Charles Darwin’s voyage of enlightenment

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Charles Darwin, Islands, and Galapagos Tortoises

Charles Darwin’s voyage of enlightenment

Imagine you’re traveling on a naval vessel off the coast of South America. In between the days of endless seasickness and lousy food, you have the opportunity to explore some relatively unknown islands and countries. You’re a sort of amateur naturalist, and can’t help but notice that the animals and plants that have the most in common are found closest together. If God had created each species at the beginning of the earth, why would he go to all the trouble to group similar organisms with each other? This is exactly the question a young, unknown English gentleman found himself asking during the 1830s.

Like many young people today, Charles Darwin was unsure about what he wanted to do with his life after finishing school. He first set out to be a doctor, but he was so squeamish about blood that he ran from the classroom whenever an operation was being performed. In his short year at medical school, Darwin nevertheless found the time to join a natural history society, attend lectures on older evolutionary theories and to learn taxidermy, the art of preserving biological specimens for study. Resigned to making a living somehow, he enrolled at the University of Cambridge to work towards a career in law or as a parson, the leader of a small English church. While at school, Darwin was introduced to beetle collecting by his cousin, and learned a lot about natural theology, the belief that biological wonder demonstrates the glory of God.

Of all the exotic locations studied by biologists, none are as famous as the quirky Galapagos Islands, where this young British man named Charles Darwin was inspired to question where different plant and animal species came from. Although he didn’t realize it until he returned to England and shared his collections with experts, the simple ecosystems and slight differences between species on each island made for the perfect natural laboratory for evolution, the inherited change in organismal traits over time. The development of the theories of evolution and natural selection, Darwin’s new idea about how evolution worked, is a classic tale of the wonder of discovery, and a testament to the power of a single man’s mind in figuring out how the natural world operates.

In Darwin’s day, there were no professional scientists. Instead, science was mostly conducted by gentlemen with inherited wealth that didn’t need a job. People of the higher classes even went so far as to look down their noses at people that had to work for their living. Because he thought he needed the money, Darwin reluctantly started looking for a job in the church after his planned scientific vacation to the Canary Islands fell through. Instead, he received a letter inviting him to serve as companion and naturalist on a British naval survey of the coast of South America. Darwin jumped at his chance, his uncle’s encouragement defeating the protests of his father, and set out on the first and only great adventure of his life.

Aside from his duty to keep the captain company, Darwin traveled up and down the coast of South America, shooting birds and collecting beetles, identifying rocks and unearthing fossils, speculating about the formation of islands and taking plant samples. Darwin noted that the fossil animals he found in South America more closely resembled those living in South America than anywhere else, and he was also struck with the similarity of island animals to those living close by on the mainland. These things pointed to a natural process of animal origins – why would God go to all the trouble to place similar animals remarkably close together?

The Galapagos Islands

Let’s take a closer look at the Galapagos and the unique animals that call these islands home. In your kit you will find a textured map and a bunch of plastic animal models. Ignore the large tortoises for now, but let’s take a look at the map. The Galapagos archipelago sits right on the equator, and consists of six large and quite a few smaller islands. Take a moment to familiarize yourself with the shape and names of the islands. The islands have Spanish names, because they are part of the country of Ecuador, where Spanish is the primary language. The islands are covered by volcanoes, (V. on the map stands for Volcan, the Spanish word for volcano) and mountains (Sierra on the map). The islands were formed from a westward-moving volcanic hot spot, meaning that the islands are made from cooled lava, and that the eastern islands are significantly older than the western ones. The large western island, Islabela, is so young that it still has more than five active volcanoes that spew molten lava fields on a regular basis. Try and get a sense for the relative size, orientation, position, and distance between islands – this will become important later. Also notice that some of the islands have a moist interior surrounded by arid seacoast. The moist lands are at higher elevation, and are ecologically very different from the dry, rocky land near the ocean and on the smaller islands.

