Geographic Distributions are Influenced by the Movement of Continents
Approximately 15 billion years ago all the matter and energy in the universe was consolidated in a tiny region smaller than a dime. Then, according to the "big bang" model, there was a violent explosion and the super-heated matter and energy began to rapidly expand outwards. The sun and its planets are thought to have formed about 4.6 billion years ago from a cloud of cosmic dust and gas. Most of this material condensed into a single compact mass, the sun. Within the remainder of the dust and gas cloud, lesser centers began to form. These became the planets, of which the earth is one.
As the materials that formed the earth came together, a stratification of its components took place, with heavier materials, such as iron and nickel, moving toward the center and lighter substances becoming more concentrated nearer the surface. As time passed, the components became further stratified into three distinct regions, with the dense iron and nickel accumulating in the center to form the core, the less dense silicates of iron and magnesium forming a partly molten mantle surrounding the core, and the lighter substances remaining near the surface (Fig. 1). As the surface of the earth cooled, the surface materials, composed primarily of the lighter silicates, solidified to form a crust. This crust is quite thin compared to the diameter of the earth—about the thickness of an eggshell compared to the diameter of an egg. As the crust solidified, it wrinkled and compacted, much like the skin on pudding, and formed massive plates resting on the semi-molten mantle.
Fig. 1. Interior of the earth. The earth consists of three zones that vary in chemical composition: the core, mantle, and crust. The core consists mostly of iron, with some nickel. The inner core is solid whereas the outer core is liquid (i.e., molten). The mantle is the largest zone; it is composed primarily of iron and magnesium silicates. The upper layer of the mantle is solid and rigid while the inner layer is partly molten and plastic. The crust forms a thin skin over the earth’s surface. It is differentiated into the oceanic crust, under the sea floor, and the thicker continental crust. The crust varies in composition. The thickness of the crust is exaggerated in this diagram.
The Earth’s Continents Are Moving
For many years both geologists and biologists assumed that the distribution of earth’s major land masses existed as it does now throughout the history of life. That assumption has been proven false by geologists as evidence began to accumulate during the 1960s that these land masses (or plates, as geologists call them) have changed positions nearly continuously throughout the earth’s history. According to the theory of continental drift, the earth’s surface is divided up into a dozen or so massive, rigid plates that rest on the earth’s more fluid mantle. Some of the plates, the oceanic plates, are under the ocean, and the others, which are much thicker and heavier, are the continental plates. Convection currents running along the top of the mantle, created by the upwelling of new crust in some places and the sinking of old crust in others, move the plates at roughly 2.5 centimeters per year (Fig. 2). Baja California, for instance, is drifting quite rapidly––it is moving at the same rate in which your fingernails are growing! Collisions between plates result in the deformation of the earth’s crust and the uplift of mountains, and cause severe earthquake and volcanic activity as well. Lateral slip between the plates also gives rise to earthquakes, as we see along the western coasts of North and South America (Fig. 3).
Fig. 2. Movement and deformation of the earth’s outer layers. The crust and upper portion of the mantle are solid and rest upon a partly molten, semisolid layer of mantle. The solid portion is broken up into smaller, rigid plates that can move about over the semisolid and plastic region. When the plates move apart (left) in the process known as sea floor spreading, the magma wells up into the gap from below and solidifies, creating new sea floor. The sea floor plate moves as if it were on a conveyer belt, and when it meets a continental plate moving toward it (right), the sea floor plate is pushed under, forming a subduction zone. The old sea floor is remelted in the subduction zone. Areas above the subduction zone may be pushed upwards to form mountains and are prone to earthquakes and volcanic activities.
Fig. 3. San Andreas fault in California. The fault marks the junction between two different plates that are moving side by side in opposite directions. Earthquakes occur when the plates scrape past one another.
Evidence suggests that about 300 million years ago all the continents had drifted together to form a single massive supercontinent, called Pangaea (Fig. 4). Approximately 200 million years ago Pangaea began breaking apart due to continental drift. The first major break was an east-west one, separating a northern supercontinent called Laurasia (composed of the land masses that would later become North America, Greenland, and Eurasia minus India) from a southern supercontinent called Gondwanaland (composed of the future South America, Africa, Madagascar, India, Antarctica, and Australia). Soon thereafter Gondwanaland began to break up, and the land masses began to move slowly apart; India drifted off to the north, and an African-South American mass separated from an Antarctic-Australian mass. The drifting apart of the plates is the sort of event that could cause speciation.
By about 65 million years ago, South America had split from Africa and was drifting westward; India had moved northward, but had not yet collided with the rest of Asia. By 40 million years ago Australia had split from Antarctica and had begun drifting northeastward into the warmer regions of the southwest Pacific. The division of Laurasia into North America and Eurasia was one of the last major continental changes to take place, occurring about 25 million years ago (though the continents remained connected at times by a land bridge). About 20 million years ago India collided with Asia, giving rise to severe earthquakes in the surrounding area and pushing up the Himalayas and the Tibetan plateau.
Geographic Distributions are Influenced by Climate
As continents moved over the course of millions of years, and their distances from the earth’s poles and the equator changed, their climates must have undergone major shifts. India, for example, moved from a position next to Antarctica all the way across the equator to its present location in the tropics of the northern hemisphere. Australia, too, moved steadily northward. But climatic changes due to causes other than continental drift have also occurred during the history of earth. Thus Antarctica, though probably always near the South Pole, has not always been the bleak, ice-covered land it is today. Indeed, fossils of temperate and tropical amphibians and reptiles have been found there. During at least part of the Mesozoic, Antarctica must have been reasonably warm, and it was probably warm again about 50 million years ago, when tropical and subtropical climates were far more widespread on the earth than they are today. By contrast, the earth was much colder only a few thousand years ago, during periods of extensive glaciation. Thus, both the past configurations of the land masses and their past climates are important in explaining the distributions of organisms on the earth.
Europe, Asia and Africa are considered an essentially continuous land mass, but you may wonder why North America is also considered to be continuous with Eurasia. The answer is that North America and Eurasia have been connected through much of their geological history. Even after they broke apart as the northern part of the Atlantic Ocean formed, a new connection, the Siberian land bridge between what are now Alaska and Siberia, provided a link. Part of this bridge is beneath water at the present time, and the continents are separated by about 90 kilometers of water. Because the climate of Alaska and Siberia is so forbidding, you might not expect many organisms to use a bridge in that region. But once again present conditions are misleading. Fossils of many temperate and even subtropical species of plants and animals are abundant in Alaska. Indeed, all the fossil evidence points to much movement, on many occasions, between Asia and North America via the Siberian land bridge. Humans crossed this bridge from Siberia into Alaska about 50,000 years ago.
Other land bridges, such as the North Sea bridge to Britain, the New Guinea-Australia bridge, and the Sunda bridge between Asia and Indonesia, have greatly influenced the present distribution of plants and animals on earth। But what caused these bridges to form and then sink? Geologic evidence shows that during the last few million years the earth has undergone periodic transformations, alternating between “greenhouse” and “icehouse” climates. About 30,000 years ago, during the last glacial age, ice sheets covered about 29 percent of the earth’s present land area; by contrast, only about 10 percent is covered today. The moisture needed to form and sustain these ice sheets comes ultimately from the oceans, and when the sheets form there is a substantial lowering of sea level on a worldwide basis. During the last glacial age, for instance, it is estimated that the world sea level was lowered about 100 meters, which was sufficient to join France to Britain, establish the Siberian land bridge, and connect Indonesia to Asia. Plants and animals were able to migrate freely between these areas. As the climate warmed, the glaciers melted and the sea level rose, once again covering the bridges. And although the last major ice age ended about 11,000 years ago, the climate has been far from stable. Research in different parts of the globe suggest that the most recent cooling (the Little Ice Age) took place between the 15th to the 19th century. During this period the climate in Europe was considerably cooler with unusually harsh winters, and the mountain glaciers in the Alps, Alaska, and New Zealand enlarged and advanced. Several other periods of minor cooling have taken place in the last 11,000 years, and the evidence suggests that the climate follows a rough cycle, swinging from cold to warm and back again about every 2,600 years. Geologists suggest that at the present time we are approaching the period of maximum warmth in the glacial-interglacial cycle, after which the climate will begin to cool and the earth will enter another glacial age.
Fig. 4. Pattern and timing of continental drift and the break-up of Pangaea.
The horizontal movements of surface water in the ocean, called currents, often have a profound influence on climate, and therefore on distribution. Like the prevailing winds, ocean currents follow a distinct pattern. Currents usually flow parallel to the coast along the edge of continents, with warm currents generally moving from the equator to higher latitudes along the eastern margins of continents, while cold water from higher latitudes flows toward the equator along the western continental margins (Fig. 5). The warm and cold currents have marked effects on temperatures of nearby land masses. For instance, the North Atlantic Drift, a northern continuation of the warm Gulf Stream, keeps Great Britain and northeastern Europe much warmer than their northern location would dictate: the average January temperature in London, England, which is at 51 degrees north latitude, is 4.5°C higher than that of New York City, which is 11 degrees farther south. And cold ocean currents flowing along the west coasts of Chile and Peru convert the tropical deserts in this region to relatively cool, damp, foggy areas.
Fig. 5. Surface ocean currents. The ocean currents (horizontal movements of surface water in the ocean) often have a profound influence on the climate of the nearby land masses. Currents usually flow parallel to the coast along the edge of continents, with warm currents (red) generally moving from the equator to higher latitudes along the eastern margins of continents, while cold water (blue) from higher latitudes flows toward the equator along the western continental margins.
The history of the major land masses and of their changing climates helps explain the present distribution patterns of organisms. A species can be found in an area if (1) it evolved there, or (2) it moved into the area. Consider the distribution of a group of organisms that occurs in South America, Africa, and southern Asia. If the group of animals is of very ancient origin (e.g., cockroaches), this distribution may indicate that the species occurred throughout the old Gondwanaland supercontinent and continued to survive in South America, Africa, and India after they drifted apart. Species such as these that exist in two or more distinct areas now separated by a barrier but lived in the entire area before the barrier was imposed are said to have a vicariant distribution. But if the group is more recently evolved (e.g., the majority of modern mammalian and bird families), which arose after Gondwanaland had broken up, it may be assumed that the species moved between the New World and the Old World via either the North Atlantic or the Siberian land bridge and between North and South America via Central America, and that it then became extinct in the north, either because of climatic changes or because of intense competition. Species that exist in different areas because they crossed a barrier are said to have a dispersal distribution. The fossil record shows that this pattern of dispersal between southern regions by way of the northern continents has occurred again and again. For example, members of the camel family occur today in South America (llamas, alpacas, vicunas, etc.), northern Africa, and central Asia, but fossils indicate that the family originated in North America, spread to South America via Central America and to the Old World via Siberia, and later became extinct in North America Horses followed a similar pattern (Fig. 6). To use another example, the marsupials of Australia are thought to have arisen in North America and migrated to Australia some 70 million years ago by way of land bridges from South America and across Antarctica to Australia (arrow, Fig. 7). Because the marsupials reached Australia long ago, and because the continent was then isolated from all others, they encountered no competition from placental mammals. This permitted marsupials to undergo extensive adaptive radiation. Because they were filling niches similar to those filled in the rest of the world by placentals, and were thus subject to similar selection pressures, they evolved striking convergent similarities to the placentals. Certain of the marsupials resemble placental shrews, others placental jumping mice, weasels, wolverines, wolves, anteaters, moles, rats, flying squirrels, groundhogs, bears, etc.
Fig. 6. Horses have a complex evolutionary history. The earliest horses evolved in North America; many lineages arose and died out, and ancestors of several of these lineages crossed into Asia over the Bering land bridge and into South America over the Central America land bridge.
Fig. 7. The marsupial trail. The marsupials of Australia are thought to have migrated from South America and across Antarctica to Australia (arrow) by way of land bridges, some 70 million years ago. Because the marsupials reached Australia long ago, and because the continent then became isolated from all others, the marsupials encountered no competition from placental mammals and underwent extensive adaptive radiation.
Anticline Syncline
