Darwin’s Finches


Mike Morley

BY 590: Biological Theories

March 5, 1999


  1. Introduction
    1. Charles Darwin's theory of evolution
    2. Early study of the Galapagos Islands' finches

    3.  
  2. The Galapagos Islands
    1. Location
    2. Geology
    3. Climate

    4.  
  3. Darwin's Finches
    1. Classification
    2. Characteristics
    3. Distribution

    4.  
  4. Evolution of Darwin's Finches
    1. Common ancestor
    2. Allopatric model of speciation

    3.  
  5. Examination of beak dimensions in Darwin's Finches

  6.  
  7. Proof of continuous adaptation in Darwin's Finches

  8.  
  9. Conclusion

  10.  
  11. Tables

  12.  
  13. Bibliography

Darwin’s Finches


    "A major task for biologists is to explain organic diversity in terms of evolutionary principles: to explain why so many different types of species have arisen, and why they vary so much in form, function, and behavior" (Grant, 1986). The history of evolutionary biology really begins in 1859, with the publication of Charles Darwin’s On the Origin of Species (Ridley, 1996). Darwin challenged the views of early evolutionary biologists that believed in species fixity and transformism. His controversial theory was that species were not fixed in form, but that one species evolved into another. It is important to remember that separate creation was the accepted theory of life during Darwin's experimentation. Darwin’s theory was proposed after spending five years (1832-1837) aboard the HMS Beagle. Having collecting species from areas such as South America and the Galapagos Islands, Darwin devised the theory of evolution by natural selection. After reading Malthus’ Essay on Population, Darwin realized that certain species were better adapted for survival and would in turn produce more viable offspring. On the other hand, those species less suited for their environment would decrease in frequency and gradually disappear. Biologists agreed with the theory of evolution, but were skeptical of natural selection because Darwin, and other biologists, had not yet developed the theory of heredity. It wasn’t until Mendel’s work resurfaced around 1920 that biologists understood the modern theory of heredity. Darwin and Mendel’s combined works formed what is called the modern synthesis, or neo-Darwinism. Neo-Darwinism is now widely accepted and unifies genetics, systematics, paleontology, and classical comparative morphology and embryology (Ridley, 1996).

    Darwin’s travels aboard the HMS Beagle brought about interesting sights and uninhabited territories. None seemed more intriguing than the Galapagos Islands. However, Darwin did not realize the impact that the small birds of the island would have on modern evolution. During his five-week stay (Weiner, 1994), Darwin only collected a small number of finches and failed to label the island on which they were found. After his return, Darwin marveled at the differences in characteristics between the species of finches collected. Grant (1986) cites Darwin as reporting that "the most curious fact is the perfect gradation in the size of the beaks of the different species of Geospiza—Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that, from an original paucity of birds in this archipelago, one species had been taken and modified for different ends." Darwin was the first person to record the finches’ habits, and question their origin, evolution, and variation. However, Darwin did not provide a detailed explanation for the evolution of the finches. He only studied nine specimens, and in fact, only classified six as species of finches (Grant, 1986). With the problems of taxonomy and having never returned to the area, Darwin left many questions to be answered by future biologists.

    Conditions to study the finches are much more suitable now. The island system is a perfect habitat to study the characteristics of a specimen. It is difficult for the specimens to leave the area and they are less likely to be affected by outside influences. Modern transportation also allows biologists to return to the area in an effort to continue the observation of the finches. Several organisms lend themselves to feasible study, an example of which is the Drosophila flies. Grant (1986) feels that Darwin’s Finches also fit into that category, stating that "they (Darwin’s Finches) are a text book example of adaptive radiation: the evolutionary diversification of a single lineage into a variety of species with different adaptive properties. What follows is an examination of adaptive radiation by Darwin’s Finches, including their remarkable diversity of form and function, especially in beak structure and associated feeding habits.

