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If we look at the Continents, we can make the following observations:
In the Americas we find Cacti in the deserts. These plants are perfectly adapted for life in the desert. Cacti are a type of plant referred to as Succulents. To survive in the desert succulents must be tough, leafless and have a fat stem to store water.
In the deserts of Africa, Australia and Asia, although similar in climate, there are no native cacti. Their place is taken by a different order of succulents called Euphorbs, which are also adapted to fill the same desert niche even though they are completely unrelated to cacti.
Why should two sets of deserts have plants that resemble each other but are unrelated? Creationists have argued that deserts may look similar but, in fact, may have subtle differences that don't allow (say) America's cacti to grow in Asian deserts. This cannot be correct because when Cacti are transferred to an Asian desert they thrive.
A better explanation is provided by the Theory of Evolution.
Animals and plants in similar habitats will be subject to similar pressures. Natural Selection will work in the same way to produce features that allow the organism to survive better. For desert plants this means small or no leaves and a fat stem for water retention.
This process is called Convergent Evolution.
For the deserts of the Americas, the cactus family have filled that niche while for the deserts of Africa, Asia and Australia a different set of plants has evolved to fill the desert niche.
There are more examples of Convergent Evolution.
The Toucans of the Americas fill the same niche as the Hornbills of Africa. Both have enormous and colourful beaks, but are unrelated to each other.
Each continent has a large flightless bird: the Rhea in the Americas and the Ostrich in Africa. If we include Australia we can mention two more large flightless birds: the Emu and the Cassawary.
Australia itself (including nearby islands) has a strange collection of wildlife: the Monotremes are two species of egg-laying mammals, the Platypus and the Echidna. The pouched mammals, the Marsupials (Kangaroos, etc) occur only in Australia and in isolated forests of South America.
Australia is missing many major groups found in the rest of the world: large carnivorous cats, bears and foxes (dingoes arrived with the first humans), even-toed ungulates (like pigs, camels), shrews, squirrels and rabbits (introduced disastrously by Europeans). In Australia, Placental Mammals which dominate the other continents are mainly absent.
Convergent Evolution means that the Marsupials of Australia have their Placental counterparts in other continents even though the two types of animal reproduce in a very different way. Marsupials have two vaginas and a double uterus. Placental mammals have a single uterus and nourish their young with the placenta. The following are examples of very similar looking animals, one of which is Placental, the other Marsupial.
On different continents, we can also see divergence between similar animals as well as complete differences in the wildlife present.
The Americas have a solitary (not living in a group) carnivorous cat called the Jaguar. The equivalent in Africa is the Leopard. In Asia it is the Tiger. These are three members of the cat family that have developed apart from each other and have evolved slight changes to each other.
There are no apes in the Americas. Instead we find New World Monkeys ((super family Ceboidea). These have prehensile tails that can grab things and hang on to tree branches. Africa and Asia have Apes and Old World Monkeys (Cercopithecidae). These monkeys have tails that are used for balance only.
There is not a single species of either of these two distinct type of monkey that is found in the "wrong" continent.
Sloths are unique to the Americas - nothing like it exists in Asia or Africa. Africa and Asia have Elephants which are unknown in the Americas.
The Northern Polar Regions contain Polar Bears and Reindeer - both completely unknown in the Southern Polar Regions (with their similar climates). Penguins are found in the Antarctic regions but are never seen in the Arctic.
The distribution of life on the continents can be perfectly understood by using the Theory of Evolution and the Theory of Plate Tectonics.
Plate Tectonics describes how continents move over periods of millions of years. About 300 million years ago, all the continent were joined together. This allowed plants and animals to move easily between them. Since then continents have moved apart. This isolation has allowed evolution to go in slightly different ways. It has allowed different plants and animals to fill the same niches even though they may be unrelated (for example the Flying Squirrel and Sugar Glider). It also allows similar animals to evolve differently (like the Jaguar and Leopard).
When continents rejoin or land bridges form (Africa and Eurasia early on and the two Americas more recently), that affects the way evolution acts as it allows mixing and migration.
The joining and subsequent isolation between the Americas and Africa (or Asia) has meant that the two types of monkeys have developed differently in the two regions. The prehensile tail developed after the split and became confined to the Americas. If all monkeys were created separately, this does not make sense. This continental isolation also explains why jaguars and leopards have diverged.
Animals that are a relatively recent development have appeared only on a single continent (like the sloth in the Americas). Polar bears only appear in the northern polar regions because they have evolved from northern hemisphere brown bears - in fact the two can still interbreed. Penguins, on the other hand, have evolved from sea birds living in the southern hemisphere.