The Galapagos is famous for its endemic species, organisms that occur nowhere else on earth. Perhaps most well-known are the marine iguanas – the only species of iguanas that regularly swim and feed under the water. The iguanas sleep, breed, and relax on land, but venture out into the surf to eat seaweed. The iguanas are ectotherms, meaning that they get their heat from the environment. Before diving in the cold water they must heat up by basking in the sunlight, and can only stay at sea for a short time or they will cool off and die. Juveniles and females stay near the shoreline, but the larger males have enough body mass that they can swim far offshore and back before getting too cold

Included are small models of the marine iguana and another endemic reptile - the Galapagos land iguana. These iguanas are the likely ancestor of the marine iguanas, are adapted to eat the tough, local cactus, and were originally so plentiful that Darwin complained he couldn’t pitch his tent without covering one of their burrows. If you visited the Galapagos today, however, you’d have to be lucky to see one in the wild. Why do you think this is?

The Galapagos is also home to a curious bird – the flightless cormorant. Cormorants hunt by swimming on and below the water’s surface to catch fish. Every other cormorant species, however, can also fly. Why do you think the isolated Galapagos flightless cormorant lost the ability to fly? We’ll discuss this strange bird more later on.

Land tortoises and Unique Species

The large and lumbering land tortoises that Darwin briefly encountered on Santa Cruz Island (called Indefatigable by the British) had been known for decades by whalers and pirates to be delicious and convenient sources of meat – they could be stored in the hull of a sailing ship and wouldn’t die for weeks. Darwin soon realized that this ability could also allow the tortoises to survive ocean travel between islands, and even explain why the tortoises were so similar to the ones he observed living in South America. Instead of being placed on each island specifically by God, he realized, they could have floated there by prevailing currents, or rafted on logs of vegetation torn from the coast during storms.

Feel the Galapagos tortoise shell models included in the kit. You’ll notice that some of them have a saddle-shaped arraignment (these tortoises have an arch in their shell high over their neck, shaped like a saddle for a horse), some have a highly domed shell, and others have a shape somewhere in between – no two sub-species are exactly the same. There are thirteen subspecies or races of Galapagos tortoise, and each occupies a different geographical part of the islands. Right now, spend a few minutes examining each tortoise sub-species model in detail. Make a mental list of the similarities and differences that you note, and try to imagine how these differences came about. Bear in mind that each turtle is found on only one, or just a few, of the many Galapagos Islands.

What is important about the presence of closely-related but unique species living on isolated islands located close together?

Darwin first noticed this pattern of slight differences between islands when he looked at the Galapagos mockingbirds. The mockingbirds on the islands are very similar to those on the mainland, but have evolved a slightly larger body, work together to hunt and gather food, and are more hesitant to fly than mainland species. In addition, the mockingbirds on Espanola Island are larger, mangier, and have a more curved beak. Scientists studying the seabirds on this island have noticed that the mockingbirds use this curved bill to break into the eggs of boobies, gulls, and albatross, and even to drink the blood of seabird chicks. Darwin immediately noticed the similarities and differences in these species and recognized that the geographic proximity meant something – it didn’t make sense that special creation would place these animals so close together. It seemed as if the birds had moved to Espanola, and then diverged slightly from the original type.

When Darwin came back to England, he shared his Galapagos specimens with ornithologists, scientists that specialize in birds, who quickly noted that all of the dull brown/black birds Darwin collected were not totally unique species as Darwin had originally thought, but were actually all closely related to birds called Tanagers on the South American mainland. The implications, as Darwin realized, were enormous. One group of birds had colonized a group of offshore islands and developed into separate types with unique bills on different islands. It didn’t make sense for God to put animals on the islands in this way – why wouldn’t he just put the same species on the islands as on the mainland? Why would he deliberately arrange the placement of the birds to make it seem like they were related to each other? Nothing made any sense at all to Darwin unless a small group of birds had traveled to the Galapagos and then radiated into separate types all on their own.

Back Home and Eureka!--Natural Selection

Darwin realized that the birds evolved from a single group almost at once, but he couldn’t figure out how it happened. How could one type of animal adapt to a new habitat and become another type? The inspiration for his answer came from a seemingly unrelated essay on populations by the mathematician Thomas Malthus. Malthus reasoned that if the human population kept increasing at its current rate, there would eventually be more people than we could grow food to feed, and most babies would not survive to become adults. Darwin supposed this was also the case in wild animals, and thus hit upon the idea of natural selection. Put simply, it suggests that because only a certain number of animals from each generation can survive, and because there are small differences within populations (some robins have ever so slightly smaller or larger beaks, for example) only the ones most adapted to their environment would be able to grow to adulthood and have children of their own.