The Galapagos archipelago is located in the Pacific Ocean, about 1,000 km west of South America, near the equator. The volcanic islands are estimated to be five million years old, much younger than its neighboring continent (Weiner, 1994). There is no geological evidence that the islands were ever linked to the mainland by either an isthmus or a chain of islands (Grant, 1986). This is important in ruling out mass migration. Isabella, the largest and highest island, is 100 km long 1700 m high, and comprised of six major volcanoes. This is unique in that most of the other major islands are comprised of only one volcano. Table 1 lists the seventeen major Galapagos Islands, in order of decreasing size (Spanish and English names).

    The islands are influenced by a particularly seasonal climate. January to May marks a hot and wet period, with cooler, drier weather the remaining months. Precipitation is greater at higher altitudes and on northern sides of high islands, because they are in the rain shadow of the prevailing southeasterly winds. Precipitation has a more profound effect on Darwin’s finches than does temperature. There is also an extreme variation in the amount of precipitation during any given month, meaning that precipitation in March may vary from 0 mm in one year to more than 200 mm in the next (Grant 1986). The total amount of precipitation can also vary from year to year.

Most sources report fourteen species of finches on the Galapagos Islands (Table 2). These fourteen species can be divided into four groups: ground finch, tree finch, warbler finch, and Cocos finch. Adult male Cocos are black, while the females are brown and streaked. Both males and females in the warbler group are a greenish color. The ground finches resemble the Cocos finch in coloration. Tree finches can be many different combinations of color, depending on species. The distribution of Darwin’s Finches is listed in Table 3. Grant describes three main features of the finch distributions: First, there is great variation shown by the species, Second, tree finches are restricted to the larger and higher islands, and Third, the larger, higher, and more centrally located the island, the larger the number of species it supports. Finch species can be distinguished by bill size and shape and by body size. However, some species are so similar that it would appear they have only recently formed from a common ancestor, and others so different in feeding habits and associated beak structure as to suggest an ancient origin, (Grant, 1986).

    We now know where the finches are located and some of their characteristics, but where they came from, and how they evolved still needs to be addressed. It is believed that all of Darwin’s Finches are descendants of a common ancestral finch that colonized the Galapagos Islands after flight from South or Central America. Because of the relative youth of the islands, this event probably occurred in the last 4.5 million years. Several continental fossils have been found that could provide the link to Darwin’s Finches. Melanospizarichardsonii, Tiaris species, and Volatiniajacarina, all have common features to the subjects in question, including plumage, musculature, and internal anatomy. However, not one of these species seems to be more related to Darwin’s Finches than the other species. Allopatric speciation is the primary model of speciation accepted in the explanation of the fourteen different species that were described earlier. This model states that gradual adaptations took place in a certain species after geographic isolation (i.e. two separate islands) from its ancestors. When the two new species converge at some point in time, they will be different enough that interbreeding will not be possible. Therefore, sometime in the past, an ancestor species of finch found it necessary to leave the original island to inhabit another one. This must have happened several times, and each time the island developed a finch that was perfectly adapted to its environment--particularly feeding conditions. "At this point the species existed as a series of moderately differentiated populations" (Grant, 1986). Then, at some point in time, the populations of an island split into two. The split happened largely because immigrants moved into an area where the native residents were not exploiting part of their food supply, thus designating food supply as the major factor influencing beak dimensions. This will in turn affect the speciation of finch populations on the different islands.

    "All fourteen species of Darwin’s Finches differ from each other in body size and/or bill size and shape" (Grant, 1986). However, most groups have a greater difference in beak size and shape than any other characteristic. Because birds use their beaks to feed, there should be a correlation between beak dimensions and the finch’s diet. Beak size is also a factor in changing finch characteristics such as such as jaw size and musculature due to the demands of different beak dimensions. There are three different feeding types among Darwin’s Finches: probing, tip biting, and base crushing. Each finch has developed a unique beak to satisfy its feeding mechanism. Heavy beaked birds with base crushing beaks will feed on larger or harder seeds that require strength for crushing, and smaller-beaked species will eat thinner shelled seeds. Heavy-beaked finches will also have the advantage of eating seeds that range from small to large in size, where the smaller-beaked finches are limited to the small seeds of the area. Finches that probe plants for food have developed a beak that can be related to needle nose pliers, allowing the beak to fit into small, narrow holes. Tip biting finches have an intermediate sized beak that allows it to feed on surface items. An example would be certain species of finches perching on the backs of the Galapagos’ reptiles to feed on ticks, much to the delight of the reptile.

