Shalnov Published in Wasteless Chemical Processing By V. Kafarov Published in Quantitative Analysis By V. Alexeyev Quantitative Analysis By V. Alexeyev Published in Borshch-Komponiets, B. Fedorov, M. Kolesnikova Published in Thermodynamics of Chemical Processes By Yu. Zhorov Published in From Crystal to Solution By G.
Kobenin Published in Diamonds Wrought by Man By B. Derjaguin, D. Fedoseev Published in Grigolyuk, V. Venikov, V. Zhuravlev and T. Filippova Published in Robotics By E. Popov and E. They also inflict wounds at the base of the sitting boobies' wing feathers and consume the blood.
On this island the finches' beaks are long. In contrast to the sharp-beaked ground finches, birds with large robust beaks, such as the large tree finch, Camarhynchus psittacula , do not probe Opuntia flowers or poke at eggs. Instead, the beak of this finch is a tool for tearing bark and crushing twigs and small branches—a beak modified for a different end.
These examples illustrate some of the ways that Darwin's finches vary in beak morphology and are versatile in their feeding habits. This versatility is fostered by ecological opportunity and impelled by food scarcity in the dry season and in dry years. As evidence of adaptive diversification, associations like this do not satisfy everyone, because causality can be argued from beaks to diets as well as from diets to beaks.
For example, it can be said that finches feed in different ways because they have different beak sizes and shapes e. They surely do, but this begs the question of why the species' beaks differ in the first place. Another objection has been raised on the grounds of insufficiency of evidence. It is a problem which might be largely solved by teams of workers dealing for a number of years with a particularly favourable example.
Without knowing about this critique at the time, we picked up the challenge 30 years ago. Our study took place on the island of Daphne Major—a particularly favorable location, although one would not think so judging from its steep topography see the photograph on p. By virtue of its small size 0. We accomplished this by capturing and measuring many finches to determine phenotypic variation, comparing offspring with their parents to determine inheritance, and following their fates across years to detect selection.
We found pronounced heritable variation in beak size and body size within populations of the medium ground finch Geospiza fortis and the cactus finch Geospiza scandens.
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We also found that when the environment changes, some of the variants in each population survive while others die. This amounts to a vindication of David Lack's views on adaptation. Birds with small beaks and small body size suffered selective mortality in , during a severe drought figure 5. The larger members of the medium ground finch population survived on a diet of large, hard seeds, which increasingly dominated the food supply as a result of an initial preferential consumption of small seeds. Smaller birds, lacking the mechanical power to crack the large seeds of Tribulus cistoides and Opuntia echios , died at a higher rate than large birds.
An evolutionary response to directional natural selection followed in the next generation figure 5 , because beak size variation is highly heritable Keller et al. Natural selection in the opposite direction, with small birds surviving disproportionately, occurred 8 years later. The abundant rain and high temperatures transformed the vegetation and food supply of the finches, and they bred for 8 months as opposed to the usual 1 or 2 months. Vines and other plants multiplied and spread, smothering the low-growing Tribulus plants and Opuntia cactus bushes.
The seed supply became dominated by small seeds, and seeds of Tribulus and Opuntia became scarce. When the island entered the next drying-out episode during the drought of , the supply of seeds fell, and so did the numbers of finches from high points in the productive years of and Large birds died at the highest rate; hence, the medium ground finches that were small, with relatively pointed beaks, were selectively favored. Thus selection oscillates in direction. We have observed this repeatedly over the full year period Grant and Grant a. As a consequence, neither the medium ground finch nor the cactus finch has remained morphologically constant or static.
In fact, the mean body size and beak shape of the two species are not the same now as they were at the beginning of the study figure 6. In an environment subject to climatic and floristic change, the finches have changed evolved. We have directly observed the sort of adaptive change that is normally only inferred from a comparison of related populations differing in mean morphology. Based on the evidence of Darwin's finches, evolutionary changes are indeed going on. Speciation begins with the divergence of a population and is completed when two populations that have diverged on different islands establish coexistence with little or no interbreeding Mayr , Grant We obtained insight into the initial process of divergence on Daphne Major, thanks to a highly fortuitous circumstance: the founding of a new population.
The large ground finch Geospiza magnirostris became a breeding member of the community in late , when two females and three males began to breed. In preceding years we had observed immigrant members of this species on the island in the dry season, but when the rains began they disappeared, presumably returning to their island of origin to breed.
The breeding birds produced 17 fledglings in —, but only one of the breeding pairs produced the next generation. A daughter bred at different times with two brothers, one of which can be ignored because the offspring did not survive to breed. Thus, the population was effectively founded by a single pair, and the next generation comprised a sister—brother pair. We have followed the fate of this population ever since Grant et al. There are now 30 to 40 breeding pairs on the island. Observations of a newly founded population go to the heart of the question of how biodiversity generation begins.