Australia has been completely isolated from the rest of the world for 40 million years and this has lead to its rather unique flora and fauna. While Placental Mammals thrived and took over on the other continents, the earlier egg-laying and pouched mammals thrived in isolation on the Australian continent.
It is instructive to look at the history of Camelids. This has been worked out using fossil records and DNA studies.
Camelids first appeared in North America about 25 million years ago. A land bridge allowed a few of these animals to cross over into Asia via Siberia about 8 million years ago. Two of their descendants survive today as the Bactrian Camel (two humps) in Central Asia and the Dromedary Camel (single hump) in India, Arabia and Africa.
Three million years ago another land bridge allowed the Camelids to migrate into South America. Their four modern descendants are the domesticated Llama and Alpaca and the wild Vicuna and Guanaco.
In the last few thousand years the North America Camelids became extinct, possibly due to over hunting by early humans or climate change during the ice ages.
The modern world has six Camelids; two larger species in Africa and Asia and four smaller versions in South America.
One of the biggest successes of the Theory of Evolution is its explanation of the unusual wildlife patterns found on islands. Before we can look at this we must first distinguish between two types of islands.
Continental Islands are islands that have recently been joined to the continents. They have since separated because of the flooding of land bridges or the movement of continental plates.
Examples include the British Isles (last joined to Europe about 300,000 years ago), Japan, Sri Lanka, Tasmania, New Guinea, New Zealand (joined to Australia 85 million years ago) and Madagascar (joined to Africa 160 million years ago).
The wildlife on these islands is generally very similar to the wildlife on the nearest continent, especially if the connection was recent. New Zealand and Madagascar were connected to continents in the more distant past and behave slightly differently, as we will see shortly.
Oceanic Islands are islands that have never been connected to any continent. These islands have arisen as a result of volcanic action below the ocean. When these islands first appeared they were covered with lava and completely barren.
Examples of these include the Hawaiian Islands, the Galapagos Islands, St Helena and the Juan Fernandez Islands.
When we look at the wildlife and plants on Oceanic Islands, we find some interesting peculiarities.
Oceanic Islands are populated with Insects and Birds but only a few groups. These few groups occur in many species.
Examples are the 15 species of Finches in the Galapagos Islands, 40-odd species of Honeycreepers and over 800 species of Fruit Flies (half the world's total) on Hawaii, and many species of Wood Weevils on St Helena which lacks many groups of insects.
There are plants that elsewhere in the world would be small and weed-like but on Oceanic Islands occur in many species and grow tree-like. The Sunflowers on the Juan Fernandez Archipelago and trees from the Daisy family on St Helena are two of the best examples.
Oceanic Islands have no native Land Mammals, Where there are mammals they will be marine or flying mammals. Examples include Sea Lions and Seals on the Galapagos Islands. Mammals dominate the continents but are essentially completely absent from Oceanic Islands. Why is this?
Islands like Hawaii, St Helena and Juan Fernandez have no native Reptiles. The Galapagos Islands have a few reptiles and what exists occurs in many species, like the Giant Tortoises which differ on each island. Why should such a long-lived and successful class of animals be mainly missing from most of the Oceanic Islands?
Even more interesting are the groups of plants and animals that are completely missing from all Oceanic Islands.
There are no native Fresh Water Fish and no native Amphibians.
Another interesting feature is that the species present on Oceanic Islands are usually closely related to similar species found on the closest mainland. The Finches and Iguanas on the Galapagos Islands have closely related versions on the nearby South American mainland.
In addition there are many unique species found on Oceanic Islands that are found nowhere else. These are called Endemic Species. The (now extinct) Dodo on Mauritius, Flightless Cormorants and the unique Marine Iguana on the Galapagos Islands and a large 13cm hummingbird (the Firecrown) on the Juan Fernandez Islands.
Many of these unique species are (or were) very tame and unafraid of early human arrivals probably due to the lack of predators. Lack of predators also explains the large number of flightless birds on Oceanic Islands.
Continental Islands that have been cut off from continents for a long time show a typical continental distribution of organisms but also show unique and endemic species. Their wildlife distribution shares features of Continents and features of Oceanic Islands.
The 90 species of Lemurs (primitive primates) found only on the island of Madagascar are the most famous example.
Although Australia is, strictly speaking, a continent its isolation has given it many endemic species like Egg-Laying Mammals and the Marsupials.
New Zealand has few mammals but does have two species of endemic Bat. It also has endemic flightless birds (Kiwis and the extinct Moa which had dozens of species).
All of the above observations can be explained by the Theory of Evolution.