    It should now be clear that food supply not only determines diets, but also which species occur on an island, as well as their fitness. Reexamination of the early dispersal of finch species is necessary to prove this theory of adaptive radiation. As immigrants landed on areas where certain foods weren’t being exploited, they discovered the ability to fill a vacant niche. Take the example of the larger-beaked finch moving into an area inhabited by small-beaked finches. The large-beaked immigrants were better adapted to feed on the larger and harder seeds of the island. Next, an environmental change will occur that will allow for the beak characteristics to evolve even more. Let’s say that a drought affects the island. Larger finches will produce larger offspring that will not have to compete with the medium to small size finches. This will lend to such a morphological difference that no interbreeding will occur, thus isolating the two groups. This is only a short-term example, but these events continued over time until the different species that we see today appeared.

    These explanations seem acceptable in theory, but can they be proven experimentally? Ridley (1996) cites an experiment that examines how beak size influences feeding efficiency. Two species were compared: Geospizamagnirostris and Geospizafortis. Both species feed on the mericarp fruit, however Geospizamagnirostris has a much larger beak than the small beaked Geospizafortis. The larger beaked finch can crack the fruit transversely, taking on average only two seconds, while exerting an average force of twenty-six kgf. It can easily eat all 4-6 mericarp seeds in only seven seconds. On the other hand, the small beak finch is not strong enough to crack the mericarp and instead twists open the lower surface applying a force of only six kgf, while taking seven seconds to reach the seeds inside. Only one or two seeds can be obtained by this method and it takes an average of fifteen seconds to extract them. From this data, it should be predicted that natural selection would favor larger-beaked finches when large food items are abundant. Furthermore, if beak size is inherited, then the average size of a population of Geospizafortis should increase when large seeds are available and vice versa when large seeds are rare. In the study that Ridley (1996) reports, the sizes of parental and offspring finches in several families were measured. Large beaked parental finches indeed produced large beaked offspring, thus proving that beak size is inherited. Next, a test of changes in beak size following changes in size distribution of food was performed. The test group was the Geospizafortis finch of the Galapagos Island of Daphne Major. The study shows that this species has undergone two contrasting evolutionary events. A dry season started in mid 1976 and lasted until early 1978. One whole wet season was missed due to the drought. The finch population collapsed from about 1,200 to about 180, with the sex ratio at five males per female. Smaller birds seemed to die at a higher rate. Smaller seeds were reduced in numbers, which favored larger finches that could survive on the larger fruit. Finch size increased because of this event. The study shows that Geospizafortis born in 1978 were about four percent larger than those born before the drought. El Nino caused a reverse in the weather conditions in 1982. The rainfall of 1983 was exceptionally heavy. Seed production was enormous, especially small seeds. If smaller finches should exploit the seeds more efficiently, then the smaller finches should survive relatively better. The study found that the sizes of Geospiza fortis in 1985 to be 2.5% larger than those born before the downpours. If adaptive changes can occur in this relatively short amount of time, imagine the possibilities of evolution over a period of 4.5 million years.

    "Natural selection by itself is not evolution. It is only a mechanism that, according to Darwin, can lead to evolution" (Weiner, 1996). The type of selection represented by Darwin’s Finches is disruptive, where the two extremes of the population are the fittest. In this case, variation does result in variation of fitness. Intermediates are not favored on the Galapagos Islands because competition decreases an organism's fitness. Feeding habits will influence beak dimensions so that populations of species will not have to compete with each other. Disruptive selection will lend itself to allopatric speciation. The finches will separate by geographic boundaries, reproduce under isolation, and eventually form two separate species over time. Adaptations of Darwin’s Finches are best seen in beak dimensions. Ridley’s (1996) examples that were mentioned earlier prove that this selection does occur as the environment changes, especially in the amount of precipitation.