Environmental change appears to have been a key factor in facilitating population establishment and subsequent exponential growth. The G. Small additional changes were caused by natural selection on beak morphology and probably by genetic drift. All of this is to be expected, but some other features were surprising.
Immigration did not occur just once but repeatedly, especially in the s. With one exception, immigrants that stayed to breed came not from the obvious and closest potential source Santa Cruz Island but mainly from one farther away Santiago , as revealed by their microsatellite DNA. A major change took place in the frequency of song types in the population. This change was initiated by a single male that bred for the first time in In addition to this nongenetic, culturally transmitted contribution, he introduced a total of 11 new alleles at the 16 microsatellite loci surveyed.
Thus, while environmental change was the key factor that triggered the founding of a new population, some idiosyncratic genetic and nongenetic factors determined the fate, development, and composition of the population. Even though one individual made a large contribution to the population, overall changes were relatively small in magnitude, for three reasons: Selection pressures were weak, the population did not remain small enough for random genetic drift to be effective, and continuing immigration would have retarded divergence.
If the case of G. Speciation is completed when two populations that have diverged in allopatry can coexist with little or no interbreeding. Medium ground finches and cactus finches occupy different ecological niches, although their diets overlap. The ecological differences presumably permit coexistence in sympatry, in an environment e. To paraphrase David Lack , the species are ecologically isolated through niche differences that evolved by natural selection in allopatry.
The differences may have been enhanced by selection in sympatry, thereby reducing interspecific competition for food.
Reflections on the Scientific Process, as Seen in Climate Studies
But how do the species maintain coexistence without interbreeding? What are the differences that keep them reproductively isolated, and how did the differences evolve? Members of the group of closely related ground finch species do not differ in plumage or courtship behavior, but they do differ in beak morphology, and they differ conspicuously in song Grant These two sets of cues, visual and vocal, have been shown in separate field experiments to be used by finches in discriminating between their own and other species Ratcliffe and Grant a , b , Thus, part of the answer to the question of reproductive isolation is that it evolves as a consequence of adaptive evolution of beak sizes and shapes in allopatry.
The other part, centered on song, is more complex. Song differences play a major role in keeping species apart.
Like beak differences, song differences presumably arise through divergence in allopatry for reasons that are not entirely clear. Song is an interesting trait because it is culturally, and not genetically, inherited. We know this from a few experiments with captive birds Bowman , supplemented by field observations of songs of offspring, parents, and even grandparents Grant and Grant , Only males sing an advertising song; it is simple and is sung unaltered throughout life, which may be as long as 16 years.
Most sons sing the same song subtype as their fathers, while a minority sing a different song subtype that is sung by other male members of the same population. Thus, song is acquired through learning early in life in a process that resembles imprinting; it is generally acquired from fathers during the period of parental dependence, in association with parental morphology.
By their pairing patterns, females give evidence of learning song at the same time and from the same sources. Rare exceptions to these rules provide additional valuable information on the role of learning and its bearing on the question of what keeps species apart. Medium ground finches and cactus finches hybridize rarely. The hybrids that were produced in backcrossed to medium ground finches in , and others that were produced in backcrossed to cactus finches in The direction of backcrossing differed because in the hybridizing male was a cactus finch that sang a medium ground finch song, whereas in the hybridizing males were cactus finches that sang cactus finch songs.
Sons of all families sang the same song as their fathers, and daughters in each case mated with males that sang the same song type as their fathers. Hybridization is sometimes the result of heterospecific singing through apparent misimprinting. The causes of misimprinting and hybridization are idiosyncratic and difficult to determine. They include extra-pair mating, interspecific takeover of nests with eggs, and dominant singing of a close neighbor.
Ecologically, the significant feature is a change in the environment that has facilitated the introgression of genes. F 1 hybrid and backcross survival is not intrinsically lower than the survival of the parental species, and there is no sign of diminished fitness when hybrids breed. In fact, in the s and up to the present, the flow of genes from the medium ground finch to the cactus finch population has contributed to a decrease in mean body size and a blunter beak morphology of cactus finches Grant and Grant a.
The barrier to gene exchange erected by song differences has been breached, and environmental change appears to have been the most important factor. To summarize, the coexistence of finch species is facilitated by divergence in beak morphology and song. Beaks diverge under natural selection, but why songs diverge is less clear. Cultural drift, a process of random change in culturally transmitted learned traits, is probably involved, and sexual selection may be involved as well.
Lamarck and Darwin: Summary of Theories
The end point of speciation is the complete absence of gene exchange. Many, if not all, coexisting populations of Darwin's finches have not quite reached that point, although they function as species by remaining distinct even in the face of occasional gene exchange. This offers two important lessons. First, species diverge in mate preferences before genetic incompatibilities evolve. Second, different populations can function as biological species before they would be recognized as species solely on the basis of genetic distinctness.