This mechanism could finally explain the change of species over time – the animals best adapted to their habitat would pass on their characteristics to future members of the species.

Alright, let’s try an activity to get a better feel for how natural selection really works. In your kit you’ll find a tray and some wooden beads. Some of them are fuzzy, and some smooth. In this game, all of the beads are animals in the same species called Squassums, and the difference in texture represents different traits present in the population. Imagine that our population of Squassums was living in a pretty open environment, breeding asexually, meaning that one squassum just divides into two new squassums, and were being terrorized with regular predation by Clawsters (you get to play the clawster). Fortunately for the Squassums, a fuzzy ground plant called Grabweed has been growing in their habitat (it’s what makes the tray fuzzy), providing protective cover for some of our Squassums. When the grabweed started growing, half of the Squassums were fuzzy, and half were smooth. Let’s see what happens after a few generations.

Start out by placing ten fuzzy and ten smooth Squassums in the habitat. Every generation you’ll simulate a Clawster attack by holding your hand in the shape of a giant claw and dragging it across the environment to grab 2-3 squassums. After that, you’ll simulate the growth of the Squssum population by grabbing 5 totally random squassums from the habitat and replacing each one you grab with the original and two other’s just like it – just as if each fuzzy squassum had two fuzzy children, and each smooth squassum had two smooth children.

Run the game for 10 generations (each dividing population and clawster attack is one generation) and then count the squassums left in the habitat. How many smooth and fuzzy squassums are there? Are the numbers different from the beginning (there were 10 of each in the beginning)? If they are different, what do you think caused the differences?

Let’s get back to our land tortoises. If natural selection caused the tortoises to develop unique shell shapes on each island, there must have been a way in which these differences could benefit each sub-group of tortoise, just like how the fuzzy coat on some of the Squassums allowed them to adhere with the Grabweed and avoid predation from the Clawsters. Biologists believe that the saddle-shaped shell design helps tortoises in the drier islands to reach vegetation out of reach to tortoises with a normal domed shell. (Feel how the saddle flare above the neck removes the part of the shell that would otherwise block the neck from becoming totally vertical). The populations on these islands may have started out with normal dome shapes, but evolved saddle-backed shells during times of drought, a perfect example of Darwin’s natural selection theory. Notice that the hoodensis tortoise, one of the most saddlebacked of all, comes from Espanola, one of the dry islands, and that porteri, one of the most domed tortoises, comes from moist Santa Cruz Island. When food was scarce, the tortoises with shells that allowed them to reach higher got more to eat, survived at a higher rate than the domed tortoises, and gave rise to more saddle-shaped offspring. This process, where one type of animal diversifies into many different types that specialize in different ecosystems is called adaptive radiation.

Alright, we have a hypothesis for why some tortoises are saddlebacks and some are not – the saddleback form evolved on drier islands where food is less available and the tortoises need to be able to reach higher for any available vegetation. But go back to your notes from before – are these the only shape changes you noticed? If you were highly observant, and it’s not always easy to notice everything, you may have noted that some of the shells were flatter, or had bigger ridges on the side, or had more rounded sides. These small traits probably have very little effect on how well the tortoises survive. Instead, these changes probably came about through a different mechanism.

Isolation and Genetic Drift

Take another look at the map of the Galapagos Islands – there is another way in which shell shape is affected by geography. In order for a species of animals or plants to diverge into two or more separate species, the original species is first usually separated into two or more populations by some sort of barrier. The barrier is usually a physical structure, such as a mountain range or a desert.

After awhile, random changes accumulate in each population make them too different from each other to reproduce and produce viable offspring, resulting in speciation, or the formation of new species. This accumulation of random changes between two species is known as genetic drift, and probably explains why the Galapagos tortoise subspecies, although they are outwardly very similar, cannot breed together today. The effect is similar to how the separation of the Atlantic Ocean has caused the English language to be spoken with a different accent in the United States and the United Kingdom. The American accent developed by the accumulation of random changes, doesn’t make it easier to communicate in the United States than in England, and if the USA and England became totally separated, could eventually become so distinct that English-speakers from the two countries could not understand each other.