    We know a great deal about the evolution of Darwin’s Finches, however, there are still questions to be answered. First, there is no general consensus to which species represent modern relatives to the finches’ ancestor. Second, biologists do not know where the original colonists came from, or which island they inhabited first. Third, historic environmental conditions are not known in any detail. Fourth, biologists are not sure if extinction of species occurred, even though it is believed to have happened. With these questions remaining, there is still a need for the study of Darwin’s Finches. Modern technology will allow for greater advances in this field of research until one day the mysteries of the Galapagos Islands are solved.


Tables:

Table 1: The Seventeen major Galapagos Islands.
 
Spanish English
Isabela Albemarle
Santa Cruz Indefatigable
Fernandina Narborough
Santiago (San Salvador) James
San Cristobal Chatham
Floreana (Santa Maria) Charles
Marchena Bindloe
Pinta Abingdon
Espanola Hood
Baltra South Seymour
Santa Fe Barrington
Pinzon Duncan
Genovesa Tower
Rabida Jervis
Wolf Wenman
Darwin Culpepper
Seymour North Seymour

Source: Grant, 1986

Table 2: Darwin’s Finch species
 
Scientific Name English Name Weight (grams)
Geospiza fuliginosa Small Ground Finch 14
Geospiza fortis Medium Ground Finch 20
Geospiza magnirostris Large Ground Finch 35
Geospiza difficilis Sharp-beaked Ground Finch 20
Geospiza scandens Cactus Ground Finch 21
Geospiza conirostris Large Cactus Ground Finch 28
Camarhynchus parvulus Small Tree Finch 13
Camarhynchus pauper Medium Tree Finch 16
Camarhynchus psittacula Large Tree Finch 18
Platyspiza crassirostris Vegetarian Finch 34
Cactospiza pallida Woodpecker Finch 20
Cactospiza heliobates Mangrove Finch 18
Certhidea olivacea Warbler Finch 8
Pinaroloxias inornata Cocos Finch 16

Source: Grant, 1986

Table 3: Species of Darwin’s Finches on the seventeen major islands (B = breeding, (B) = probably breeding, E = extinct, (E) = probably present as a breeding population formerly, now extinct.
 
 
G. magnirostris
G. fortis
G. fuliginosa
G. difficilis
G. scandens
G. conirostris
C. Psittacula
C. pauper
C. parvulus
P. crassirostris
C. pallida
C. heliobates
C. olivacea
Seymour
-
B
B
-
B
-
-
-
-
-
-
-
B
Baltra
-
B
B
-
B
-
-
-
-
-
-
-
B
Isabela
B
B
B
(E)
B
-
B
-
B
B
B
B
B
Fernandina
B
B
B
B
-
-
B
-
B
B
(B)
B
B
Santiago
B
B
B
B
B
-
B
-
B
B
B
-
B
Rabida
B
B
B
-
B
-
B
-
B
B
-
-
B
Pinzon
B
B
B
-
(B)
-
E
-
B
E
B
-
B
Santa Cruz
B
B
B
E
B
-
B
-
B
B
B
-
B
Santa Fe
B
B
B
-
B
-
B
-
B
(E)
-
-
B
San Cristobal
E
B
B
(E)
B
-
-
-
B
-
B
-
B
Espanola
-
-
B
-
-
B
-
-
-
-
-
-
B
Floreana
E
B
B
E
B
-
B
B
B
B
-
-
B
Genovesa
B
-
-
B
-
B
-
-
-
-
-
-
B
Marchena
B
B
B
-
B
-
B
-
-
B
-
-
B
Pinta
B
B
B
B
B
-
B
-
(B)
B
-
-
B
Darwin
B
-
-
B
-
-
-
-
-
-
-
-
B
Wolf
B
-
-
B
-
-
-
-
-
-
-
-
B

Source: Grant, 1986


Bibliography

Grant, P.R. 1986. Ecology and evolution of Darwin’s Finches. Princeton University Press Princeton.

Ridley, M. 1996. Evolution. Blackwell Science Inc., Cambridge.

Weiner, J. 1994. The beak of the finch. Knopf, New York.