Sunday, March 7, 2010

Geography and Evolution

As we all learned from our ecology unit, evolution involves species variation, adaptation, and eventually evolution. However, whenever we think of evolution, we often think of it as a predator vs. prey adaption. In Jerry Coyne's book Why Evolution is True, he talks about how different geographical factors could also play in evolutionary change--especially in terms of convergent evolution (pg 92). Give examples of two different species that show similar traits despite developing in separate yet similar environments, and how this could have occurred.


  1. The Virginia opposum is an interesting exception to the general rule of thumb that marsupials are only found in Australia. Like other marsupials, the opossum gives birth to incompletely developed embryos, which then develop fully while being carried in the pouch of their mother. However, unlike most other marsupials, the opossum is found not in Australia but in North America. It would seem that the unique traits of marsupials could develop independently on two very distant continents. However, there is an explanation: continental drift.
    Continental drift was first developed in full by geologist Alfred Wegener in 1912 (Wikipedia), when Wegener observed that South America and Africa appeared as though they could fit together like a jigsaw puzzle. The development of plate tectonics in the 1960s proved continental drift, as it became clear that the continents had not always looked as they do now. In fact, when the first fossils of marsupials are found (80 million years ago, in North America), South America was actually connected to Australia via Antarctica. Clearly, over millions of years, ancient marsupials migrated southward through South America (about 40 million years ago) and Antarctica (30-40 million years ago), finally arriving in Australia about 10 million years ago (pg. 94-95). In Australia, the climate favored the characteristics of marsupials, as it did in North America and did not in South America and Antarctica (where they have since died out). Having selected a suitable body for the environment in both Australia and North America (in the opossum's case), evolution has not changed the marsupials very much over the last 10 million years. Thus, such an odd group of animals is found on two very distant continents.

  2. Two species that appear similar but really evolved similar traits in different environments are the flying phalanger and flying squirrel. Both species evolved to be tree dwellers that have flaps of skin between their arms and legs that allow them to glide from tree to tree. Both species are also nocturnal because they are not good at escaping birds that hunt during the day. While the flying phalanger evolved from a primitive rodent in Australia, the flying squirrel evolved from a primitive rodent in other parts of the world. This evolution of two species that have similar traits in different environments is called convergent evolution. The flap of skin in both the phalanger and the squirrel was a selective advantage that evolved because it was beneficial to be able to travel short distances through the air by traveling instead of having to use more energy to walk to another location.
    As Zach said, continental drift allowed similar species to evolve in different areas. The continents were initially connected allowing for the movement of animals including the primitive rodent that led to the flying phalanger and flying squirrel. The southern flying squirrels that now reside in North America came from Asia across the Bering Land Bridge 25 million years ago during the Miocene era while the northern flying squirrel came across the same bridge about 12 million years ago. Flying squirrels migrated to Central America from North America about 100,000 years ago during the Pleistocene era. The environment of North America, Central America, and Australia were well suited to the evolution of a rodent into an animal that could use its flap of skin to move from tree to tree.