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
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
- Successive appearance and disappearance of different forms without genetic connection
- Transmutationism: direct lineal relationships between ancestral species and
- The latter became better known as evolution. Darwin preferred the phrase
"Descent with Modification".
Darwin & Wallace independently discover Natural Selection as the mechanism of
descent with modification. Three underpinning observations behind Natural Selection:
- Variability: There is variation in all populations
- Heritability: Some (but not all) variation is inherited. (Causal mechanism of
inheritance unknown in Darwin's time. Mendelian genetics and discovery of DNA came later)
- Superfecundity: Organisms produced far more offspring than can possibly
survive (application of Malthus' reproductive excess to Nature)
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.
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.
"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
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:
- Trends: Traditional image of evolution: series of ancestors and descendants with
accumulation of traits favorable to a new way of life.
- Can be behavioral: pre-human Galápagos animals totally naïve of people; as those
ones were easily killed off, those who are slightly to totally wary survive. On other
archipelagos, naïve animals were all wiped out by Darwin's time.
- Another behavioral example: short-eared owl (Asio flammeus galapagoensis)
on Genovesa Island cannot be a nocturnal winged hunter, as no local mammals. So only those
owls able to stalk prey during daytime could survive.
- Galápagos lacks good fossil record, so tracing major morphological evolutionary
trends is difficult there
- Divergence: Major (but often overlooked) contribution of Darwin is recognition
that an ancestor can (and often does) give rise to multiple different lineages. Each
acquires their own set of modifications through time, leading to very different forms as
changes accumulate (see quote above from Variation)
- Example: Ambylrhynchus
are each other's closest relatives,
but neither is identical to mainland iguanas. Ancestral population rafted over to
Galápagos, and there two divergent types survived: those adapted to live in the dry
interior, and those adapted to life at sea
- Adaptive Radiation: As above, but where many different lineages of different
life habits evolve from single common ancestor in very short period of time.
Darwin's finches are THE classic example of this!!
- Local flora includes similar (Opuntia cacti and Scalesia)
- Convergence: two or more lineages of animals evolving very similar
morphological or behavioral traits due to similar life habits.
- Many of Darwin's finches are convergent on different types of mainland birds: in fact,
Darwin didn't even recognize that they were finches!
- Another example: mangroves (not all plants with mangrove adaptations are closely
- Vestigialization: reduction in structure or behavior when no longer
functionally significant (where presence requires use of resources that might be otherwise
more gainfully utilized).
- Flightless cormorant (Nannopterum harrisi)
has greatly reduced, non-flying wings, while relatives
are good fliers.
- Behavioral vestigialization in "nests"
of a few twigs constructed by Masked and Nazca boobies (Sula dactylatra and S. granti)
- Insular Gigantism: with no large predators, animals on islands can often grow
larger than on mainland.
- Example: dodo vs. land pigeons.
- Galápagos tortoises (Geochelone nigra)
MAY be insular giants (closest living relatives on mainland are smaller), but in
geologically recent past there were giant mainland species of Geochelone, which
might conceivably be the ancestors.
- Sexual Selection: selection not for survival, but for greater reproductive
success. Often recognized in display structures that might otherwise interfere with
survival, but which the opposite sex are attracted to.
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:
- Endemic: evolved within, and unique to, the Archipelago
- Native: not unique to Archipelago, but arrived without the aid of humans
- Exotic/Introduced: brought to islands by humans, whether intentionally or
Colonization of the islands and modes of dispersals:
- Rafting (vegetation uprooted by storms, could hold many sorts of animals in its
- Floating (tortoises, plants)
- Blown in (birds, insects, plants, bats)
- Brought in ON the birds, insects, bats: seeds, insects, etc.
- Given direction of wind and air currents, favor forms from southern and central
coastal South America
- Some inhabitants are simply within normal migration routes (sea birds, marine organisms)
Factors controlling island diversity:
- Surface Area: number of species per island goes up with surface area
- Probably just a proxy for habitat richness plus available resources
- Rate of immigration decreases as a new island is colonized
- As more species have colonized, less chance that a given immigrant is a species that hasn't
- As more species fill up the island, fewer available spots in ecosystem
- Rate of extinction increases with increasing species diversity
- More competition as diversity increases
- More available species to become extinct as diversity increases
- Where immigration and extinction cross, Equilibrium Number of Species (S)
- Smaller area means greater possibility a species can become extinct,
so larger area means larger S
- Greater distance from mainland means slower rater of immigration, so greater
distance means lower S
Predictions about volcanic island diversity:
- Greater endemism in terrestrial species vs. aerial (esp. sea feeding) or aquatic
- Birds should be common (blown out to islands); non-avian reptiles fairly common
(salt-resistant physiology and able to survive for long times without food); mammals
(not salt-tolerant, high metabolic rates requiring lots of food) and amphibians
(extraordinarily not salt-tolerant) vanishingly rare