The once-barren Ocean Islands can only be reached by species that can either fly, float, swim or be carried across large areas of ocean.
This is a relatively rare phenomenon and so only a few animals and plants make the journey. That is why there are so few orders of native plants, birds and insects on Oceanic Islands.
The most likely species to make the journey are ones from nearby continents or occasionally along the path of a storm. This explains why many island species are related to ones from nearby continents. Insects can fly or be blown to the islands. Plant seeds can be carried by wind, in the feathers of birds or in their guts.
Tree seeds are generally too heavy to make the journey by air and are usually killed by salt water. The one exception is the coconut which can float and survive on sea water. That explains the ubiquitous palm trees covering islands in the Pacific and Indian Oceans.
Many plant seeds found an environment with no trees to compete with them and went on to develop hard trunks and became tree-like themselves. The trees on St Helena are plants of the Daisy family.
If Oceanic Islands are reached by only a few birds or a few insects or plant seeds, they will find a large number of empty niches so they will specialise in different environments. This will eventually lead to a single species diversifying into many closely related species. This is called speciation (or species radiation). It explains the Finches of the Galapagos Islands and the Honeycreepers of Hawaii.
There is evidence that this type of dispersal is possible. Caribbean seeds have turned up in Scotland carried by the Gulf Stream. Insects have been collected on ships and aeroplanes far from land over the ocean. A bird called the Purple Gallinule, which was an occasional visitor to the Tristran da Cunha Islands in the Atlantic Ocean, began to breed there in the 1950s. North American Douglas Fir has arrived in Hawaii and, remarkably, South American logs have ended up in Tasmania.
The volcanic island of Krakatoa was destroyed by a volcanic eruption in 1883. Within a hundred years, it was once more a forested island with many species of birds and insects.
Amphibians and Fresh Water Fish cannot cross large areas of ocean as sea water kills them. Hence no Amphibians or Fresh Water Fish are indigenous to these islands.
Mammals cannot swim great distances and cannot cross the ocean unless they are marine or they can fly.
Creationists have explained the absence of Amphibians and Mammals on Oceanic Islands by postulating that Amphibians and Mammals cannot live on the Oceanic Islands because the environment is not right for them, hence they were not created onto these islands.
This is not a viable answer because when humans have introduced these animals onto Oceanic Islands, they often thrive to the detriment of the local wildlife. Cats and Goats introduced to the Galapagos Islands have decimated the local wildlife which had never evolved a defence against predators. In Hawaii, the Cane Toad, a poisonous Amphibian, was introduced in 1932 and thrives so well it has become a pest.
Reptiles can float on rafts during storms. They don't need to eat or drink as much as birds or mammals so can survive a journey of weeks. This explains the presence of reptiles on some Oceanic Islands. An example occurred in 1995 in the Caribbean during a hurricane when a raft containing a group of fifteen Green Iguanas was carried 300km to the island of Anguilla where they did not formerly live.
The lack of predators makes animals tame. It also allows them to grow large, hence the Giant Tortoises of the Galapagos and Mauritius. Many birds lose the ability to fly on islands without carnivorous cats or bears. This explains unusual species like the Flightless Cormorants, the Kiwi, the Dodo and the Moa.
Incidentally, the wildlife distribution of Oceanic islands is shared by other isolated habitats, like Lake Baikal in Siberia or the Tepuis (table mountains) of South America. They too will have many similar species but few animal groups.
The study of Oceanic Islands provide one of the best verifications of the Theory of Evolution.
The native wildlife found on these islands makes perfect sense if these islands were "seeded" by life arriving from elsewhere and diversifying into new species. Only those plants and animals that can survive a long sea crossing are represented. Many viable life forms are missing because the journey was not possible for them.
Birds and insects found many unoccupied niches and diversified into many species to exploit them giving rise to a few groups with many species. Small weed-like plants used the lack of trees to develop hard trunks and become tree-like. Isolation has led to unique species, found nowhere else.
The lack of predators made much of the wildlife over-tame and many birds lost their ability to fly. These adaptation have lead to the demise of many species when humans and their attendee animals and plants arrived.
The Theory of Evolution explains the biogeography of Oceanic Islands in a consistent and logic way. If all species were created individually, the distribution makes little sense.
Biologist Jerry A Coyne put it like this:
"The biogeographic evidence for evolution is now so powerful that I have never seen a creationist book, article or lecture that has tried to refute it. Creationists simply pretend that the evidence doesn't exist."
Introduction to Biogeography.
Plate Tectonics and the moving continents.
The Galapagos Islands, explored by Charles Darwin.