Let’s play another game to better understand how this could have happened. Take out the poker chip monsters – note that there are different types – 10 each of monsters A, B, and C. Throw all of them on the table and scatter them around. Next, we’ll pretend a mountain range grew up in the middle of the table. Without feeling to see which type of monsters you’re grabbing, separate the monsters into three groups. Next, count the types of monsters left in each smaller group. Does each group all have the same number of each type of monster? Why not? What does this mean for the future of the poker chip monsters?
If you were a Galapagos tortoise on one of the islands in the Galapagos, there would be two main barriers to your mating with any other Galapagos tortoise – the ocean, and impassable fields of razor-sharp lava. Biologists believe that the ancestral tortoise population landed on either Espanola or San Cristobal island (the islands closest to the mainland), and then fortuitous currents or, many thousands of years later, sailors transported tortoises from island to island. These events were extremely rare, but given the long time scales involved (tortoises have been on the islands for at least 2 million years) are almost certain to have occurred. Once a population got started out on a new island, it was totally isolated for all practical purposes. There were not enough tortoises accidentally floating back and forth to keep the two groups connected, and the populations diverged.

Another cause in which tortoise populations could be separated is the relatively active geology of the archipelago, or island group. Examine the largest island on your map, Isabella, which also happens to be one of the youngest. The island has five major volcanoes, Volcans Wolf, Darwin, Alcedo, Sierra Negra, and Azul. When these volcanoes erupt, molten hot lava flows out of the craters, covering the islands in thick spreads of sharp rock, sterilizing the environment and creating an impassible zone for tortoises. Not only is the region a navigational challenge to the slow-moving tortoises, but the new lava flows are devoid of all plant life for many years, and therefore there’s no food for the tortoises in these regions.

Sometimes, because the habitat on either side of the barrier is different, populations undergo speciation as the result of adaptive radiation and genetic drift working together. Take another look at the map of the Galapagos Islands - can you imagine how this sort of evolution could occur here?

Evolutionary History of Galapagos Tortoises

We’ve now discussed two mechanisms that can explain the difference in tortoise shells in the Galapagos – adaptation to the local environment, and speciation from isolation, called allopatric speciation. Feel the models you’ve been given, and keeping these two factors in mind, try to figure out an evolutionary history of the tortoise species. Grab the map again - you’ll need to know where the tortoises are found on the islands:

Tortoise subspecies Distribution

Becki Volcan Wolf, Isabela

Chathamensis San Cristobal

Guntheri Sierra Negra, Isabela

Hoodensis Espanola

Porteri Santa Cruz

Are you having trouble figuring it out? It’s pretty difficult. Biologists have been struggling to figure out the lineage of these tortoises for many years, and only recently has DNA evidence been used to shed light on the confusion. We know that the first tortoises to land in the archipelago had to be dome-shaped, because all of the tortoises on the mainland of South America are dome-shaped. The first tortoises probably landed on the island of Espanola or San Cristobal, the closest islands to the mainland, and also the oldest (remember – we read about the moving hot spot and the formation of the Galapagos Islands). The tortoises on San Cristobal have a shell shape intermediate in between the domed and saddle shaped varieties, and those on Espanola are among the most extreme saddle shaped races. From here, tortoises moved to the rest of the islands by prevailing winds and currents. There is evidence that a now-extinct sub-species of domed tortoises was established on San Cristobal, meaning that it is the site of original colonization.

