GEOL 388: Field Natural History of the Galápagos Islands

Summer Semester I 2004
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

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 & 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. *

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))

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.

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 dispersals:

Factors controlling island diversity:

Predictions about volcanic island diversity: