Essay 3 10/13/2003: Evolutionary Biology, Geology

Implicating Continental Drift in Speciation

The rearrangement of land masses over geologic time has helped to create biological
diversity on our planet. Without the profound effects created by geological
reconfiguration life on earth might have been very different from the way that it is now.

Continental drift is the movement of land masses due to the effects of plate tectonics.
Originally all of the world’s surface land was located in one region on the globe,
Pangea. This supercontinent supposedly began to separate late in the Triassic Period
(245 to 208 million years ago) into a southern landmass, Gondwanaland, and a northern
landmass Laurasia.  Gondwanaland was composed of the modern day continents of
India, Africa, Australia, Antarctica and South America. These continents began to break
up and head towards their present day locations only some 130 million years ago.
Because these two land masses were the only two continents on the face of the earth
for about 520 million years they are perhaps two of the most important geological
structures of the last billion years. Our modern understanding of continental drift, and the
structures that it has produced, has inexorably impacted our understanding of the fossil
record and has given us detailed information about the evolutionary history of animal

The earth is populated by tens, perhaps hundreds of millions of animal species. We can
attribute this fantastic diversity, in part, to an evolutionary trend called speciation.
Speciation is a phenomenon that normally takes place when a group of animals of the
same species find themselves isolated from one another. They can be isolated
geographically by great distances, rising mountains or large bodies of water.They can
also be isolated by biological or behavioral barriers.

One organism is distinguished from another if they are unable to create viable offspring
together- this is the definition of animal species. Once a group of animals of the same
species becomes split apart or isolated, they begin to be changed, molded and
fashioned by the hand of natural selection to more properly fit in with their surroundings.
After a period of time these two groups begin to be so different anatomically and
genetically that soon it becomes impossible for them to procreate. This inability for two
animals, that were once the same species, to create viable offspring is called

Before 130 million B.C. the landmass of Gondwanaland was the home to many types of
mammals. Thanks to plate tectonics it was split in two to create modern day South
America and Australia. Many mammals called Gondwanaland their home and after it
split, two very different types of mammals emerged. One order of mammal began to
both thrive in Australia and die out in South America. These were marsupials, or
pouched mammals, and they evolved into modern day kangaroos, koalas, wallabies,
wombats and tasmanian devils. The only type of marsupial species able to survive in
the South American continent after the break up of Gondwanaland was the opossum.
On the other hand, on the isolated continent of Australia, almost all of the indigenous
mammals are in fact marsupials.

Very soon placential mammals, (the order that we belong to) and marsupials were on
opposite sides of the globe. Today there are no marsupials that can mate with
placential mammals at all. Their genetic and anatomical differences created
unsurmountable barriers, making them sexually incompatible. The story of the order
marsupialia is perhaps one of the most dramatic demonstrations of the effects of
continental drift on speciation.

The different types of primates with whom humans were able to mate have long since
died out. At one time we were very similar anatomically and genetically to some of the
other species of primates on the planet. It is thought that humans and Neanderthals
were sexually compatible, meaning that functionally we were not two different species.
Because of behavioral differences, probably due to social and intellectual
discrepancies, we simply did not mate with their kind very much at all.  DNA tests reveal
that there are very little or no Neanderthal genes in modern human populations.

Animal speciation can be caused by geographical barriers, sexual barriers, behavioral
barriers -and this example makes it obvious that social barriers can prove causal as
well. If some of these barriers had not existed, and if our geographical configurations
were only slightly different then humans may never have evolved the way that they did, or
may never have evolved at all.

Evolution: noun
Change in the genetic composition of a population during successive generations. The
development of species resulting from the way natural selection acts on the genetic
variation among individuals.

Gondwanaland: noun
The hypothetical protocontinent of the Southern Hemisphere that, according to the
theory of plate tectonics, broke up into India, Australia, Antarctica, Africa, and South

Neanderthal: noun
An extinct human species (Homo neanderthalensis) or subspecies (Homo sapiens
neanderthalensis) living during the late Pleistocene Epoch throughout most of Europe
and parts of Asia and northern Africa and associated with Middle Paleolithic tools.

Order: noun
Biology. A taxonomic category of organisms ranking above a family and below a class.

Pangea: noun
A hypothetical supercontinent that included all the landmasses of the earth.  Before the
Triassic Period Pangaea broke up into Laurasia and Gondwanaland.

Speciation: noun
The evolutionary formation of new biological species, usually by the division of a single
species into two or more genetically distinct ones.

Triassic Period: noun
The period from 190 million to 230 million years ago. Marked by dinosaurs, marine
reptiles; volcanic activity.
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