This discussion brings up an important point about islands – their formation. Islands can form in two ways: either by separating from a continent and floating out to sea, as in the case of Madagascar, or being formed as part of a unique volcanic event – many islands are actually just the tips of huge underwater volcanoes. Believe it or not, the volcano that forms the island of Hawaii is actually the tallest mountain on earth, stretching over 10,000 feet higher from the floor of the ocean than Everest rises above the land. When volcanic islands such as the Galapagos and Hawaii form, they’re initially devoid of life. Everything living on these islands now (except for species that humans have accidentally or purposefully introduced) had to make it to the islands by chance. In the 1970s, the volcano Krakataua exploded in the Indian Ocean in one of the largest volcanic eruptions ever recorded. As part of the eruption, a new island of fresh lava was formed, called Anakrakatau, or son of Krakatau. Now-famous biologists, including E.O. Wilson, jumped at the chance to watch a new environment being formed before their eyes, and documented the species landing on the island over time. The growth started out slow, a few species a month, but then began to accelerate. Eventually, an environment reminiscent of the older Kraataua developed and after a while, the total number of species starting remaining the same, even though new species kept showing up. How could this be? Well, it turns out some of the species were going extinct.

The distance/area effect

After an island comes to equilibrium, the number of species living on the island stays relatively the same, but it’s not necessarily the same assembly of species forever. In addition, scientists have noticed that different islands have different numbers of species. To understand why, it’s important again to think about geography. Two main factors affect the number of species on a given island: the immigration rate (how often new species become established on an island), and the extinction rate (how often established species die off). What do you think influences these factors?

First, the immigration rate depends on the distance between the island and other land masses. This makes sense, because the further an island is from a continent, the less likely it is that animals and plants will be lucky enough to land there. If you fire a shotgun at a target 10 feet away, a lot more of the pellets are going to hit than if your target is 50 feet away, and this explains why Madagascar gets a lot more new species than anywhere in the Hawaiian Islands. The biggest factor affecting the extinction rate is simply island size – bigger islands can support more species, because there are more niches on larger islands and the chance of each organism hitting rough times and dying off is lessened It’s sort of similar to how larger cities have more diverse restaurants – small towns don’t have enough diners to support many different kinds of food options.

Pull out the chart of the species-area-distance curve. The rate of extinction is plotted against the right axis, and rate of immigration plotted against the left axis. Think about what the chart is saying about the total number of species likely to be present on any given island. What kind of island would support the greatest number of species?

Consider an island. You’re probably picturing a small piece of land in the middle of the ocean with gentle waves and palm trees. Instead, imagine central park in NYC. It’s sort of like an island, isn’t it? It’s a small piece of natural area surrounded by the ocean of urbanization that is New York City. If an animal wants to get in or out of central park, it has to cross blocks of downtown city traffic – quite a feat for a raccoon or a moose. Birds and insects can readily fly into the park from other natural areas close to the city, but what if New York City was much larger?

Uniqueness of island fauna

Islands have remained important in biology even after the days of Darwin and Wallace. Indeed, a whole subset of biology, called island biogeography, has been started to understand the distribution of plants and animals on island systems. First off, several curious effects of islands on animal species have been noted. To start, island species seem to become either much larger or much smaller than their mainland ancestors. On the Galapagos, the tortoises are gigantic, weighing up to 880 pounds and growing up to 6 feet in length, much bigger than their mainland ancestors. Can you think of an explanation for the gigantism on islands? There are many other examples, including the giant Galapagos centipedes, and the larger mice and rats found on the Channel Islands in California.

As science writer David Quammen has noted, “Islands are havens and breeding grounds for the unique and anomalous. They are natural laboratories of extravagant evolutionary experimentation.” Animals on islands tend to evolve very bizarre tendencies. The famed marine iguanas of the Galapagos are a fantastic example. Not only do they grow to sizes far in excess of their mainland ancestors, they have adapted to spend a lot of time in the ocean, and are the only known aquatic iguanas in the world. Why do you think islands are good places for weird animals to evolve?

Furthermore, island species often lose their capacity for dispersal. Most animals and plants have a lot of abilities in order to spread out over the landscape: plants have seeds that get stuck onto the hair of animals and get carried to new locations, and birds can easily fly to new places. On islands, however, this seems to be reduced. Many island birds become flightless. The Galapagos is the home of the world’s only flightless cormorant, a diving seabird closely related to loons, and the mockingbirds on every island fly much less than their ancestors, preferring to hop quickly along on the ground. What about islands do you think causes animals to lose their dispersal ability?

