ALLOPATRIC AND SYMPATRIC ISOLATION

 

EVOLUTIONARY MODEL

If the evolutionary model is correct, geography matters. Mountain ranges, oceans, rivers, canyons, and islands can isolate species in a unique environment. As a population adapts to a local environment, geographical barriers can separate it from parent populations. As a result, differences between the two populations can accumulate over time and foster the development of new species, new genera, and new families. Much of earth's current and past biodiversity would have been influenced by the geography of the areas where it evolved.

CREATIONISM MODEL

If the creationists model is correct, geography doesn't matter. No matter what environmental conditions existed in North America, South America, Africa, Australia, etc. originally, the ancestral members of each kind of organism were present in the Middle East only a few thousand years ago where they interacted with the first humans. Shortly afterwards, all kinds of organisms were present in the Middle East to board a large ark and, following the destruction of virtually all life on earth, all kinds of organisms disembarked off the ark in the Middle East. From there, every type of modern organism migrated to its current habitat. Thus, the first members of each group could only adapt to the habitats of the Middle East they experienced. Given that these species then somehow were able to travel from the Middle East to North America, South America, Australia, Hawaii, Madagascar, and every inch of this planet, they must have had the ability to overcome the barriers presented by oceans, mountains, rivers, etc. There is no reason to expect that the diversity of these highly mobile Middle-Eastern organisms would be determined by the local environmental conditions of any single part of the world.

INTELLIGENT DESIGN


If intelligent design's concept of "irreducibly complexity" is applied to allopatric speciation, geography shouldn't matter much. Local geography would provide opportunities for gradual adaptations given that environmental changes can occur gradually (deserts form or continents drift over long periods of time) and that there are transitional areas between the desert and the grassland or the forest and the savanna which would provide opportunities to intermediate forms. Intelligent design claims that supernatural influence must be responsible for those processes in which gradual changes would be of no benefit to intermediate forms.

ALLOPATRIC SPECIATION

The earth is a big place. A great percentage of the modern and past biodiversity seems to have resulted from the geographic isolation of species.
Populations on islands are geographically isolated from mainland populations and can adapt to new conditions. Much of the biodiversity of islands (such as Hawaii, the Galapagos, etc.) are endemic, occurring there and nowhere else. Speciation which has occurred on islands includes the great diversity of Hawaiian birds, the lemurs of Madagascar, a quarter of the world's species of Drosophila which diversified on Hawaii, and the marine iguana of the Galapagos. Some island species increased in size such as the giant Galapagos tortoises, a meter long insectivore, or world's largest lizard, the Komodo dragon. Some island species decreased in size such as several lineages of dwarf elephant that lived in the past. Many island species lost the ability to fly such as the diverse species of moas from New Zealand, the dodo, a large flightless goose existed on New Zealand (Cnemiornis calcitrans), a flightless parrot from New Zealand (the kakapo), and a flightless comorant from the Galapagos.

Allopatric isolation is the key to speciation in many species of birds, especially island species(Grant, 1997). In New Guinea, the Papuan kingfisher populations of islands around New Guinea are more varied than those in the diverse habitats of New Guinea itself (Raven, 2002). Darwins finches include six species feed on the ground primarily on seeds (although some have a longer bill which allows them to also feed on cactus flowers), four species of insect-eating tree finches, one tree finch which uses twigs as tools to probe for insects, a warbler finch which possesses a very thin beak and preys on insects on vegetation, and a vegetarian finch with a thick bill to pull buds from branches (Raven, 2002).

Outside the oceans, the greatest diversity of freshwater fish in the world occur in the Neotropics, where about 8,000 species reside. Many species are endemic to specific areas and their distribution is best explained in light of the geological events of the Cenozoic involving the uplifts of tectonic plates and the resulting changes in drainage basins (Hardman, 2006).

Estimates of the number of modern cichlid species vary from 1400 to 1700. The speciation of cichlids correlates with the breakup of the former southern continent of Gondwana and the water systems created by tectonic activity on individual southern continents. From Africa, one lineage traveled to South America and was the source of the monophyletic Neotropical clade (Murray, 2001). Central American cichlids were colonized from South America and some South American cichlids belong to the clade which colonized Mesoamerica. One tribe (Heroini) diversified in Mesoamerica where they can compose a quarter of fish species (Concheiro Perez, 2007; Martin, 1998; Husley, 2004). During the Miocene, tectonic uplift in East Africa changed water drainage patterns and created the great lakes which were the centers of cichlid diversity (Murray, 2001). Most African cihlids are thought to have arisen from the paleo-basin of Lake Tanganyika (including the extinct Lake palaeo-Makgadikgadi) and the greatest diversity of modern cichlids is endemic to East Africa (Katongo, 2007). More than a thousand species of cichlid fishes inhabit the three African Great Lakes, Tanganyika, Malawi and Victoria (Watanabe, 2007).

Many catfish species diversified as plate tectonics changed the waterways and the geography of the southern continents. When the Indian subcontinent collided with Eurasia 50 to 45 million years ago, it began the formation of the Tibetan plateau, the largest and highest plateau on Earth (5 million square kilometers and an average height of 5 km). The resulting geologic changes allowed for the speciation of diverse fish lineages endemic to the area, such as the glyptosternoid catfishes (Peng, 2006). More than 50 fish species are endemic to the Maracaibo basin of northwestern Venezuela which was formed by a rising section of the Andes 8 to 10 million years ago (Hardman, 2006). As the Death Valley region dried, the few remaining springs were colonized by past species that adapted to that spring. Many fish (such as pupfish) have evolved into separate species whose distribution may be limited to one spring only (Campbell, 2003).

