The Galapagos Islands: Crucible of Evolution.
I. The Archipelago. 1. Remote - About 600 miles west of SA.
2. Small (13 main; 6 small- er); arid.
3. Of recent volcanic origin (5-10 Mya): “every height crowned with its crater and the boundaries of most lava flows still dis- tinct.” – (Voyage of the Beagle ).
II. The Biota. 1. Large fraction of endemics. a. About half of the birds; b. About 1/3 of the plants.
2. “Specialties” include a. Tortoises – 14 subspe- cies. b. Marine iguanas. c. Geospizid finches.
3. Tortoises, finches, etc ., manifest inter-island varia- bility at subspecific, specific and generic level.
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Marine iguanas are unique to the Galapagos. As the name sug- gests they spend a lot of their time in the water and, in fact, pos- sess unique adaptations that allow them to dive to depths of up to 30 feet while foraging for algae on the sea bottom.
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Galapagos tortoise, Geochelone nigra , is the world’s largest tor- toise and another Galapagaean endemic with at least a dozen subspecies. “The inhabitants,” Darwin wrote, “state that they can distinguish the tortoises from the different islands; and that they differ not only in size, but in other characters. Captain Porter has described those from Charles and from the nearest island to it, namely, Hood Island, as having their shells in front thick and turned up like a Spanish saddle, whilst the tortoises from James Island are rounder, blacker, and have a better taste when cooked.”
3 III. From Darwin’s published Journal and Remarks (1839): 1. “The natural history of this archipelago is very remarkable: it seems to be a little world within itself; the greater number of its inhabitants, both vegetable and animal, being found nowhere else.”
2. “… the organic beings found on this archipelago are peculiar to it; yet their general form strongly partakes of an American char- acter. … This similarity of type … would be explained, according to the views of some authors, by saying that the creative power had acted according to the same law over a wide area.”
IV. From The Voyage of the Beagle (1845):
1. “… a little world within itself, or rather a satellite of America , whence it has derived a few stray colonists.”
2. “Considering the small size of the islands, we feel the more astonished at the number of their aboriginal beings, and at their confined range.”
3. “…we are led to believe that within a period geologically recent the unbroken ocean was here spread out. Hence, both in space and time, we seem to be brought somewhat near to that great fact – that mystery of mysteries – the first appearance of new beings on this earth.”
4 V. The Finches.
1. 14+ closely related spe- cies.
a. One on Cocos Island; remainder in Galapa- gos.
b. Dull, nondescript.
c. Distinguished by bill size / shape. Variation in bill size and shape d. Genera: in Darwin’s finches. 1. Large ground finch. 2. Medium ground finch. 3. Small tree i. Geospiza – ground finch. 4. Warbler finch. From finches. Lack, D. 1947. Darwin’s Finch- es . ii. Camarhynchus – tree finches.
iii. Platyspiza (prev. Camarhynchus ) – vegetarian finch.
iv. Certhidea – warbler finch.
v. Pinaroloxias – Cocos finch.
5 e. Bills range from finch-like ( Geospiza ) to warbler-like (Certhidea ). f. Diets varied:
• Heavy-billed Geospiza specialize on seeds;
• Others eat seeds, in- sects, buds, flowers, pulp, etc .
• Camarhynchus palli- dus (woodpecker finch) an ecological The woodpecker finch and its vicar. stick.
6 2. Example of insular adaptive radiation driven by
a. Colonization;
b. Opportunity – ecological vacuum;
c. Competition;
d. Chance – founder effect, hybridization, extinction.
3. Darwin’s personal journey from Special Creation to Transmutation
a. Reflected by different treatments in Journal and Voyage .
b. Propelled by i. His own observations ii. Subsequent determinations by Gould and other professional taxonomists.
7 VI. From The Voyage of the Beagle :
1. “The most curious fact is the perfect gradation in the size of the beaks … in the Geospiza , from one as large as that of a hawfinch to that of a chaffinch and (if Mr. Gould is right in including his sub- group Certhidea in the main group) even to that of a warbler.”
2. “Seeing this graduation 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.”
3. “The distribution of tenants of this archipelago would not be nearly sow wonderful if, for instance, one island had a mocking thrush, and a second island some other quite distinct genus … But it is the circumstance, that several islands possess their own species of the tortoise, mocking-thrush, finches and numerous plants, these species having the same general habits, … and obviously filling the same place in the natural economy of this archipelago that fills me with wonder.”
4. “… neither the nature of the soil, nor height of the land, nor the climate, nor the general character of the associated beings … can differ much in the different islands.”
8 5. “Reviewing the facts given here, one is astonished at the amount of creative force … displayed on these small, barren and rocky islands, the more so at its analogous action on points so near each other.”
6. “I have said that the Galapagos archipelago might be called a satellite attached to America, but it should rather be called a group of satellites, physically similar, organically distinct, yet in- timately related to each other …”
9 VII. David Lack : Emphasized importance of competition as a driver of ecological divergence.
1. Observed “character dis- placement” in bill size
2. Interpreted either as re- sponse to competition for food or hybridization.
VIII. Peter and Rosemary Grant extended Lack’s work.
1. Redid the phylogeny.
2. Measured a. rates of resource produc- tion / availability; Character displacement in the b. survival of individual genus Geospiza . birds;
3. Documented two examples of selection for altered bill size consequent to dearth of available seeds.
4. Studied determinants of reproductive isolation, hy- bridization, etc .
5. Observed a new Darwin’s finch in statu nascendi – isolation by song.
10 IX. Relevant Ecological Concepts.
1. Hutchinson’s niche.
a. Defined in terms of tol- erances.
b. Fundamental vs. real- ized niches.
c. Competitive exclusion. i. Species with iden- tical niches can’t coexist. ii. E.g. , Barnacles.
2. MacArthur’s niche.
a. Defined in terms of re- source utilization.
b. Character displace- ment.
c. Species packing – min- ͘ imum Ɵ ratio.
11 3. Morphology as an indicator of diet.
Beak size and performance in two populations of G. fortis on Daphne Island. From De Leon et al . (2011).
12 4. Community structure.
a. Competition vs. random assemblages.
b. Null hypothesis wars.
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