Broadly speaking, speciation takes 1 of 2 opposite directions: specialization or generalization. Geographics often tilts the vector decisively.
Because of their tricky environment, some places engender speciation. The Galápagos Islands, owing to the rapid transformation of the volcanic islands there, is the classic example that inspired Darwin.
The Galápagos Islands have many singular species, arising from unique geophysical circumstances. These islands are born by volcanoes and driven to their demise by tectonics.
A volcanic hotspot near the equator, 980 km off the west coast of South America, creates Galápagos Islands in a production line. The Galápagos hotspot originated an estimated 25 to 90 MYA.
The tectonic Nazca Plate ferries the hotspot’s production east-southeast at 6.4 cm per year; a slow-motion geological conveyor belt.
As islands move away from the hotspot, volcanic activity declines into eventual dormancy. The islands eventually subduct under the South American plate which is defined by South America’s western coastline.
Volcanism has so far created 15 large islands, 3 smaller ones, and 107 islets. Total land area is 8,000 km2.
The farthest western island, and hence the youngest, is Fernandina, which sports an active shield volcano. Fernandina is too new to have much plant life in the interior. Mangrove forests abound around the shoreline. Marine iguanas live there, as do other animals dependent upon the ocean, including pelicans, Galápagos penguins, and sea lions. Rice rats and land iguanas also manage a living.
Located toward the western edge of the island chain, and hence a young island, Isabela is the largest: 4,640 km2. Isabela was formed by the merger of 6 shield volcanoes. 5 of the 6 Isabela volcanoes are still active.
A shield volcano builds islands by steady accumulation of lava sheets. Shield volcanoes erupt low-viscosity mafic lava, which travels farther than from more explosive volcanoes. The term shield volcano derives from such a volcano producing a broad, gently sloping base from a central dome, resembling a warrior’s shield.
The largest shield volcano chain in the world produced the Hawaiian Islands. In Hawaii, each volcano has a distinct duration of activity as the Hawaii hotspot moves under that portion of the Pacific plate. After passing the hotspot, a volcano becomes dormant.
In contrast, Galápagos has concurrent volcanism over a wide area. Of the 21 emergent volcanoes at the Galápagos hotspot, 13 are still active.
Activity from the Galápagos hotspot coalesced into a vast undersea platform, abruptly rising from the ocean floor. The ridge up to the Galápagos platform interrupts the cold Cromwell Current coming from the west, creating an upwelling of mineral-laden water which seeds the island chain with nutrients for life below the waterline, thereby enhancing life above.
Upwelling zones attract sea life. As the cold, nutrient-rich water comes to the surface it encourages the growth of planktonic algae which bring animals that feed on the algae, including various crustacea, mollusks, and jellyfish.
Loggerhead sea turtles love such fare, especially jellyfish, which can be hard to find. An upwelling zone is just the spot.
Such zones are often temporary, local phenomena, and scattered about. They can be quite hard to find. Fortunately for the turtles, upwelling zones stink of something they readily smell.
When crunched into lunch by algae eaters, phytoplankton exude dimethylsulfoniopropionate (DMSP). DMSP rapidly degrades into dimethyl sulfide (DMS) and methanethiol, releasing oxygen in the process. (DMSP: (CH3)2S+CH2CH2COO−. DMS: CH3SCH3. Methanethiol: CH3SH.)
Marine bacteria quickly quaff methanethiol but are not so fast on the uptake of DMS, which has an overwhelming smell redolent of cabbage. DMS is the salient chemical clue to an upwelling. Loggerheads follow their nose.
4 ocean currents flow to the Galápagos, with seasonal or sporadic force, and to much different effect. The Cromwell Current is life-giving, as is the submerged Humboldt Current, which ferries frigid water from Antarctica. The upwelling from these currents brings welcome nutrients.
The westward-flowing South Equatorial Current runs on top of the Cromwell Current: warm water that runs along the equator to the islands, bearing little benefit to life.
The other surface current is the Panama Flow, flowing southwestward from the Central American coast. Few nutrients come with it. When the Humboldt Current flags, and the Panama Flow strengthens, life on the eastern islands suffers.
El Niño is a quasi-periodic climate pattern that forms in the tropical Pacific Ocean, roughly every 5 years. Its arrival can bring catastrophe to the Galápagos.
During the powerful El Niño of 1997–1998, Galápagos’ air was unusually warm. More taxing to life was a stronger Panama Flow of warm water, while the cold-water current upwellings were dramatically reduced. On top of that, heavy rains fell, flooding the land.
Sea animals, and those dependent upon them, starved. Ground nests were disastrously flooded.
Vegetation flourished, as did mosquitos. The flush of insects drove many sea birds to abandon their nests and chicks. Meanwhile, insectivorous birds – mockingbirds and finches – thrived, their populations soaring.