Part of it works like this. Because islands are usually devoid of species in the beginning, especially volcanic islands like the Galapagos, there are usually almost no competitor species present; animals that somehow make it to islands are released from all of the pressures they experienced at home. Birds have fewer dangers to fly away from, and at the same time there aren’t a lot of places to fly to – the animals are isolated on a small patch of land. Therefore, with the major benefits of flight eliminated, the costs of developing flight structures become a handicap and flightlessness becomes an adaptive trait.

In the same manner, the lack of established communities on islands allows animals to evolve strange lifestyles in these places. Because there are no native woodpeckers in the Galapagos, a species of Galapagos finch has evolved from being a classic seed-eater to probing cactus plants with its bill and cactus spines to seek out insects. The finch isn’t as effective in this job as a classic woodpecker, but it gets along in the absence of competitors. Similarly, in the absence of goats and other mammal herbivores, the tortoises in the Galapagos were able to evolve gigantic size to become the dominant plant-eaters on the islands.

As a result, we can see that islands can essentially function as biodiversity factories. Biodiversity is a measure of the variation of life forms in an ecosystem or geographic region, and can be based off the total variation in species, individuals, communities, or genes in the area. Because each island is separated from every other, each one can evolve its own unique species found nowhere on earth, contributing to biodiversity. Remember how the Galapagos tortoises were able to diverge into sub-species just between the separate islands in the archipelago? Well, this same process occurs worldwide, with every unique island ecosystem generating tons of endemic species (those species found nowhere else). These species are often specialists, well-adapted to a very specific way of life, with adaptations that allow them to extract a way of life from a very specific biological niche. A great example of this is the mangrove finch, one of Darwin’s finches, which has adapted to pluck insects from the isolated coastal mangrove forests on the Galapagos Islands of Fernandina and Isabela.

Unfortunately, animals that become specialized to exploit a very narrow habitat often become completely dependent on a rare ecosystem and are therefore highly vulnerable to changes in the environment. Galapagos land iguanas specialized to feed almost entirely on the cactus on the islands, and nearly went extinct when introduced goats denuded the area of cactus plants.

An addition concern for specialized island animals is that, because they have evolved in an ecosystem devoid of competitors and predators, they often lose the defensive mechanisms that other species take for granted. For instance, the introduction of rats and goats onto the Galapagos Islands has proved devastating for native tortoise and land iguana populations. The goats out-compete the tortoises for plant foods, and the rats feed on their eggs, preventing them from reproducing. For these species to be able to survive in the age of modern transportation, human colonization, and jet aircraft, they have to be vigorously protected by man. Without a successful captive breeding program and the strict regulations about who and what is allowed to land on the Galapagos Islands, the native tortoises and land iguanas would surely go the way of the dodo.

Artificial selection and the Age of the Earth

When Darwin got back to England, he settled down to a quiet life in the countryside and assembled his thoughts. You’d think he would have been ready to tell the world about his theories, but he held back. In Darwin’s day, the notion that each species was not specially created by God was held as heretical, and in promoting it, he would have isolated himself from his colleagues, supporters, and scientific heroes. He knew if he was going to publish such an outrageous theory that he would need to have overwhelming evidence. Darwin spent the next twenty years conducting experiments on seeds and barnacles – he wanted to see if plants could reproduce after their seeds had been at sea for months, and he wanted to document the high level of variation necessary for natural selection to occur in another type of animal. Darwin also marshaled evidence from domestic species. Meeting with eccentric pigeon breeders in dusty taverns, Darwin argued that if puny human meddling could bring into existence the great variety of domestic dog and bird varieties, surely eons of natural selection could create all of the naturally occurring varieties of plants and animals.

This notion of eons brings up an important point – in Darwin’s day it wasn’t well-established that the world was all that old. A literal interpretation of the bible put the earth at only 6,000 years old, not even a fraction of the time necessary for biological diversity to be explained by evolution as Darwin conceived it. More scientifically, the eminent geologist William Thompson, later known as Lord Kelvin, by calculating the rate of cooling of the earth from lava, estimated the age of the earth could be no greater than 400 million years old. This estimate remained a major objection to a natural explanation of biological diversity until the discovery of nuclear physics – the production of heat from the breakdown of elemental particles in the earth’s core allows for a much slower rate of cooling, and the actuate 4.5 billion year age of the earth. This helps to explain the lackluster support for natural selection in the past – although the concept of evolution in general was accepted by scientists almost immediately after the publishing of Darwin’s work, biologists thought he’d gotten the mechanism wrong well into the 1930s.

