The Elements of Evolution – Speciation

Speciation

All things must change to something new, something strange. ~ American poet Henry Wadsworth Longfellow

Variation in organisms of the same species is ubiquitous, in every facet of existence, from genotype to phenotype and mentotype. Diversity is an essential aspect of Nature. From an evolutionary perspective, diversity enhances survivability during times of stress.

More variable populations are less vulnerable to environmental changes, show decreased fluctuations in population size, have superior establishment success, larger distribution ranges, and are less extinction prone. ~ Swedish evolutionary biologists Anders Forsman & Lena Wennersten

While the specific mechanics and effects of individual variation are multifarious, variations ultimately originate with the environment: both within an organism and externally. The genome may define the deck of playable cards, but the cards played are dealt by intimate ecological interaction – in a word: adaptation.

New species arise through a variety of evolutionary mechanisms. ~ American biologist Margaret Ptacek & American zoologist Shala Hankison

Speciation is the process of a new species originating, as a population of similar organisms establish a collective pool of reproductive identity.

Speciation is a series of processes. ~ Margaret Ptacek & Shala Hankison

During speciation, sex chromosomes often accumulate interspecific genetic incompatibilities faster than the rest of the genome. ~ American geneticist Colin Meiklejoh et al

Speciation is both prodded and checked by the environment. In other words, environmental influences parameterize evolution. Intrinsic factors include an organism’s own preferences and tolerances, and its ecological interactions: with its own kind, competitors, possible cohorts, prey/foodstuffs, and predators.

Interactions between species are important catalysts of the evolutionary processes that generate the diversity of life. ~ Canadian evolutionary biologist Jeffrey Joy

In animals, differing local food supplies can drive speciation between populations, even those live close by. Closely related animal species seldom prefer the same food. This lessens competition between them, even as there is often overlap in what related species will eat.

Diet affects gut flora composition, which has cascade effects throughout the body. Environmental exposures frequently alter the microbiome. This can sway host behaviors and preferences. As microbes are particular about their associations, they may induce speciation in their hosts to avoid intermingling with what they consider undesirables. While it is known that the microbiome influences host behaviors, the processes in toto are little understood besides that communications at the cellular level are involved.

Extrinsic factors provoking speciation are environmental upheavals which influence prospects for survival: habitat changes of all sorts, including climate. These most immediately affect nutrient supply and can challenge the tolerances that an organism has.

History

In 1559, species referred to different kinds of wine. English cleric Edward Topsell first used the term biologically in the 1608 book The Historie of Serpents. By 1700, species of plants and animals was in common use.

Distinct species were easy to spot when life forms were largely thought to be unchanging kinds of creatures created by the Almighty. God-fearing naturalists carried the banner to describe species, though explaining how they came to be was a given: they were the products of divine creation.

Getting past creationist bias took time. German evolutionary biologist Ernst Mayr broke the mold with his 1942 book Systematics and the Origin of Species, redefining species not as just a morphologically similar population, but as a group that exclusively bred among themselves.

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Variation blossoms into speciation after interbreeding generally ceases. Populations diverge into distinct species by relative isolation, however achieved, and by adaptation to niche habitats. Isolation may happen geographically or behaviorally. Behaviors such as feeding strategy, mate selection, and even breeding timing form bases for speciation. Over time, associative selection and genetic drift differentiates a new species. In referring to allele frequency, genetic drift is also called allelic drift.

In many instances, what starts as behavioral differences and breeding choices turn into mechanical isolation: the original and descendant species are no longer breeding-compatible. Sometimes hybrids do not develop properly. Crosses of sheep and goats do not survive to reproductive age. In other instances, sterility/infertility results. Mules, the offspring of a male donkey and a mare (female horse), cannot reproduce; and so with hinnies, which are the offspring of a stallion (male horse) and a female donkey.

 Woodrats

Desert and Bryant’s woodrats are closely related and live in neighboring biomes. They can interbreed, but seldom do.