Even for vegetation, extra rainfall from El Niño can be too much of a good thing. During the 1982–1983 El Niño, highland forest trees suffered major diebacks from root rot.
The bioelements of each island dramatically change through time. Life struggles to establish itself on young islands. Lava cactus arise inland, while salt-tolerant mangroves grow on the shore, having floated in from afar. These hardy pioneers provide a start for other life that follows.
Mangroves are a keystone species: anchoring their resident habitats throughout the tropical and subtropical regions of the world. Every continent between 5° N and 5° S latitude has mangrove swamps.
Trade winds blow toward Galápagos from the American mainland, bringing lightweight invaders: microbes and the spores of mosses, lichens, and ferns. Rain showers drop off others.
Most plants have seeds too heavy to fly long distance, but orchid sunflower seeds readily stay airborne. Birds bring seeds in their stomachs, stuck to their feathers, or in the mud on their feet.
Plants are master adapters. Of the 560 indigenous plants in Galápagos, 180 are endemic.
A few of the larger, more mature Galápagos Islands have lush forests. As with all Galápagos life, these are the product of immigration and evolution. Caught up in a storm and dumped ashore, insects and other larger life have been carried to the Galápagos from time to time, most often on vegetative rafts.
Galápagos’ species numbers are low. Today there are only 2 species of bat and 4 of rat. The 1,700 native insect species is a low count for any less-remote land mass.
Of the 29 resident bird species, 22 are endemic to the island, including Galápagos hawks, rails, doves, mockingbirds, and finches. Finches have especially speciated into specialist habitat niches. A single group of finches that originally colonized the islands a few million years are now 13 species.
There are 2 endemic species of land iguanas in the Galápagos. These are large lizards – up to 13 kg and 1 m long – with places where unwelcome lodgers take up residence. The iguanas struck up a mutualist relationship with some finches and mockingbirds: a lizard will stand tall and let a bird clean the parasites off it.
Land iguanas eat mostly invertebrates when young but become vegetarians into adulthood. Cactus flowers and pads are constantly on the menu in the desert lowlands where iguanas manage to live. Though it looks painful, sharp spines don’t deter iguanas from munching cactus pads.
The comparably sized marine iguana is the only truly marine lizard in the world. Adult males can swim through the rough surf and dive to 10m, where they hang onto the rocks with powerful claws and graze on algae. Smaller, less-rugged individuals must content themselves with what can be found in rock pools, and algae exposed on the shore at low tide.
Special glands let a marine iguana tolerate the large doses of salt that they receive from their feeding forays. They blow the excess out their noses as concentrated salt sprays; oceanic dragons with white fire out of their nostrils.
A few turtle species call the Galápagos home, including the largest concentration of green sea turtles in the world. But the celebrity with a shell is the giant tortoise, which may weigh up to 300 kg and grow to 1.3 m. Giant tortoises may live to be 150 years old.
The giant tortoise appeared 75 MYA: descended via saltation from a reptile linage that arose 250 MYA. 1 million years ago they reached the Galápagos Islands.
During the 16th and 17th centuries, pirates plied the Galápagos, preying on Spanish treasure ships. They filled their ships’ holds with tortoises: a tasty meat source. Between 1831–1868, 67 boats took 10,000 tortoises in 151 visits. This decimated giant tortoise populations: completely eradicated on some islands, as all were taken. These magnificently monstrous tortoises would be extinct if not for a last-minute effort to stop the ongoing slaughter.
Along with the ordinary plunder of everything considered of value, various invasive species have been brought to the Galápagos, including plants, ants, and other insects, along with rats, pigs, goats, and dogs. Their proliferation has been at considerable cost to the native species, which had attained an ecological balance, as ecosystems naturally do.
Goats, first brought by buccaneers as a reliable food source, have been especially vexing. Goats outcompete giant tortoises for the same plants, thus starving the tortoises as the goats thrive.
On his historic visit to the Galápagos, Darwin complained that he could hardly pitch a tent for all the iguanas. On several islands, their numbers have shrunk to a small fraction of that now.
The human population continues to grow on the Galápagos while indigenous life declines. Among numerous examples, Galápagos penguins have been under siege for decades from the dogs, cats and rats brought to the islands by man.
As the Galápagos Islands age and slowly sink in geological time, diminishing in size as they submerge, habitats become increasingly harsh again. In their old age, these islands become carriers for creatures whose true home is the ocean. Sea birds and sea lions are typically the last residents before these rocks crumble into the sea.
Wind and water shape Galápagos biotic possibilities. Far removed from continental landmasses, the islands fickle climate is largely determined by the complex pattern of ocean currents which are driven by trade winds.
Tumultuous geology and climate heavily influenced the evolution of life on Galápagos, as well as making it an especially challenging place to live. As elsewhere, mankind has only added misery.