude, longitude, island area or its degree of isolation. Nor do coefficients of variation show stroug geographical trends. Several of the results confirm the findings ofother workers from simpler and nonstatistical comparisons. In addition we have shown that the tree finches have relatively long legs (tarsi), and that these finches as well as the ground finches which spend most time scratching on the ground or climbing in cacti also have a relatively long hallux (hind toe). To interpret the various morphological patterns a knowledge is required of inter-island variation in food supply, feeding habits and the incidrnce of genetic exchange between populations. Recent field studies have provided some of this needed information, which helps to explain, among other things, why several populations of Darwin’s finches are so unusually variable.

The idea that interspecific competition is an important process in structuring communities stems largely from David Lack's work with Darwin's Finches (Lack, 1940, 1945, 1947, 1969). Lack made ecological inferences about feeding niches from analyses of bill sizes and shapes. He listed several instances where the niches of coexisting species were different, and where the niche of an absent species appeared to be occupied by one or more species present. These examples he interpreted as evidence of competitive displacement and exclusion. Lack's aim was to offer a coherent theoretical framework for understanding the adaptive radiation of Darwin's Finches, not to test specific hypotheses. In contrast, our aim is to test the hypothesis of interspecific competition. This is needed because the evidence in general for interspecific competition as an important factor in determining niche relationships and community patterns has been critically debated recently (e.g., Connell, 1975, 1978, 1980; Wiens, 1977; Connor and Simberloff, 1978, 1979; Simberloff, 1978; Strong et al., 1979; Abbott, 1980; Grant and Abbott, 1980; Ricklefs and Travis, 1980; Rotenberry and Wiens, 1980; Wiens and Rotenberry, 1980). We can return to Darwin's Finches to perform such tests without circularity by obtaining and applying quanRevised October 20, 1981

Nestling growth was studied in six species of Darwin’s finches, and in two subspecies of one of them, on four islands of the Galápagos archipelago. The results show how the different beak and body size proportions among the adults of the species are brought about in ontogeny. Large species lay large eggs which give rise to large hatchlings, and in general these have higher growth rates than do the nestlings of smaller species. Geospiza difficilis hatches at a proportionally larger size than the other species and then grows at a slow rate. Variation with species tends to decline with nestling age, but this may be partly artefactual. Adult levels of variation are reached by about day 9, but in G. magnirostris bill depth variation increases significantly from day 9 to adulthood owing to differential mortality or growth. Bill depth variation is heritable in G. conirostris , the best-studied species, on every day from day 2 to day 10; heritable variation was demonstrated towards the end of the nestling phase of growth of this species in bill width, tarsus length and mass but not in bill length. In terms of relative growth, i. e. variates in proportion to body mass, there is only minor variation among species in growth rates of wing and tarsus but substantial variation in relative growth of bill dimensions. By extrapolation, this is one source of indirect evidence that pre-hatching growth differs among the species. The other evidence is that bill proportions differ at hatching relative to final adult size among species. In a multiple discriminant function analysis, the best discrimination among the three Geospiza species on Isla Genovesa was achieved by bill dimensions at all stages throughout the nestling period. Rates of relative growth were fastest in those bill dimensions that are most pronounced in adults. Each species changes in beak shape during nestling growth in a unique manner, except that G. difficilis on I. Genovesa and G. fuliginosa on I. Marchena, which are convergent as adults, have the same growth trajectories. The two subspecies of G. difficilis also have the same growth trajectories, and the differences in adult proportions are the allometric consequences of different stopping points along these trajectories. After fledging there is more growth remaining in bill length than in bill depth and width, but G. magnirostris is unique in first growing relatively quicker in bill depth, then in bill length. These results show that the adaptive differentiation of Darwin’s finch species in adult bill morphology and size has been brought about by evolutionary changes in embryonic, nestling and post-fledging growth characteristics. A more comprehensive theory of the differentiation than exists at present could be developed by integrating ontogeny and phylogeny.

Synopsis A biota of sphaeromorph and nematomorph cryptarchs has been discovered in a grey shale unit of the Aultbea Formation, on Tanera Beg, Summer Isles. The grey shale unit is 4.9 m thick but microfossils are contained only within very thin fine grained intercalated laminae. The composition and lithological association of the biota suggests sedimentological control of its character.

(1) There is a close association between Opuntia helleri (cactus) and Geospiza conirostris (cactus finch) on Isla Genovesa, and between Opuntia echios and Geospiza scandens on Isla Daphne Major. The two finch species consume nectar and pollen, pollinate the obligatorily out-crossing flowers, consume the aril around the seeds, crack the seeds and occasionally disperse them. (2) In the dry season the two finch species open Opuntia flower buds, thereby gaining early access to pollen. But in the process, and at partially open flowers, they snip the style and destroy the stigmas in up to 78% of the flowers. (3) Stigma snipping prevents fertilization of the ovules and the development of seeds which are an important food item later in the year when food supply is likely to limit finch population sizes. Stigma snipping almost ceases in the main part of the wet season, corresponding with an increase in the density of Opuntia flowers and other food types, and a decrease in the proportion of feeding time spent on Opuntia flowers. Stigma snipping neither increased the time flowers remained open nor influenced nectar flow or the number and duration of subsequent visits to flowers by potential pollinators. This last result precludes the possibility that snipping is a means of marking the flower to signal a previous visit to the flower and pollen removal. (4) We conclude that the two finch species snip stigmas to facilitate removal of pollen from the central incurving stamens which are masked by the stigma in a closed or partially open flower. The finches gain a short-term benefit from the removal of stigmas in easy access to pollen and possibly nectar. They potentially suffer in the long term through diminished dry season food supply (seeds), particularly in drought years but we were unable to detect any fitness decrement suffered by the individual finches which do the snipping. (5) An important implication of these results and interpretations is that a behavioral trait which confers a short-term benefit to the individual may increase the chances of the population going extinct.

(1) We report some effects of an annually variable and unpredictable rainfall upon Darwin's Finches on the Galápagos. Finch numbers, foraging and food supply were studied on I. Daphne Major in December of 1973 and 1977, and compared. 1973 was the second of two successive wet years and 1977 was a drought year. (2) Seed numbers and biomass were approximately one order of magnitude lower in the drought year than in the wet year. Small and soft seeds were absolutely and relatively rarer in the drought year than in the wet year. (3) Similarly finch numbers and biomass were approximately one order of magnitude lower in the drought year than in the wet year. Numbers of G. scandens declined less than did number of G. fortis. Both species exhibited unabalaced sex ratios, in favour of males, in the dry year in contrast to balanced sex ratios in the wet year. Male scandens were heavier on average in the wet year, but male fortis were heavier in the dry year. (4) The foraging of scandens, a cactus (Opuntia) specialist, was similar in the two years. The foraging of fortis in the dry year differed from foraging in the wet year in three important respects: fortis devoted a disproportionate amount of time to feeding on small seeds while tending to avoid seeds of Opuntia, they fed more on floral and extra-floral parts of Opuntia and they fed on Tribulus cistoides, a large and hard fruit which was absent from their diet in the wet year. As a consequence of feeding more on Opuntia, fortis foraging was more similar to scandens foraging in the dry year than in the wet year. (5) The results are discussed in relation to expectations from competition theory. The decline in numbers in relation to a decline in food supply was expected, but a convergence in diets was not. The convergence is attributed to the recent renewal of a single resource, Opuntia flowers, against a background of general resource scarcity. Diet overlap and limitation of numbers by food provide indirect evidence of interspecific competition; scandens, with an included niche, was competitively superior to fortis.