In the Mojave, desert woodrats live in the valley amid desert scrub. Up the hill is the sierra vegetation where Bryant’s woodrats reside (shown).

Making their homes and foraging in distinct habitats offers limited mating opportunities between the 2 woodrats. Another factor limits interbreeding: personality conflicts.

Desert woodrat females are intimidated by the large, aggressive Bryant’s woodrat males. But Bryant’s females, which are larger than their desert cousins, find either woodrat species appealing. The more docile desert woodrat males are pleasant enough.

For logistical and preference reasons, few hybrid mating encounters occur. Those that do create disadvantaged offspring. Only 10% of hybrid pups make it to their 1st year of adulthood, compared to 22% of desert rats and 33% of Bryant’s rats.

Being less aggressive, hybrid males do not compete well for denning sites, and falter in territorial foraging battles. This pushes them out to live in less promising places.

 Sympatric Speciation

Ecological isolation is allopatric speciation: populations become geographically separated. This is a common dynamic for speciation.

By contrast, sympatric speciation transpires among populations within the same habitat. A subpopulation that is not physically separated from the main population strikes out on its own for whatever reason.

Sympatric speciation was long thought unlikely and rare, as the conventional Darwinist view about speciation was that populations needed physical separation to prevent interbreeding.

Sympatric speciation happens more often than commonly thought. Mole rats in Israel, palms on Lord Howe Island off Australia, and apple maggots in North America are other examples of rapid sympatric speciation.

Under suitable conditions, allopatric and sympatric speciation can occur with similar ease. ~ American biologist Justin Meyer

 Lake Constance Sticklebacks

Threespine sticklebacks were introduced into Switzerland’s Lake Constance around 1860. Since then they have split into 2 distinct species. One lives in the main lake. The other lives in the streams that flow into the lake.

These speciated sticklebacks continue to breed in the same streams at the same time of year. They still interbreed. But the two are genetically and physically distinct. The lake dwellers are bigger, with longer spines and tougher armor.

The sticklebacks’ genetic differences are concentrated on parts of chromosomes that are not prone to recombination. Hence, the gene variants that give the two their distinct traits are less likely to get mixed up.

 Orca Culture

Orca live in all the world’s oceans, without any geographical barriers to keep their populations from interbreeding. But separate groups do not interbreed, despite living in close proximity to one another. These separate groups differ in choice of prey and how they hunt. Orca choose mates that share their customs. It is an ongoing speciation based upon culture which is resulting in physiological changes.

 Ducks

In the lab, North American mallard and pintail ducks will mate; but not in the wild. The drakes of these species have similar coloration, but mallard females dress quite differently than pintails, who have a long, thin tail.

Coloration was not part of the selection process that led to speciation. Mallard drakes and pintail drakes have different mating displays which make for a lady duck’s pick.

 Plants

Sympatric speciation is common in plants. To adapt to an environmental stress, such as soil changes, plants often resort to autopolyploidy: creating a new set of similar (homologous) chromosomes. Polyploidal offspring share the same habitat as parent plants but are reproductively isolated. Throughout evolutionary history, plants have productively employed polyploidy for adaptive radiation. The diversity and success of flowering plants owes much to autopolyploidy.

Wheat has been modified and hybridized for millennia by humans. There are now strains that are diploid (2 sets of chromosomes), tetraploid (4 sets), and hexaploid (6). Durum wheat, also known as macaroni wheat, is tetraploid, while bread wheat is hexaploid.

 Parapatric Speciation

Populations in nearby areas may separate by preferential choice. This is parapatric speciation.

The North American threespine stickleback is typical of parapatric speciation. Some spend their time feeding near the surface (limnetics), while others prefer the lake floor (benthics). As sunlight filters through water depth, these 2 habitats have different lighting. Over time, the deep and shallow females respectively chose to mate with males at their depth, leading to reproductive isolation.