The present is known; the past is inferred. In the absence of fossils, genes are our best source of information about this history Price et al. The closest genetic relatives of Darwin's finches on the South American continent, in Central America, and in the Caribbean are a group of seed-eaters Tiaris and relatives allied to tanagers Sato et al. Darwin's finches diverged from them in the last 2 million or possibly 3 million years, according to calculations based on an assumed molecular clock applied to mitochondrial DNA and allozyme data Grant The recent origin of Darwin's finches helps to explain why they are still capable of exchanging genes.
Situated km from continental Ecuador on the Nazca plate and moving imperceptibly toward the mainland, the archipelago is a remote place for birds to visit. Colonization is an improbable event. Nevertheless, according to one calculation, ancestral Darwin's finches arrived in a moderately large flock or several small ones. Modern finches are genetically diverse at the major histocompatibility complex locus, and Vincek and colleagues used the allelic diversity of class II genes to calculate that the original colonists numbered at least 30 individuals. Improbable events may arise in improbable and hence rare circumstances.
What might those circumstances have been? Any answer must be speculative, even if rooted in current phenomena. If the unusual dispersal activity from the mainland followed similar patterns, it may have been induced by unusual volcanic activity in the Andes. Burning of the forests in one such episode would be followed by the establishment of large areas of shrub and secondary growth.
With the buildup of finch populations in secondary forest, and another round of fires and burning, large numbers of finches and other birds in coastal regions would fly out to sea to escape the flames and smoke. The most marked shift in climate seems to have occurred at 2. This may be when the ancestral Darwin's finches arrived. To understand the past, the second question that needs to be addressed is how the islands have changed, if at all, since the first finches arrived.
The simplest possible answer would be that the islands have always been much as they are today in terms of geography, climate, and vegetation. If this is correct, the finches' adaptive radiation can be viewed as a process of differentiation that results in the occupancy of all ecological niches present at the outset—rather like filling empty boxes, one species per box. Indeed, this is how early writers on the subject viewed the radiation of the finches, stressing the absence or scarcity of competitors in the boxes as a facilitating factor Huxley , Simpson , Lack , Grant In the modern version, the boxes are replaced by adaptive peaks in a more or less fixed landscape.
This view implies that diversification is rapid early in the radiation and then slows down as ecological opportunities diminish. Some radiations of other organisms elsewhere conform to this pattern Schluter However, taken at face value, the phylogeny of Darwin's finches shows exactly the opposite pattern: a slow start followed by recent, rapid diversification figure 7. The species occupy adaptive peaks, but the fixity of the ecological landscape that allowed them to do so is questionable. In fact, the islands and their inhabitants have changed radically. Three million years ago, there were far fewer islands than today, perhaps only five Grant and Grant The number of islands increased as a result of volcanic activity centered on and near a hotspot beneath the western island of Fernandina.
Climate changed as well. The average air temperature declined but, more important than this, the world's climate underwent a sharp transition 2. In the last million years the climate has oscillated between glacial and interglacial conditions at about ,year intervals.
This has been revealed by the isotopic oxygen signatures of temperature in Andean ice cores Seltzer et al. Most of the contemporary finches evolved in the last million years, so it is important to know what happened during these cycles. The two major effects of glacial—interglacial cycles were the depression of temperatures and of sea level relative to contemporary levels. Sea level fell with the temperature. The decline was gradual from the current level about , years ago to about 60 meters m lower about 30, years ago ; then an abrupt further decrease occurred to about m below the current level Lambeck and Chappell As a consequence, islands were larger and distances between them were shorter.
Thus, both temperature and sea level were usually lower and never much higher than they are today. The food of finches—plants and arthropods, the latter feeding on the plants and on each other—must have been affected by these geophysical and climatic changes. First, new species of plants and arthropods would have arrived by immigration. We do not know when this happened or which species were involved. That situation needs to be rectified. Second, other species, perhaps influenced by the immigrants, would have become extinct.
Severe and extensive droughts would have been particularly crucial and cruel times for many organisms, including finches. Third, the temperature- and moisture-dependent altitudinal zonation of plants would have shifted upward and downward at different times; hence, the highest and lowest habitats would have been most vulnerable to elimination, as noted elsewhere Prodon et al. Plant distributions would have changed across the archipelago. The evidence of a change in the upland forests is the relative scarcity of endemic plants Johnson and Raven The evidence of a change in the lowest zone is the anomalous occurrence of plants on some islands with apparently unsuitable climates and soil.
For example, the tree Erythrina velutina occurs at mid elevations on the high islands of Santa Cruz, Santiago, and Isabela. A single tree of this species occurs on the low arid island of Genovesa, and fewer than a dozen occur on the low, arid, and even more remote island of Wolf. These may be the remnants of much larger populations on these islands under cooler, wetter, or less seasonally arid conditions than those that prevail today.