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Biology 103
2000 First Web Report
On Serendip

Location and Evolution: Habitats Creating Inhabitants

Katie Gallagher

In order for a new species to evolve, there are several factors that must be present. One of the most important factors is isolation. For a new species to come about, it must be reproductively isolated from other populations with which it has bred in the past. There are several isolating mechanisms, including ecological, temporal, behavioral, and mechanical; this paper will focus on ecological. This type of isolation occurs when the creatures live and reproduce in different habitats. However, it is important to note that this mechanism alone is not a genetically-based isolating mechanism (1). Two excellent examples of ecological isolation are the Galapagos Islands and Lake Baikal.

The Galapagos Islands are known for Darwin's work there in creating his theory of the mechanisms of evolution, which concentrated especially on the diversity of finches. There are many species that are found only on these islands, such as the Lava heron (Ardeola sundevalli). It is descended from the Striated heron (Andreola striata). The Lava heron has evolved to adapt to its environment through its coloring: while the Striated heron has more distinct markings (pale neck and breast, a black cap), the Lava heron is a darkish gray all over. The Lava heron's coloring enables it to blend in with its environment and therefore hide from predators (2).

Another remarkable creature found only here is the Marine iguana (Amblyrhynchus cristatus), the only aquatic lizard in the world. The population of the Marine iguana is greater than the population of any other single creature on these islands, with a density of 4,500 lizards per mile of coast. Like many other speciations, the evolution of the Marine iguana was driven by the hunt for food. They eat mostly marine algae; back when they could not dive, they were forced to wait for low-tide. This gave them a window of approximately only two hours per day when they could eat, and food became scarce due to the size of the population, so they began to venture into the tide for algae. They progressed deeper and deeper, until eventually they evolved into the modern Marine iguana, which can dive up to 40 feet and stay underwater for an hour. Darwin observed, that "a seaman on board sank one, with a heavy weight, thinking to kill it directly; but when, an hour afterwards, he drew up the line, it was quite active." Another noteworthy result of evolution is the salt-excreting glands, located just above the nostrils. They are the most efficient glands of all reptiles because the salt intake of the Marine iguana's diet is so high. The iguanas sneeze frequently to get rid of the salt, giving the otherwise black lizards a "whitewashed" look (3) (4).

Exactly how these iguanas evolved is not certain. While it has been assumed that the Marine iguana evolved from the Land iguana, there is also some data that indicates that the two types are not related to each other any more than other iguanas. Crossbreeding does occur, but it is not known whether the offspring is fertile. If the offspring is not fertile, that would be a clear indicator that they are, in fact, quite separate species. However, the opposite is not true-there are some closely related but different species that can produce fertile offspring, such as some of the finches that Darwin studied. Molecular clock studies (tests involving the rate of amino acid replacement) have estimated that the Marine iguanas branched off from other iguanas around eight million years ago. Since the oldest island of the Galapagos is four million years old, this means that either the Marine iguana evolved on the mainland and then moved to the Galapagos, or that there are older Galapagos islands that have sunk beneath the sea. Although nothing is certain, there has been some evidence of the latter (5).

Lake Baikal is not only the world's deepest lake, reaching depths of 1,620 m., it is the oldest lake, dating back 25-30 million years ago. It contains one-fifth of all the Earth's surface freshwater supply, and has a number of islands in it. Here, in southeastern Siberia, is the home to some of the most diverse creatures in the world, the results of an isolated environment (7) (9).

Nerpa is the Russian word for "seal," and it is also the nickname of the Baikal seal (Phoca sibirica), the only one existing in freshwater. Because the nerpa has more genetic characteristics in common with the Arctic seal, it is a puzzle to scientists how it came to pass that the nerpa's habitat is a freshwater lake thousands of miles away from the sea (6). Scientists believe it is possible that during the Ice Age, the Arctic Ocean and Lake Baikal were connected by the Lena, Yenisei, and/or Angara Rivers. The seals may have traveled inland in search of food, adapted gradually to the fresh water, and have been breeding there for the past 22 million years after plate tectonic movement separated Lake Baikal from the ocean (8) (10) . During its evolution, the nerpa also became more graceful, its skin became a silvery-grey color, and it gained two liters of blood which enabled it to swim underwater for nearly 70 minutes (6).

Another unique creature is the golomyanka, or oil fish. Although these fish are the most numerous fish in Lake Baikal, they do not swim in schools. They live for the most part very far down in the lake, so far down that light does not reach there, and they have become completely blind. They are small, no bigger than 24 cm. Because their bodies are 35% vitamin-A-rich oil, they are opalescent and translucent in color. The oil also makes the fish very sensitive to the temperature of the water-they are comfortable up to +5 degrees Celsius, anything higher than that they try to avoid, and +10 degrees Celsius is fatal (6). Their body composition is over 50% fat (12), allowing them to swim freely at depths of 1,000-1,400 meters, a depth where the water pressure is so great that a cannon cannot fire. Golomyankas are also viviparous-they give birth to live young, about 3,000 fries at a time. The females often die after child-bearing, but it is possible for them to give birth two or three times, although that is extremely rare (6). It is amazing that these fish are able to exist at all-in every other deep lake in the world, hydrogen sulfide and other gases make conditions impossible for life, but thermal springs at the bottom of Baikal blanket the bottom in fresh oxygen. The hot, rising, oxygenated water is mixed through the lake by horizontal currents, creating an environment more conducive to living (11).

Although it is not the only factor, geographical location is a key part of evolution. The environment affects its inhabitants; those who can work with it survive to produce offspring. Sometimes the effect of the environment is enormous, changing body composition; other times, it just changes the color of the creature. Everything is constantly evolving... but what about humans? Location is not really a factor in today's world; people of all genealogies travel all over the globe. What does this mean? Have humans stopped evolving? Well, the important thing to remember is that location is not the only factor, so even if location were not as vital as it is in the Galapagos Islands or Lake Baikal, humans would still continue to evolve. However, I believe that location still plays a part. People are reproducing with people from different locations. For example, if one asks an American "what" he/she is, chances are that he/she will rattle off at least three nationalities. This means that our genetic pool is widening; we have genes in us that are adapted to all climes, to various diseases, to every type of environment. We are still evolving, but instead of adapting to one specific place, perhaps we are becoming genetically more flexible so that we can adapt to any place on the face of the planet.

WWW Sources

1) Observed Instances of Speciation , a little about evolution itself

2) Virtual Galápagos: Wildlife - Coastal Zone - Herons , Lava Heron

3) Virtual Galápagos: Wildlife - Marine Iguanas , Marine Iguanas

4) The Galapagos Marine Iguana , Study: Marine Iguanas

5) Galapagos Land Iguana , Land Vs. Marine Iguanas

6) Animals and fishes of the lake Baikal , Lots of species

7) About lake Baikal - details from Encyclopedia , Info about Lake Baikal

8) Nerpa , Nerpa info

9) Lake Baikal, Russia , Lake Baikal

10) Russia Experience , Lake Baikal

11) Facts about Baikal , Lake Baikal

12) Country Profile: Republic of Buryatia - Natural Wonders of the Lake Baikal , Lake Baikal




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