A similar reproductive isolation has occurred with sea lions in the Galápagos Islands. Central island sea lions prefer feeding in shallower waters, while those near the northwestern islands feed deeper. These sea lions are ostensibly the same species and could interbreed, as they live in overlapping foraging ranges, but they choose not to.

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Regardless of geography or reason, subpopulations that refuse to interbreed become separate species. Over time, certain adaptations create even greater differences between these species.

Even though the sources of genetic diversity are quite different between bacteria and sexual eukaryotes, the process by which adaptive diversity spreads and triggers ecological differentiation seems very similar. ~ American evolutionary biologist Jesse Shapiro

Speciation by timing is most mysterious. The closely related star coral and boulder star coral differ mainly by different timing in release of sperm and eggs, by 1.5 to 3 hours; enough to preclude cross-fertilization, given the ocean currents. How the schedules diverged is not known.

Coastal California pine speciated similarly. Monterey pine anthers (which release sperm) become active several months before Bishop pine stigmas (which hold eggs) become active, and so the 2 pines do not mate.

Pink salmon only breed in alternate years, after spending 2 years in the high seas. The salmon remain, for now, a single species, but in time may diverge by this reproductive isolation.

 Monkey-Flowers

The monkey-flower is a colorful plant, with about 150 species worldwide. The largest species group is in western North America.

There are 2 nearly selfsame varieties of monkey-flower in California which live in the hills at different elevations. Low-elevation monkey-flowers are red, pollinated courtesy of hummingbirds. Up the slope are pink monkey-flowers, pollinated by bees.

A difference in pollinator is a common element of plant reproductive isolation. Honeybees on the hills don’t carry their pollen down, and hummingbirds prefer the lowlands.

In between the different elevations hybrid monkey-flowers do exist. They are not as robust and produce fewer seeds.

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Interactions between species is a constant evolutionary driver. Sometimes a successful species falls prey to one that prefers the same dietary fare but lacks the skill to exploit it on its own. Such relationships can be complicated.

 Unhired Protection

The enemy of my enemy is my friend. ~ ancient Hindu proverb

With large colonies superbly organized, ants are a natural attraction to other species less endowed. Though relatively free of infectious diseases, thanks to their habitual cleanliness, ant societies are often invaded by social parasites.

Many ant pilferers escape punishment by passing themselves off as belonging to the colony, by having the proper scent to escape detection. Others are more brutish, in either forcing their way in or by brandishing chemical weapons. One particularly devious ant genus – Megalomyrmex – produces alkaloid venoms that repel and poison potential adversaries.

Some Megalomyrmex species get by on their own in small colonies. Others are raiders that loot the fungus gardens that attine ants cultivate.

Megalomyrmex agro-predators create havoc when they attack colonies. Fortunately for their victims, such raids are not very common. Only 1.5–14% of attine colonies are roiled by such parasitic raids.

Other Megalomyrmex settle in for the long haul. M. symmetochus has a cozy life as an unwelcome lodger in the nest of Sericomyrmex, an attine ant genus. Over 80% of Sericomyrmex colonies suffer the objectionable guest.

A newly mated Megalomyrmex symmetochus ant queen enters a host colony by stealth. She establishes herself in the fungus garden, where her growing band of parasites consume fungus and host brood for years.

The presence of M. symmetochus reduces the health of the host nest by slowing colony growth and reducing reproduction. The lodger clips the wings of host gynes (virgin queens), preventing mating flights. Clipped gynes become workers.

Like all lodger ants, M. symmetochus produces its own worker caste. In contrast, other social parasites exploit host workers without producing their own.

Once established, M. symmetochus never leave. The fate of their small colony is tied to that of their host.

Sericomyrmex is also subject to raider ants which can devastate a colony. When faced with such a threat, Megalomyrmex symmetochus earns its keep. The lodger attacks raiders with ferocity. Not only do they kill the invaders, they discombobulate them with their toxin. The raiding party may violently turn on itself, self-destructing because they no longer recognize their own kind.