Wallace and Biogeography

While Darwin was puttering around his gardens measuring the length of barnacle’s various anatomical novelties, little did he know there was another brilliant amateur naturalist hot on the trail of natural selection. Alfred Russell Wallace, forced to support himself without the family inheritance that Darwin enjoyed, had led an exciting an adventurous life full of the hardship of tropical travel in the Victorian age. Constantly victim to rashes, insect stings, and frequent bouts of malaria, Wallace nonetheless collected many rare and valuable specimens in the Amazon. Returning to England, his ship caught fire, destroying almost all of his work. With a paltry 200 pounds in insurance, Wallace set off again – this time for the Malay Peninsula. Collecting butterflies, birds, and monkeys, he noticed something geographically quite curious. The islands of Bali, Borneo and Sumatra were full of species that were quite similar. Just across a narrow straight to the island of Lombok, however, the animals suddenly became quite different, and strongly resembled animals from Australia. To the west of the straight, the trees were full of screaming and howling monkeys. To the east, the arboreal realm was filled instead by the relatively quiet munching of tree kangaroos.

Wallace was an intelligent man, intent on generating an explanation for the “mystery of mysteries” – the source of new species of life on Earth. Working backwards, Wallace realized that the animals east of the mysterious boundary must be have originated with the Australian creatures, and the others in Asia. The islands must have been connected in the past, with the ocean waters only recently intervening. The split along the line must be much more significant – these animals had to have originated independently. Modern geology has borne out Wallace’s thinking, revealing that the water separating the islands on either side of his line is much deeper than the waters in between the other islands. When water levels were lower in the past, the area consisted of two landmasses instead of a bunch of islands.

Wallace knew that the distribution of animals and plants on these islands indicated that organisms were related to each other, and that they therefore must have a common ancestor, but until an especially virulent bout of malaria confined him to his bed with nothing but his thoughts, he couldn’t figure out the mechanism of natural selection. After his epiphany, he sent a draft of his ideas to Darwin which must have sent the elder naturalist into hysterics – his caution might just have been his downfall. The independent discoveries were presented together at a meeting of the Royal society without much aplomb. Not until the publishing of Darwin’s “big book” On the Origin of Species, would evolution become a household concept and the target of widespread criticism.

Island biogeography and conservation practice

Let’s go back to central park. Remember, a park is essentially an island of wildness in the ocean of urban development. Using the relationships scientists have discovered from real islands, we can predict what will happen to central park. If the urban part of New York City expanded, the immigration rate to the park would decrease. As an island, central park is pretty small, about 850 acres, roughly the size of one of the smaller British Virgin Islands. We might be a little concerned about the extinction rate, especially because Central Park isn’t totally wild. Over the long term, we might start to see extinctions in Central Park. First, some of the more rare flowers and trees might start to disappear. Maybe after one rough winter there would be no more raccoons. As the tree communities become less complex, the diversity of breeding bird species would decrease. Without connections with other wild areas, we might predict that New York’s central park could be a haven only for rats, pigeons, grass, and maple trees by the year 2150. Using this same way of thinking about parks as islands, let’s take a look at the map of the United States.

Part of the mission of the national park system is to preserve national biodiversity through the management of public lands. To this end, the Park Service administers 84.5 million acres of wilderness area, in conjunction with the bureau of land management (BLM) and the national forest service. If all of these lands were pooled together into one giant park, it would be huge and support a lot of wildlife, even if the rest of the United States became one giant urban sprawl. Unfortunately, the park system is spread out into isolated parks in order to preserve landmarks and ecosystems of important cultural, scenic, or biological importance. As a result of this policy, however, the parks are in danger of becoming islands in the sea of the greater United States. If they become disconnected, they could start to lose their species, as the area effect takes hold. You might find it hard to believe that parks on land will act the same as islands. So did many biologists, until another smart naturalist had a particularly bright idea in the rain forest.

