GEOL 388: Field Natural History of the Galápagos Islands

Summer Semester I 2008
On the Origin of Species: Evolution, Speciation, and Island Biogeography

"The natural history of these islands is eminently curious, and well deserves attention. Most of the organic productions are aboriginal creations, found nowhere else; there is even a difference between the inhabitants of the different islands; yet all show a marked relationship with those of America, though separated from that continent by an open space of ocean, between 500 and 600 miles in width. The archipelago is a little world within itself, or rather a satellite attached to America, whence it has derived a few stray colonists, and has received the general character of its indigenous productions. Considering the small size of the islands, we feel the more astonished at the number of their aboriginal beings, and at their confined range. Seeing every height crowned with its crater, and the boundaries of most of the lava-streams still distinct, 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." - Darwin, Voyage of the Beagle, Chap. 17

"It was the consideration of such large groups of facts as these which first led me to take up the present subject. When I visited during the voyage of H.M.S. Beagle, the Galapagos Archipelago, situated in the Pacific Ocean about 500 miles from South America, I found myself surrounded by peculiar species of birds, reptiles, and plants, existing nowhere else in the world. Yet they nearly all bore an American stamp. In the song of the mocking-thrush, in the harsh cry of the carrion-hawk, in the great candlestick-like opuntias, I clearly perceived the neighbourhood of America, though the islands were separated by so many miles of ocean from the mainland, and differed much in their geological constitution and climate. Still more surprising was the fact that most of the inhabitants of each separate island in this small archipelago were specifically different, though most closely related to each other. The archipelago, with its innumerable craters and bare streams of lava, appeared to be of recent origin; and thus I fancied myself brought near to the very act of creation. I often asked myself how these many peculiar animals and plants had been produced: the simplest answer seemed to be that the inhabitants of the several islands had descended from each other, undergoing modification in the course of their descent; and that all the inhabitants of the archipelago were descended from those of the nearest land, namely America, whence colonists would naturally have been derived. But it long remained to me an inexplicable problem how the necessary degree of modification could have been effected, and it would have thus remained for ever, had I not studied domestic productions, and thus acquired a just idea of the power of Selection. As soon as I had fully realised this idea, I saw, on reading Malthus on Population, that Natural Selection was the inevitable result of the rapid increase of all organic beings; for I was prepared to appreciate the struggle for existence by having long studied the habits of animals." - Darwin, Variation of Animals and Plants Under Domestication, "Introduction"

Pre-Darwinian Concepts
Scientists recognized the succession of species through time by the late 1700s. Apparent that the living component of the Earth changed through time. Two different hypotheses for this:

Natural Selection
Darwin recognizes Galápagos as site of new species (see quote above). Was, and is, excellent site for study of evolution, because they were "insulae rasae" ("blank islands"), and have a relatively low diversity.

Darwin & Wallace independently discover Natural Selection as the mechanism of descent with modification. Three underpinning observations behind Natural Selection:

So, all other things being equal, those variants in a population with some trait that allows them to survive better and/or have a better than average chance of reproducing will preferentially have descendants in the next generation. If the variation that allows them to survive better and/or have a better change of reproducing is heritable, than that trait will preferentially be represented in the next generation.

Hence,

* Natural Selection is the differential survival and reproduction of variants in a population resulting in a net change in phenotype of the descendants. *

(Short form: "Natural selection is the differential survival and reproduction of variants in a population.")

(Even shorter form with a 20th Century slant: MUTATION PROPOSES, SELECTION DISPOSES)

Key points of Natural Selection:

Darwin and Wallace demonstrated the reality of Common Ancestry and Divergence through Time:

"Survival of the Fittest"?: Not as such. Phrase not in the earlier editions of the Origin, nor was it coined by Darwin. Comes from economist/philosopher Herbert Spencer:

Note that phenotypic changes can be behavioral as well as morphological:

Reproductive Isolation & Speciation
Darwin recognized that varieties and subspecies are themselves incipient species, and that speciation (origin of new species) is a continuation of the same processes that produce varieties: change in overall frequency of particular mutations as selected in response to the external environment. Consequently, no definite point when geographically-distinct populations are unquestionably different species: speciation is a process, not an event.

Thus some non-consensus on taxonomy of various forms (i.e., Nazca boobies (Sula granti) traditionally considered subspecies of Masked boobies (S. dactylatra); are the Galápagos tortoises a single species or 14 species? Do they belong in Geochelone or should that great genus be split into many different genera (in which case the Galápagos tortoises fall into Chelonoidis).

In post-Darwin time, recognize the importance of reproductive isolation (increasing the likelihood of spread of new mutations through a population). Several types of reproductive isolation are possible. In Galápagos, occurs between organisms arriving at mainland (rafting, blown off course, etc.) and their ancestral populations, or within archipelago as islands sink or lava flows put barriers between populations. All these result in (potentially) allopatric speciation. ("Allo-" = "other"; "patria" = homeland). Some species may arise from populations on the fringes of others; these peripheral isolates may become new species through peripatric speciation ("peri-" = "edge" or "fringe"). Still others might actually become isolated reproductively (maybe because of chromosomal or behavioral differences) within the homerange of the ancestor: this would be sympatric speciation ("syn-" = togehter).

Reproductive isolation might begin by allopatry, but is reinforced by morphological, behavioral, genetic, and other isolating mechanisms. Closely related species may evolve isolating, species-specific color patterns or displays, for instance.

Macroevolution
"These complex affinities and the rules for classification, receive a rational explanation on the theory of descent, combined with the principle of natural selection, which entails divergence of character and the extinction of intermediate forms. How inexplicable is the similar pattern of the hand of a man, the foot of a dog, the wing of a bat, the flipper of a seal, on the doctrine of independent acts of creation! how simply explained on the principle of the natural selection of successive slight variations in the diverging descendants from a single progenitor!" Darwin, Variation of Animals and Plants Under Domestication, "Introduction"

Macroevolution: term for evolutionary patterns at and above species level. Since "species-level" is a difficult thing to define in a consistent way that actually applies to Nature, macroevolution can be thought of as higher-level effects of evolutionary change.

Some of the major macroevolutionary patterns and processes:

Island Biogeography
Darwin recognized that all the inhabitants of the Archipelago are descendants of animals and plants from other places (esp. South America). How did the organisms get there?

Three general categories of species within the islands:

Colonization of the islands and modes of dispersal:

Factors controlling island diversity:

Predictions about volcanic island diversity:

The general pattern of dispersal (both from the mainland to the archipelago and from island to island within Galápagos is called sweepstakes dispersal.

As mentioned above, not all the species currently in the archipelago evolved with the present-day configuration of the islands. Some (many?) (most?) evolved millions of years earlier, before Fernandina and Isabella and other young islands had formed. These species have migrated to colonize the more newly-formed islands as the Nazca plate moves eastward. This pattern has been termed escalator hopscotch.

Last modified: 13 June 2008