Evolution of Freshwater Fishes
Ardan Huck July 6, 2002

The earth is about 4.5 billion years old! This is known through the study of geology, rocks, fossils, layers of the earth, and radiometric dating.

Evolution is the process by which organisms change their genes and pass the changes on to the next generation. This is different than just "adapting" to change. The mechanisms causing change in the genes include:
A. Natural Selection: This has to do with the amount of offspring that parents produce, how fit that offspring is to survive, availability of food, and the "sexual attractiveness of the species."

B. Genetic Drift: The change in genes due to chance alone.

C. Mutations of the genes: When the genes are copied to offspring, mistakes can happen.

D. DNA Recombination: Recombining different DNA from the mother and the father results in different offspring.

E. Gene Flow: This is when a new organism migrates from one population to another and cross-breeding occurs.
It is theorized that when Earth first began, life started in the sea. Chemicals combined to form the first RNA. The first cells were probably anaerobic (without oxygen), according to fossils dating back 3.5 billion years. Photosynthesis, using the sun to convert CO2 to O2 by plant like organisms, started 3.4 billion years ago. Bacteria and algae were present at this time.

Animal organisms first appeared 600 million years ago as "jellyfish-like animals" in what is now Australia!

500 million years ago, coral-like animals appeared. In this period of time, over 400 marine "families" were present. The first groups of fish-like organisms were similar to a tadpole; having sense organs, a tail, a very basic digestive system, and a simple nervous system. They ate by bottom sucking or filter feeding. The first vertebrates, animals with backbones, appeared.

Plants and fungi came into being 400 million years ago. Fish with jaws appeared about this time, with the jaws evolving from the gill structure. They also possessed a pair of mobile fins. (440 million years ago, called the Silurian Period).

During the "Devonian Period," which lasted 50 million years, ancestors of modern fishes evolved. This period started about 395 million years ago. The earliest forms of fish were saltwater, with freshwater fish evolving later. The earliest fish were only 1 to 2 inches long and tapered from the head to the tail. The body was covered with scales, they had gills, and the eyes were well developed. Early fish "lungs" evolved into swim bladders.

About 250 million years ago, the largest extinction in history occurred, wiping out greater than 95% of all living species. It is thought that during this time, there was only one land mass, surrounded by sea. This land mass began to break apart and drift into what are now the separate continents (65 million years ago, dinosaurs became extinct). This is why different species are found in different parts of the world. They continued to evolve into today’s fishes.

There are about one million species of animals on earth. Only 160,000 live in the sea.

Of all the vertebrates, most are aquatic, with over 24,000 species of living bony fish.

The Classification Tree of Symphysodon Discus, including sub-families and tribes as proposed by Kullander, 1998.

Animal Kingdom
  Phylum: Chordata
    Subphylum: Vertebrate (with backbone)
      Class: Osteichthyes (bony fish)
        Subclass: Actinopterygii
          Infraclass: Teleostei (ray finned fishes)
            Order: Cichlidae (Cichlids)
              New World Cichlids
                  Heroini (> 4 anal fin spines, >15 anal rays total)
                    Symphysodon Discus

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