Huge amounts of the Amazon rain forest are deforested every day, both to open up new land for agriculture, and for the harvesting of tropical lumber. Because the forest was going to be lost anyway, a bright ecologist named Thomas Lovejoy decided to conduct an experiment. If only he could get the logging companies to cooperate, the rain forest could be deforested in a way that mimic the formation of islands. Using a pretty complicated map, Lovejoy and the Smithsonian Institution had the loggers create a patchwork of rainforest patches. As soon as the chainsaws fell silent, hordes of scientists and graduate students descended on the plots to list all of the species present in each patch of forest. Using a network of volunteers, Lovejoy was able to monitor how these numbers changed over time, and the results were staggering. Almost immediately, species started to disappear from the plots. The first to go were the wide-ranging species that need a log of land – jaguars, monkeys, and harpy eagles. The small plots were quickly reduced to very simple ecosystems of very common species – they simply weren’t large enough to support very many species, and almost no rare ones.

This study, called the biological dynamics of forest fragments project, helped to bring resolution to the single-large or several-small (SLOSS) debate, which argued about whether it was better to create one huge reserve or several smaller and more unique preserves to protect the most biodiversity. For example, the United States could put all its efforts into protecting a huge area near Yellowstone National Park, but then it would be less able to protect endangered salamanders in North Carolina, the high alpine ecosystem in the Sierra Nevada, or the small remaining patches of native prairie in the middle states. What do you think the conservation mission of the United States should be?

Only recently have conservation planners agreed that the conversation of the greatest degree of biodiversity should be a conservation goal. Unfortunately, this sounds much easier in writing than it is in practice. If all conservation effort focused on creating one giant park, small biodiversity hotspots like the Galapagos wouldn’t be protected. If planners tried to protect all of the hotspots, however, they might wind up conserving less total biodiversity. What do you think is the best way to solve this problem?

If you can’t make up your mind, rest assured, neither can most biologists or conservation planners. Thankfully, there are a few things that can be done to blend the two ideas together. The key tenet of biologists favoring one massive reserve is that it allows ecosystems to be more connected. To address this concern, conservationists are working on creating wildlife corridors between natural areas that already exist. If mountain lions from Yellowstone can get into the Salmon and Bitterroot wilderness in Idaho, both populations will remain healthier from the sharing of genetic material. If the oak trees on the east coast are all part of one breeding group, it’s far less likely that one of them will go extinct. Furthermore, if a really devastating winter challenges the survival of bison in Yellowstone, they’ll do much better if they can move to other grassland north of the park through a protected corridor. Human barriers represent a significant challenge to these wildlife movements, but can be made less daunting by the construction of wildlife passages under roads and pipelines, and the construction of fish ladders at hydroelectric dams. How else do you think that connectivity between protected areas can be improved?


We’ve seen how Charles Darwin started unlocking the keys to modern diversity by thinking critically about his observations in the Galapagos Islands. Let’s review – how did both natural selection, isolation, and genetic drift cause the land tortoises to develop different shell shapes?

We’ve also seen how the study of islands has been important in conservation practice. How are nature preserves and parks like islands? What does the size and position of islands mean for the continued existence of the animals and species that live on them? How can modern conservation planners use these lessons to design effective parks? What can you do to prevent extinction?

Review Questions

What led Charles Darwin to question the origin of species in the Galapagos Islands?

How does natural selection lead to changes in species over time?

What is the evolutionary process called that leads to change through random chance?

How did Darwin’s discovery lead to a revolution in geology?

How did the study of biogeography lend support to Darwin’s theory?

Why are so many unusual species found on islands? What kind of islands specifically are they found on?

What was the major insight about the biology of islands first discovered by E.O. Wilson and Robert MacArthur?

How does island biogeography inform conservation efforts?

What are the plus and minuses of creating either one large park or several smaller ones?

Have the dynamics of island biogeography ever been tested on land?

What are some of the major causes of extinction amongst island fauna?

What can we do to protect species that live on isolated islands?

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