M. symmetochus is so feared that its scent alone may dissuade raiders from even making an attempt on a nest.

Niches

Speciation rate is ultimately set by niche filling (that is, ecological competition for resources), rather than by the rate of acquisition of reproductive isolation. ~ American evolutionary biologist Trevor Price et al

Multiple species occupy the same niche when they extensively consume the same resources in the same area. Sharing a habitat creates competition. There can also be competition between subpopulations of a single species for the same resources. This has been extensively seen in human tribes.

Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants; and it follows from this, that as the favored forms increase in number, so generally will the less favored decrease and become rare. ~ Charles Darwin

The idea of competitive exclusion is that rivalry for selfsame resources progresses to dominance by one of the ‘competitors’. Situations seldom develop as simply as Darwin portrayed. Organisms don’t necessarily reproduce to carrying capacity (as humans are apt to), nor do “favored forms” increase at the expense of “less favored” (the notion of such favoritism being some ersatz ideal of predeterminism).

That said, some species may appear to overwhelm others, as is sometimes seen with ‘invasive’ plants. This dynamic relates to local population numbers and may not directly affect speciation or extinction.

The alternative to becoming rare from competitive exclusion is to adapt to a different niche by consuming a distinctive mix of resources. In creating variation that leads to speciation, competition begets a degree of isolation. To avoid decline from food competition, closely related bird species do not coexist in the same area.

As we have already seen, variation can take many routes, driving adaptation in different directions. A confluence of adaptations may take place that put a population on the path to speciation via a variety of altered traits.

Taking up residence in more inhospitable terrain may result in augmenting defenses against predators and/or greater tolerances to adversity in whatever form. It may also lead to taking advantage of new resource opportunities that create a much different lifestyle.

Niche filling has stopped species from getting big ranges. ~ Trevor Price

Specialization has its risks. Performance may improve in a new niche, but if speciation has occurred via extirpation, that species may be especially sensitive to ecological disturbance.

The short-term functional advantages of a novel trait do not always translate into a lineage’s long-term survival or diversification. Adaptation contributes to a good fit between an organism and its current environment. What it cannot do is anticipate the future. ~ Geerat Vermeij

Generalization may decrease specific exploitation performance but offer a robustness that affords survival in a changing environment. Expanding the diet to include other foodstuffs may reduce direct competition. Adaptations to optimize nutritional extraction from alternative food sources may result.

Just as the availability of unexploited resources drives nichification, radiative speciation is constrained by lack of opportunity. The rate at which niches are created and occupied limits diversification. In absence of such opportunity, new species may arise from mating preferences, but these are relatively rare.

 Competition in Guadalupe Canyon

In the 1870s, “Old Man” Clanton’s cattle ranch was one of the most profitable in the area. The ranch was not much more than a day’s ride out of Guadalupe Canyon, which straddles the Animas Valley of New Mexico and San Bernardino Valley of Arizona, just north of Mexico’s border.

Clanton did not own a brand; a legal requirement for cattle operations. But law was somewhat scarce in these parts. No brand made mixing in cattle smuggled from Mexico a lot easier.

In July 1881, a band of Mexican smugglers headed for Tucson or Tombstone gave up their silver, and their lives, in an ambush in nearby Skeleton Canyon. In the wake of that massacre, the Mexicans dispatched troops.

A few weeks later, on August 13, just after sunrise, a warm wind rustled Clanton, who was asleep on the Guadalupe Canyon floor in his bedroll, along with 6 other men and a stolen herd of cattle. The Mexican troops come upon Clanton and his companions.

Only 2 of Clanton’s men made it out alive to tell the tale. “Old Man” Clanton was as old as he was going to get. He had rustled his last.

The canyon is more peaceful now, humming with life. Literally.