Reptiles and amphibians thrive in the tropical regions of the world but mountain ranges, rivers, and oceans often form geographic barriers that they cannot cross. The salamander species Ensantina escholotzii is a ring species which has produced a number of populations in different sites around the mountains that form a barrier for it in California. These populations vary and are in the process of becoming separate species (Campbell, 2003). In areas of the American southwest, populations of species of lizards, rodents, and insects that inhabit dark rock formations have darker colors than populations which inhabit the sand of the surrounding desert (Raven, 2002). About half of all modern amphibian species inhabit tropical regions in the Western hemisphere. More than 800 species of frog are classified as eleutherodactyline frogs which undergo direct development. This group originated in South America in the early Cenozoic. Much of the subsequent speciation in South America occurred while the Andes uplift was altering habitats. Additional speciation occurred as ancestral species traveled over water to reach Central America and the West Indes (Heincke, 2007).

Many birds are endemic to specific areas. New World vultures and Old World vultures arose from unrelated groups of ancestral birds in different parts of the world. Virtually all suboscines occur in North and South America and the greatest number live in the tropics. Fossil and genetic evidence suggests that passerine birds evolved on the southern supercontinent of Gondwana. (Slack, 2007, Edwards, 2002).

Oscine passiformes are the predominant group of birds in the Northern Hemisphere. Oscine passiform groups form a nested hierarchy of related families. Birds in related families can be quite similar to each other, such as the Old World and New World warblers.


Geographic barriers can separate populations, such as the antelope squirrels on opposite sides of the grand canyon (Campbell, 2003).

Many mammals species evolved in isolated areas and were able to diversify and migrate after their lineage had been established (such as horses and camels from North America or elephants and hominids from Africa). Obviously, the unique marsupial fauna of Australia owes much of its diversity to the isolation from competition from placental animals. Madagascar ’s natural habitats experience extremes in precipitation levels which leads to unpredictable patterns of flowering and fruiting. Selection pressure has resulted in wildlife that have evolved ways of tolerating these extremes (Dewar, 2007).


South America was isolated from North America from the Late Cretaceous to the Late Pliocene, about 3 million years ago. The South American fauna was unique and many of the primitive groups of mammals survived there without pressures from advanced placental predators. When the isthmus of Panama formed to fuse the two American continents 3 million years ago, the effect on the South American fauna was enormous. The North American immigrants were very successful and diversified into new species although not all of the migrating species survived to modern times (such as a few three-toed horses, bears, and elephants). The South American ungulates and terror birds became extinct as did many of the edentates and marsupials, in part because of the advanced placental predators. More than half of the modern South American mammalian genera are descended from the North American species of this interchange.


Even throughout the fossil record, speciation was fostered by geographic isolation. Different groups of predatory dinosaurs thrived in the northern and southern continents. Of the largest dinosaurs, T. rex is known from North America, Gigantosaurus from South America, and Carcharodontosaurus from Northern Africa. North and South American sauropods diversified.


SYMPATRIC SPECIATION
If evolution is true, then natural mechanisms can allow the adaptation of populations to an environment, reproductively isolate populations, and foster speciation.
If creationism is correct, natural mechanisms are incapable of producing new species of life or beneficial adaptations to local environments.
Since speciation and adaptation can result in considerable complexity, intelligent design would argue against any natural mechanisms for speciation or adaptation.
There are small isolated lakes which contain species of closely related fish that are thought to have arisen sympatrically given that the small size and uniform environments of the lakes would not allow allopatric speciation. For example, the cichlid species of two very small Nicaraguan volcanic-crater lakes or the related fish species in small glacial lakes in Iceland seem to represent recent lineages which diverged sympatrically (Coyne, 2007).
Some islands possess plant species which seem to have diverged from a recent common ancestor, such as the palm trees of the small Pacific island, Lord Howe Island (Coyne, 2007).


In birds, it is thought that behavioral changes (such as differences in song between populations) preceded genetic changes which decrease the viability or fertility of hybrids. In birds, courtship song is a premating isolation mechanism which can reproductively isolate incipient species. In many species of birds, some aspects of song are imprinted early in life and thus are determined by an environmental component (Grant, 1997; Balakrishnan, 2006). In indigobird species which can only be distinguished by song, males responded most vigorously to males which the same songs as a sign of sympatric isolation (Balakrishnan, 2006).


HOST SPECIALIZATION
Given that much of the specialization of parasites is an adaptation to a specific host, it is thought that host-switching is an important factor in the speciation of parasites (Huyse, 2007). For example, the fly Rhagoletis pomonella has separated into two populations: one in which the larvae feed on hawthorns and a second in which larvae feed on apples, which were introduced to North America several centuries ago. These populations are reproductively isolated and have begun to accumulate variations which adapt them to the different host plants (Feder, 1997).


The apple maggot fly Rhagoletis pomonella seems to have split into U.S. and Mexican races about 1.5 million years ago. These races began to accumulate chromosomal inversions which included genes which affected larval diapause. This separation adapted the U.S. populations to make a host switch to apples when apples were later introduced. Thus, the speciation of Rhagoletis pomonella seems to have occurred through sympatric separation of populations which had already developed different allopatric distribution patterns (Feder, 2003).