The arid scrub of Guadalupe Canyon is not nectar-rich, but 4 species of hummingbird live there: black-chinned, broad-billed, violet-crowned, and Costa’s hummingbirds, to be precise. The black-chinned hummingbird nests 5 meters up in Arizona sycamores that grow above bare creek bottoms. Violet-crowned hummingbirds live upstairs in the sycamores, at 7 meters. Broad-billed hummingbirds nest low, only a meter or so above ground, preferring the north slope of the canyon, near rocks. Costa’s hummingbirds prefer to nest in the dry arroyo tributaries, adjacent to the main canyon, or on the south side.

As time went by, variation went to speciation. Each found a niche to hang on to.

Only rarely does a battle break out for a choice nectar spot. When that happens, the violet-crowned hummingbird dominates, by aggression and size: 6 grams to the others’ 3 grams.

Outside of insects, hummingbirds have the highest metabolism per weight of all animals; making a fight for nectar all the more understandable.

 Hummingbirds

Everything about hummingbirds is extreme. They have this incredible hovering flight, with wing beat frequencies of 60 times per second, which is nuts. They have the highest metabolic rate for their size of any vertebrate. They are little machines that run on oxygen at a high rate. It is amazing that evolution can take an animal to such extremes. ~ American zoologist Jimmy McGuire

Hummingbirds evolved 22 MYA. There are 338 known extant species in 9 clades.

As nectarivorous pollinators hummingbirds are a classic coevolution: plants willingly feeding animals in return for distance matchmaking between male plants with sperm-bearing pollen and egg-bearing females.

Hummingbirds have flower preferences by color and size, but those preferences can change as opportunities arise.

Hummingbirds are at an extreme in adaptive radiation. As many as 25 species can coexist in the same habitat.

The Andes mountains have been an especial hotspot for hummingbird evolution: speciating with the uplift of those peaks over the past 10 million years.

The smallest bird is the 5-cm bee hummingbird, which weighs only 1.6–2 gram. This tiny hummingbird is endemic to the Cuban archipelago.

 Birds

Even as humans have exterminated ~130 bird species since the 17th century, around 10,000 are still extant, in most every terrestrial habitat, from the Arctic to the Antarctic.

As the most speciose tetrapod, birds most vividly illustrate adaptive radiation with notably narrow niches. Resource competition and mating preference drive avian sympatric speciation.

The Greenish warblers spread out around the Tibetan plateau are a ring species: reproductively isolated populations, but not by geographic boundary or habitat. Birds in adjacent areas live within flying distance of each other, and are genetically similar enough to interbreed, but don’t.

Over time, populations developed their own distinctive songs. Speciation happened as females preferred males that sang local tunes.

 Flies in Evolutionary Flight

Most insects are specialists. ~ American biologist Andrew Forbes

Herbivorous insects are the most speciose animal group. Speciation is driven by 2 pressures: food and foes – plants and predation. Overbearing overlap on the same food source is a formula for shortage. Moreover, plants adapt to put the hurt on overpowering predators. Better to find something to eat that is not in high demand.

The tropics are full of flies; so many different flies; each species nibbling on a different plant.

The tropics are also full of parasitic wasps, specialized to lay their eggs in a specific fly. Larval offspring will not survive if a wasp lays its eggs in the wrong species.

As each fly is targeted by a specific wasp, changing food source lets a fly escape its nemesis, at least until the wasp finds it again, or another wasp adapts to take advantage. It is an evolutionary game of hide-and-seek, driving dodgy diversity amid the verdure allure.

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Broadly speaking, speciation takes 1 of 2 opposite directions: specialization or generalization. Geographics often tilts the vector decisively.

Evolutionary Hotspots

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.

 Galápagos Islands

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.

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Broadly speaking, speciation takes 1 of 2 opposite directions: specialization or generalization. Geographics often tilts the vector decisively.

Evolutionary Hotspots

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.

 Galápagos Islands

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.